This work was financially supported by a Grant-in-Aid for Specially Promoted Research from the Japan Society for Promotion of Science (JSPS) KAKENHI Grant Number 24000010, 17H04969, and JP17J02602, the US Office of Naval Research Global (N62909-17-1-2038). Y.T. is a JSPS Research Fellow and supported by JSPS through the Program for Leading Graduate Schools (MERIT).

1. Formation of a Monolayer Biofilm of S. oneidensis MR-1 on an ITO Electrode (Figure 1)
NOTE: To prevent the contamination of the electrochemical reactor with other microbes, all the media, implements, and components of the electrochemical reactor should be sterilized in advance. When using S. oneidensis MR-1 cells and constructing the electrochemical reactors, all the procedures should be conducted on a clean bench.
<ol>
	<li>Cultivation of S. oneidensis MR-1 cells 
	NOTE: A monolayer biofilm of S. oneidensis MR-1 was formed on an ITO electrode following the conditions reported previously4.
	<ol>
		<li>To grow S. oneidensis MR-1 cells, add one colony of S. oneidensis MR-1 grown on an agar plate comprising Luria-Bertani (LB) (20 g/L) and bacto agar (15 g/L) into 15 mL of LB medium (20 g/L) at 30&#160;&#176;C for 24 h in aerobic conditions with shaking at 160 rpm.</li>
		<li>Centrifuge the cell suspension at 6,000 &#215; g for 10 min, and resuspend the resultant cell pellet in 15 mL of defined medium (pH 7.8) (DM: NaHCO3 [2.5 g/L], CaCl2&#183;2H2O [0.08 g/L], NH4Cl [1.0 g/L], MgCl2&#183;6H2O [0.2 g/L], NaCl [10 g/L], and 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid [HEPES; 7.2 g/L]), supplemented with 10 mM lactate and 0.5 g/L yeast extract as the source of carbon, and trace elements for S. oneidensis MR-1, respectively.</li>
		<li>Further cultivate the cell suspension aerobically at 30&#160;&#176;C for 12 h with shaking at 160 rpm and centrifuge again at 6,000 &#215; g for 10 min. Wash the resultant cell pellet twice with DM medium by centrifugation for 10 min at 6,000 &#215; g before the electrochemical experiment.</li>
	</ol>
	</li>
	<li>Construction of a three-electrode electrochemical reactor (Figure 1)
	<ol>
		<li>Put an ITO substrate&#160;as the working electrode at the bottom of the reactor.</li>
		<li>Subsequently, insert a glass cylinder (diameter of 2 cm) and a polytetrafluoroethylene (PTFE) cover. Then insert Ag/AgCl (KCl saturated) and a platinum wire into the reactor as the reference and counter electrodes, respectively. 
		NOTE: To prevent the leakage of air and solution, insert a butyl rubber sheet between each component.</li>
		<li>Add 4.0 mL of DM supplemented with 10 mM lactate and 0.5 g/L yeast extract into the electrochemical reactor.</li>
		<li>After confirming that there is no leakage from the electrochemical reactor, flow the nitrogen gas into the electrochemical reactor over 20 min to maintain anaerobic conditions inside the electrochemical reactor. 
		NOTE: To prevent contamination with other microbes, the gas should be filtered before it flows into the electrochemical reactor.</li>
		<li>Connect the electrochemical reactor to a potentiostat and apply +0.4 V (versus standard hydrogen electrode, SHE) to the ITO electrode, keeping the temperature of the electrochemical reactor at 30&#160;&#176;C using an external water circulation system. 
		NOTE: To prevent the effect of an external electric field, the electrochemical reactor should be placed in a Faraday cage.</li>
	</ol>
	</li>
	<li>Electrochemical cultivation of S. oneidensis MR-1 cells (Figure 1 and Figure 2)
	<ol>
		<li>Adjust the cell density of the suspension obtained in step 1.1 to an optical density of 1.43 at 600 nm (OD600) with DM supplemented by 10 mM lactate and 0.5 g/L yeast extract. 
		NOTE: To obtain the correct OD600, apply the cell suspension of OD600 &#8804; 0.8 to a UV-vis spectrometer prior to adjusting the OD600 to 1.43.</li>
		<li>Add 0.3 mL of the cell suspension into the electrochemical reactor through the injection port using a syringe: the OD600 in the reactor changes to 0.1. 
		NOTE: The addition of 0.3 mL cell suspension with OD600 = 1.43 into electrochemical reactor containing 4.0 mL medium results in 4.3 mL of solution with an OD600 = 0.1. When using other reactors with different volumes, the calculation of the cell density is required.</li>
		<li>Continue the potential application at +0.4 V (versus SHE) to the ITO electrode for 25 h. 
		NOTE: For the formation of a monolayer biofilm on an ITO electrode, ensure that the produced current exhibits a deviation less than 50% from Figure 2.</li>
	</ol>
	</li>
</ol>
2. Replacement of the Supernatant with Fresh DM Medium with 10 mM Lactate (Figure 3)
<ol>
	<li>Stop the potential application and disconnect the electrochemical reactor from the potentiostat and the water circulation system.</li>
	<li>Replacement of the supernatant with fresh DM medium with 10 mM lactate
	<ol>
		<li>Flow the nitrogen gas into a bottle containing DM with 10 mM lactate over 20 min to remove oxygen from the medium.</li>
		<li>Slowly remove all the supernatant inside the electrochemical reactor using a syringe, under flowing nitrogen gas (Figure 3a, b). 
		NOTE: To avoid breaking the biofilm on the ITO electrode by nitrogen gas, the gas should flow above the liquid surface.</li>
		<li>Add 4.0 mL of fresh DM containing 10 mM lactate using a syringe (Figure 3c). 
		NOTE: To avoid breaking the biofilm on the ITO electrode, slowly inject the medium along the wall of the electrochemical reactor. To keep the biofilm wet, the medium should be added immediately after the removal of supernatant. Injection of the medium upto 1 min after the removal of supernatant does not impact the current production from the biofilm of S. oneidensis MR-1.</li>
		<li>Slant the electrochemical reactor to remove all of the supernatant attached to the wall of the electrochemical reactor.</li>
		<li>Repeat the steps 2.2.2-2.2.4 three times in total.</li>
	</ol>
	</li>
	<li>Stop the gas flow and connect the electrochemical reactor to the potentiostat again, applying +0.4 V (versus SHE) to the ITO electrode at 303&#160;K.</li>
</ol>
3. Addition of Deuterium Water to Measure the KIE on the EET Process (Figure 4)
<ol>
	<li>Confirm that the current production from a monolayer biofilm of S. oneidensis MR-1 is stable and does not increase rapidly. If the current increases steeply, wait until the current stabilizes within a 5% increase over 10 min. 
	NOTE: The formation of a monolayer biofilm on an ITO electrode without contamination of other microbial strains was confirmed by rDNA sequences and a scanning electron microscopic image of the biofilm on an ITO electrode, as reported previously4. For confirmation of the rate limitation by the EET process, monitor the effect of adding shuttling electron mediators such as 100 &#181;M anthraquinone-1-sulfonate ( &#945;-AQS). See the section of Representative Results and reference15 for further details.</li>
	<li>Add 40 &#181;L of anoxic 50% (v/v) D2O into the electrochemical reactor using a syringe such that the concentration is 0.5% (v/v) D2O in the reactor. 
	NOTE: To prevent damage to the biofilm by D2O addition, inject the D2O drop-wise.</li>
	<li>Wait for the current to stabilize, and subsequently add the D2O up to 4.0% (v/v). 
	NOTE: To obtain the KIE value (the ratio of current production in the presence of D2O and H2O), check the effect of the same volume of H2O addition on the current production.</li>
</ol>

<ol>
	<li>Nealson, K. H., Saffarini, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Iron+and+Manganese+in+Anaerobic+Respiration+-+Environmental+Significance,+Physiology,+and+Regulation.">Iron and Manganese in Anaerobic Respiration - Environmental Significance, Physiology, and Regulation.</a> Annu. Rev. Microbiol. 48, 311-343 (1994).</li><li>Bretschger, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Current+production+and+metal+oxide+reduction+by+Shewanella+oneidensis+MR-1+wild+type+and+mutants.">Current production and metal oxide reduction by Shewanella oneidensis MR-1 wild type and mutants.</a> Appl Environ Microb. 73 (21), 7003-7012 (2007).</li><li>Richter, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cyclic+voltammetry+of+biofilms+of+wild+type+and+mutant+Geobacter+sulfurreducens+on+fuel+cell+anodes+indicates+possible+roles+of+OmcB,+OmcZ,+type+IV+pili,+and+protons+in+extracellular+electron+transfer.">Cyclic voltammetry of biofilms of wild type and mutant Geobacter sulfurreducens on fuel cell anodes indicates possible roles of OmcB, OmcZ, type IV pili, and protons in extracellular electron transfer.</a> Energy Environ. Sci. 2 (5), 506-516 (2009).</li><li>Okamoto, A., Nakamura, R., Hashimoto, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In-vivo+identification+of+direct+electron+transfer+from+Shewanella+oneidensis+MR-1+to+electrodes+via+outer-membrane+OmcA-MtrCAB+protein+complexes.">In-vivo identification of direct electron transfer from Shewanella oneidensis MR-1 to electrodes via outer-membrane OmcA-MtrCAB protein complexes.</a> Electrochim. Acta. 56 (16), 5526-5531 (2011).</li><li>Strycharz, S. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Application+of+cyclic+voltammetry+to+investigate+enhanced+catalytic+current+generation+by+biofilm-modified+anodes+of+Geobacter+sulfurreducens+strain+DL1+vs.+variant+strain+KN400.">Application of cyclic voltammetry to investigate enhanced catalytic current generation by biofilm-modified anodes of Geobacter sulfurreducens strain DL1 vs. variant strain KN400.</a> Energy Environ. Sci. 4 (3), 896-913 (2011).</li><li>Lovley, D. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bug+juice:+harvesting+electricity+with+microorganisms.">Bug juice: harvesting electricity with microorganisms.</a> Nat. Rev. Microbiol. 4 (7), 497-508 (2006).</li><li>Coursolle, D., Gralnick, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reconstruction+of+extracellular+respiratory+pathways+for+iron(III)+reduction+in+Shewanella+oneidensis+strain+MR-1.">Reconstruction of extracellular respiratory pathways for iron(III) reduction in Shewanella oneidensis strain MR-1.</a> Front. Microbiol. 3, (2012).</li><li>Franks, A. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Novel+strategy+for+three-dimensional+real-time+imaging+of+microbial+fuel+cell+communities:+monitoring+the+inhibitory+effects+of+proton+accumulation+within+the+anode+biofilm.">Novel strategy for three-dimensional real-time imaging of microbial fuel cell communities: monitoring the inhibitory effects of proton accumulation within the anode biofilm.</a> Energy Environ. Sci. 2 (1), 113-119 (2009).</li><li>McLean, J. S., Ona, O. N., Majors, P. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Correlated+biofilm+imaging,+transport+and+metabolism+measurements+via+combined+nuclear+magnetic+resonance+and+confocal+microscopy.">Correlated biofilm imaging, transport and metabolism measurements via combined nuclear magnetic resonance and confocal microscopy.</a> ISME J. 2 (2), 121-131 (2008).</li><li>Busalmen, J. P., Esteve-Nunez, A., Berna, A., Feliu, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=C-type+cytochromes+wire+electricity-producing+bacteria+to+electrodes.">C-type cytochromes wire electricity-producing bacteria to electrodes.</a> Angew. Chem. Int. Ed. 47 (26), 4874-4877 (2008).</li><li>Nakamura, R., Ishii, K., Hashimoto, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electronic+Absorption+Spectra+and+Redox+Properties+of+C+Type+Cytochromes+in+Living+Microbes.">Electronic Absorption Spectra and Redox Properties of C Type Cytochromes in Living Microbes.</a> Angew. Chem. Int. Ed. 48 (9), 1606-1608 (2009).</li><li>Myers, C. R., Nealson, K. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Respiration-Linked+Proton+Translocation+Coupled+to+Anaerobic+Reduction+of+Manganese(IV)+and+Iron(III)+in+Shewanella+putrefaciens+MR-1.">Respiration-Linked Proton Translocation Coupled to Anaerobic Reduction of Manganese(IV) and Iron(III) in Shewanella putrefaciens MR-1.</a> J. Bacteriol. 172 (11), 6232-6238 (1990).</li><li>Tokunou, Y., Hashimoto, K., Okamoto, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Extracellular+Electron+Transport+Scarcely+Accumulates+Proton+Motive+Force+in+Shewanella+oneidensis+MR-1.">Extracellular Electron Transport Scarcely Accumulates Proton Motive Force in Shewanella oneidensis MR-1.</a> Bull. Chem. Soc. Jpn. 88 (5), 690-692 (2015).</li><li>Okamoto, A., Tokunou, Y., Saito, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cation-limited+kinetic+model+for+microbial+extracellular+electron+transport+via+an+outer+membrane+cytochrome+C+complex.">Cation-limited kinetic model for microbial extracellular electron transport via an outer membrane cytochrome C complex.</a> Biophysics and physicobiology. 13, 71-76 (2016).</li><li>Okamoto, A., Tokunou, Y., Shafeer, K., Hashimoto, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Proton+Transport+in+the+Outer-Membrane+Flavocytochrome+Complex+Limits+the+Rate+of+Extracellular+Electron+Transport.">Proton Transport in the Outer-Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport.</a> Angew. Chem. Int. Ed. 56, 9082-9086 (2017).</li><li>Hammes-Schiffer, S., Stuchebrukhov, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Theory+of+Coupled+Electron+and+Proton+Transfer+Reactions.">Theory of Coupled Electron and Proton Transfer Reactions.</a> Chem. Rev. 110 (12), 6939-6960 (2010).</li><li>Cleland, W. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+use+of+isotope+effects+to+determine+enzyme+mechanisms.">The use of isotope effects to determine enzyme mechanisms.</a> J Biol. Chem. 278 (52), 51975-51984 (2003).</li><li>Kouzuma, A., Kasai, T., Hirose, A., Watanabe, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Catabolic+and+regulatory+systems+in+Shewanella+oneidensis+MR-1+involved+in+electricity+generation+in+microbial+fuel+cells.">Catabolic and regulatory systems in Shewanella oneidensis MR-1 involved in electricity generation in microbial fuel cells.</a> Front. Microbiol. 6, (2015).</li><li>Kushner, D. J., Baker, A., Dunstall, T. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pharmacological+uses+and+perspectives+of+heavy+water+and+deuterated+compounds.">Pharmacological uses and perspectives of heavy water and deuterated compounds.</a> Can. J Physiol. Pharm. 77 (2), 79-88 (1999).</li><li>Xie, X. S., Zubarev, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+Low-Level+Deuterium+Enrichment+on+Bacterial+Growth.">Effects of Low-Level Deuterium Enrichment on Bacterial Growth.</a> Plos One. 9 (7), e102071 (2014).</li><li>Okamoto, A., Hashimoto, K., Nealson, K. H., Nakamura, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rate+enhancement+of+bacterial+extracellular+electron+transport+involves+bound+flavin+semiquinones.">Rate enhancement of bacterial extracellular electron transport involves bound flavin semiquinones.</a> Proc. Natl. Acad. Sci. U. S. A. 110 (19), 7856-7861 (2013).</li><li>Edwards, M. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Redox+Linked+Flavin+Sites+in+Extracellular+Decaheme+Proteins+Involved+in+Microbe-Mineral+Electron+Transfer.">Redox Linked Flavin Sites in Extracellular Decaheme Proteins Involved in Microbe-Mineral Electron Transfer.</a> Sci. Rep. 5, 11677 (2015).</li><li>Saito, J., Hashimoto, K., Okamoto, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flavin+as+an+Indicator+of+the+Rate-Limiting+Factor+for+Microbial+Current+Production+in+Shewanella+oneidensis+MR-1.">Flavin as an Indicator of the Rate-Limiting Factor for Microbial Current Production in Shewanella oneidensis MR-1.</a> Electrochim. Acta. 216, 261-265 (2016).</li><li>Guo, J. B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reduction+of+Cr(VI)+by+Escherichia+coli+BL21+in+the+presence+of+redox+mediators.">Reduction of Cr(VI) by Escherichia coli BL21 in the presence of redox mediators.</a> Bioresource Technol. 123, 713-716 (2012).</li><li>Nealson, K., Saffarini, D., Moser, D., Smith, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Spectrophotometric+Method+for+Monitoring+Tactic+Responses+of+Bacteria+under+Anaerobic+Conditions.">A Spectrophotometric Method for Monitoring Tactic Responses of Bacteria under Anaerobic Conditions.</a> J Microbiol. Meth. 20 (3), 211-218 (1994).</li><li>Myers, C. R., Myers, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell+surface+exposure+of+the+outer+membrane+cytochromes+of+Shewanella+oneidensis+MR-1.">Cell surface exposure of the outer membrane cytochromes of Shewanella oneidensis MR-1.</a> Lett. Appl. Microbiol. 37 (3), 254-258 (2003).</li><li>Lower, B. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Antibody+Recognition+Force+Microscopy+Shows+that+Outer+Membrane+Cytochromes+OmcA+and+MtrC+Are+Expressed+on+the+Exterior+Surface+of+Shewanella+oneidensis+MR-1.">Antibody Recognition Force Microscopy Shows that Outer Membrane Cytochromes OmcA and MtrC Are Expressed on the Exterior Surface of Shewanella oneidensis MR-1.</a> Appl. Environ. Microbiol. 75 (9), 2931-2935 (2009).</li><li>Chen, X. X., Ferrigno, R., Yang, J., Whitesides, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Redox+properties+of+cytochrome+c+adsorbed+on+self-assembled+monolayers:+A+probe+for+protein+conformation+and+orientation.">Redox properties of cytochrome c adsorbed on self-assembled monolayers: A probe for protein conformation and orientation.</a> Langmuir. 18 (18), 7009-7015 (2002).</li><li>McMillan, D. G. G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+impact+of+enzyme+orientation+and+electrode+topology+on+the+catalytic+activity+of+adsorbed+redox+enzymes.">The impact of enzyme orientation and electrode topology on the catalytic activity of adsorbed redox enzymes.</a> Electrochim. Acta. 110, 79-85 (2013).</li><li>Dinh, H. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Iron+corrosion+by+novel+anaerobic+microorganisms.">Iron corrosion by novel anaerobic microorganisms.</a> Nature. 427 (6977), 829-832 (2004).</li><li>McGlynn, S. E., Chadwick, G. L., Kempes, C. P., Orphan, V. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single+cell+activity+reveals+direct+electron+transfer+in+methanotrophic+consortia.">Single cell activity reveals direct electron transfer in methanotrophic consortia.</a> Nature. 526 (7574), 531-535 (2015).</li><li>Okamoto, A., Nakamura, R., Nealson, K. H., Hashimoto, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bound+Flavin+Model+Suggests+Similar+Electron-Transfer+Mechanisms+in+Shewanella+and+Geobacter.">Bound Flavin Model Suggests Similar Electron-Transfer Mechanisms in Shewanella and Geobacter.</a> Chemelectrochem. 1 (11), 1808-1812 (2014).</li><li>Okamoto, A., Hashimoto, K., Nealson, K. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flavin+Redox+Bifurcation+as+a+Mechanism+for+Controlling+the+Direction+of+Electron+Flow+during+Extracellular+Electron+Transfer.">Flavin Redox Bifurcation as a Mechanism for Controlling the Direction of Electron Flow during Extracellular Electron Transfer.</a> Angew. Chem. Int. Ed. 53 (41), 10988-10991 (2014).</li><li>Tokunou, Y., Hashimoto, K., Okamoto, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Acceleration+of+Extracellular+Electron+Transfer+by+Alternative+Redox-Active+Molecules+to+Riboflavin+for+Outer-Membrane+Cytochrome+c+of+Shewanella+oneidensis+MR-1.">Acceleration of Extracellular Electron Transfer by Alternative Redox-Active Molecules to Riboflavin for Outer-Membrane Cytochrome c of Shewanella oneidensis MR-1.</a> J Phys. Chem. C. 120 (29), 16168-16173 (2016).</li><li>Rowe, A. R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tracking+electron+uptake+from+a+cathode+into+Shewanella+cells:+implications+for+generating+maintenance+energy+from+solid+substrates.">Tracking electron uptake from a cathode into Shewanella cells: implications for generating maintenance energy from solid substrates.</a> bioRxiv. , 116475 (2017).</li></ol>

The authors have nothing to disclose.

Our whole-cell electrochemical assay has several technical advantages compared with protein electrochemistry. While protein purification requires multi-step time-consuming procedures, our whole-cell method takes one day of self-organized biofilm formation after cell culture. To achieve a stable interaction between OM c-Cyts and the electrode, we need only sterilization and cleaning of the electrode surface; it does not require electrode modification for organizing the orientation of proteins4</...

Electrochemical techniques to directly characterize a redox protein in an intact bacterial cell have recently emerged since the discovery of metal-reducing microbial strains, such as S. oneidensis MR-1 or Geobacter sulfurreducens PCA, which have outer membrane c-type cytochrome complexes (OM c-Cyts) exposed to the cell exterior1,2,3,4,5. The OM c-Cyts mediate electron transport from the respiratory chain to solid substrates located extracellularly. This transport is referred to as extracellular electron transport (EET)1,6 and is a critical process for emerging biotechnologies, such as microbial fuel cells6. Therefore, to understand the underlying EET kinetics and mechanisms, and its link to microbial physiology, OM c-Cyts have been investigated using whole-cell electrochemistry4,7, combined with microscopy8,9, spectroscopy10,11, and molecular biology2,4. In contrast, methods to investigate the impact of EET-associated cation transport, e.g., protons, on EET kinetics in living cells have been scarcely established, despite proton transport across bacterial membranes having a critical role in signaling, homeostasis, and energy production12,13,14. In the present study, we describe a technique to examine the impact of proton transport on EET kinetics in the S. oneidensis MR-1 cell using whole-cell electrochemical measurements, which requires the identification of the rate-limiting step in microbial current production15.
One direct way to evaluate the contribution of proton transport on the associated EET is the deuterium kinetic isotope effect (KIE). The KIE is observable as the change in electron transfer kinetics upon the replacement of protons with deuterium ions, which represents the impact of proton transport on electron transfer kinetics16. The theory of KIE itself has been well established using electrochemical measurements with purified enzymes17. However, since current production in S. oneidensis MR-1 results from multiple, diverse, and fluctuating processes18, one cannot simply identify EET as the rate-limiting process. To observe the KIE on proton transport processes coupled with EET, we need to confirm that the microbial current production is limited by electron transport via OM c-Cyts to the electrode. For this purpose, we replaced the supernatant solution with fresh medium containing a high concentration of lactate as an electron donor at the optimal pH and temperature conditions before KIE measurement; this replacement served two roles: (1) it enhanced the rate of the upstream metabolic processes compared to the EET, and (2) omitted the swimming cells in the supernatant released from the monolayer biofilm of S. oneidensis MR-1 on the working electrode (indium tin-doped oxide (ITO) electrode). The presented detailed protocol is intended to help new practitioners maintain and confirm that the EET process is the rate-determining step.

<table><tbody><tr><td>Glass cylinder</td><td>N/A</td><td>N/A</td><td>Custom-made, used as the electrochemical reactor</td></tr><tr><td>PTFE cover and base</td><td>N/A</td><td>N/A</td><td>Custom-made, used as a cover and a foundation of the electrochemical reactor</td></tr><tr><td>Buthyl rubber</td><td>N/A</td><td>N/A</td><td>Custom-made, inserted between each component of electrochemical reactor</td></tr><tr><td>Septa</td><td>GL Science</td><td>3007-16101</td><td>Used as an injection port of electrochemical reactor</td></tr><tr><td>Indium tin-doped oxide (ITO) electrode</td><td>GEOMATEC</td><td>No.0001</td><td>Used as a working electrode, 5&amp;Omega;/sq</td></tr><tr><td>Ag/AgCl KCl saturated electrode</td><td>HOKUTO DENKO</td><td>HX-R5</td><td>Used as a reference electrode, &amp;Phi;0.30mm</td></tr><tr><td>Platinum wire</td><td>The Nilaco Cooporation</td><td>PT-351325</td><td>Used as a counter electrode</td></tr><tr><td>Luria-Bertani (LB) Broth, Miller</td><td>Becton, Dichkinson and Company</td><td>244620</td><td>Medium for precultivation of &lt;em&gt;S. oneidensis&lt;/em&gt; MR-1</td></tr><tr><td>Bacto agar</td><td>Becton, Dichkinson and Company</td><td>214010</td><td></td></tr><tr><td>Anthraquinone-1-sulfonate (&amp;alpha;-AQS)</td><td>TCI</td><td>A1428</td><td></td></tr><tr><td>Flavin mononucleotide (FMN)</td><td>Wako</td><td>184-00831</td><td></td></tr><tr><td>NaHCO&lt;sub&gt;3&lt;/sub&gt;</td><td>Wako</td><td>191-01305</td><td>Used for defined medium (DM)</td></tr><tr><td>CaCl2 &amp;middot; 2H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Wako</td><td>031-00435</td><td>Used for DM</td></tr><tr><td>NH&lt;sub&gt;4&lt;/sub&gt;Cl</td><td>Wako</td><td>011-03015</td><td>Used for DM</td></tr><tr><td>MgCl&lt;sub&gt;2&lt;/sub&gt; &amp;middot; 6H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Wako</td><td>135-00165</td><td>Used for DM</td></tr><tr><td>NaCl</td><td>Wako</td><td>191-01665</td><td>Used for DM</td></tr><tr><td>2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES)</td><td>DOJINDO</td><td>346-08235</td><td>Used for DM</td></tr><tr><td>Sodium Lactate Solution</td><td>Wako</td><td>195-02305</td><td></td></tr><tr><td>Bacto Yeast Extract</td><td>Becton, Dichkinson and Company</td><td>212750</td><td></td></tr><tr><td>Deuterium oxide (D, 99.9%)</td><td>Cambridge Isotope Laboratories, Inc.</td><td>DLM-4-PK</td><td>Additive for kinetic isotope effect experiments</td></tr><tr><td>Incubator</td><td>TOKYO RIKAKIKAI CO. LTD.</td><td>LTI-601SD</td><td>Used for precultivation</td></tr><tr><td>Shaker</td><td>TAITEC</td><td>NR-3</td><td>Used for precultivation</td></tr><tr><td>Autoclave machine</td><td>TOMY SEIKO CO. LTD.</td><td>LSX-500</td><td>Used for sterilization of the electrochemical reactor and the medium</td></tr><tr><td>Clean bench</td><td>SANYO</td><td>MCV-91BNF</td><td>Used to prevent the contamination of the electrochemical reactor and the medium with other microbes</td></tr><tr><td>Centrifuge separator</td><td>Eppendorf</td><td>5430R</td><td>Rotational speed upto 6000&amp;times;g is required</td></tr><tr><td>Nitrogen gas generator</td><td>Puequ CO. LTD.</td><td>PNTN-2</td><td>Nitrogen gas cylinder can also be used instead of gas generator</td></tr><tr><td>UV-vis spectrometer</td><td>SHIMADZU</td><td>UV-1800</td><td>Used for optimization of cell density</td></tr><tr><td>Potentiostat</td><td>BioLogic</td><td>VMP3</td><td>Used for biofilm formation and kinetic isotope effect experiments</td></tr><tr><td>Thermal water circulator</td><td>AS ONE</td><td>TR-1A</td><td>Used for maintanance of temperature of electrochemcial reactor</td></tr><tr><td>Faraday cage</td><td>HOKUTO DENKO</td><td>HS-201S</td><td>Used for electrochemical experiments</td></tr></tbody></table>

electrochemical detection of deuterium kinetic isotope effect on extracellular electron transport in shewanella oneidensis mr-1

Direct electrochemical detection of c-type cytochrome complexes embedded in the bacterial outer membrane (outer membrane c-type cytochrome complexes; OM c-Cyts) has recently emerged as a novel whole-cell analytical method to characterize the bacterial electron transport from the respiratory chain to the cell exterior, referred to as the extracellular electron transport (EET). While the pathway and kinetics of the electron flow during the EET reaction have been investigated, a whole-cell electrochemical method to examine the impact of cation transport associated with EET has not yet been established. In the present study, an example of a biochemical technique to examine the deuterium kinetic isotope effect (KIE) on EET through OM c-Cyts using a model microbe, Shewanella oneidensis MR-1, is described. The KIE on the EET process can be obtained if the EET through OM c-Cyts acts as the rate-limiting step in the microbial current production. To that end, before the addition of D2O, the supernatant solution was replaced with fresh media containing a sufficient amount of the electron donor to support the rate of upstream metabolic reactions, and to remove the planktonic cells from a uniform monolayer biofilm on the working electrode. Alternative methods to confirm the rate-limiting step in microbial current production as EET through OM c-Cyts are also described. Our technique of a whole-cell electrochemical assay for investigating proton transport kinetics can be applied to other electroactive microbial strains.

After 25 h of potential application at +0.4 V (versus SHE), a monolayer biofilm was formed on the working electrode of ITO glass, which was previously confirmed by either a scanning electron microscopy or a confocal microscopy4. The representative time course of current production from the S. oneidensis MR-1 during the formation of a monolayer biofilm is shown in Figure 2. Although the current alters in every measurement, the ...

Watch this Scientific Journal Video about Electrochemical Detection of Deuterium Kinetic Isotope Effect on Extracellular Electron Transport in Shewanella oneidensis MR-1 at JoVE.com

Electrochemical Detection of Deuterium Kinetic Isotope Effect on Extracellular Electron Transport in Shewanella oneidensis MR-1

Here we present a protocol of whole-cell electrochemical experiments to study the contribution of proton transport to the rate of extracellular electron transport via the outer-membrane cytochromes complex in Shewanella oneidensis MR-1.

Electrochemical Detection of Deuterium Kinetic Isotope Effect on Extracellular Electron Transport in Shewanella oneidensis MR-1

Direct electrochemical detection of c-type cytochrome complexes embedded in the bacterial outer membrane (outer membrane c-type cytochrome complexes; OM ...

electrochemical-detection-deuterium-kinetic-isotope-effect-on

Research

JoVE Journal

Biochemistry

9.9K Views.  University of Tokyo. Here we present a protocol of whole-cell electrochemical experiments to study the contribution of proton transport to the rate of extracellular electron transport via the outer-membrane cytochromes complex in Shewanella oneidensis MR-1.

Video: Electrochemical Detection of Deuterium Kinetic Isotope Effect on Extracellular Electron Transport in Shewanella oneidensis MR-1

Waste Water Derived Electroactive Microbial Biofilms: Growth, Maintenance, and Basic Characterization

The growth of anodic electroactive microbial biofilms from waste water inocula in a fed-batch reactor is demonstrated using a three-electrode setup controlled by a potentiostat. Thereby the use of potentiostats allows an exact adjustment of the electrode potential and ensures reproducible microbial culturing conditions. During growth the current production is monitored using chronoamperometry (CA). Based on these data the maximum current density (jmax) and the coulombic efficiency (CE) are discussed as measures for characterization of the bioelectrocatalytic performance. Cyclic voltammetry (CV), a nondestructive, i.e. noninvasive, method, is used to study the extracellular electron transfer (EET) of electroactive bacteria. CV measurements are performed on anodic biofilm electrodes in the presence of the microbial substrate, i.e. turnover conditions, and in the absence of the substrate, i.e. nonturnover conditions, using different scan rates. Subsequently, data analysis is exemplified and fundamental thermodynamic parameters of the microbial EET are derived and explained: peak potential (Ep), peak current density (jp), formal potential (Ef) and peak separation (&#916;Ep). Additionally the limits of the method and the state-of the art data analysis are addressed. Thereby this video-article shall provide a guide for the basic experimental steps and the fundamental data analysis.

Chronoamperometric growth of anodic electroactive microbial biofilms in a fed-batch reactor using a three-electrode setup, controlled by a potentiostat, is demonstrated. The extracellular electron transfer is characterized using cyclic voltammetry in the presence of the electron donor and in the absence of the electron donor. Fundamental data analysis is demonstrated.

The growth of anodic electroactive microbial biofilms from waste water inocula in a fed-batch reactor is demonstrated using a three-electrode setup ...

Environment

Time-resolved ElectroSpray Ionization Hydrogen-deuterium Exchange Mass Spectrometry for Studying Protein Structure and Dynamics

Intrinsically disordered proteins (IDPs) have long been a challenge to structural biologists due to their lack of stable secondary structure elements. Hydrogen-Deuterium Exchange (HDX) measured at rapid time scales is uniquely suited to detect structures and hydrogen bonding networks that are briefly populated, allowing for the characterization of transient conformers in native ensembles. Coupling of HDX to mass spectrometry offers several key advantages, including high sensitivity, low sample consumption and no restriction on protein size. This technique has advanced greatly in the last several decades, including the ability to monitor HDX labeling times on the millisecond time scale. In addition, by incorporating the HDX workflow onto a microfluidic platform housing an acidic protease microreactor, we are able to localize dynamic properties at the peptide level. In this study, Time-Resolved ElectroSpray Ionization Mass Spectrometry (TRESI-MS) coupled to HDX was used to provide a detailed picture of residual structure in the tau protein, as well as the conformational shifts induced upon hyperphosphorylation.

Conformational flexibility plays a critical role in protein function. Herein, we describe the use of time-resolved electrospray ionization mass spectrometry coupled to hydrogen-deuterium exchange for probing the rapid structural changes that drive function in ordered and disordered proteins.

Intrinsically disordered proteins (IDPs) have long been a challenge to structural biologists due to their lack of stable secondary structure elements. ...

Bacterial Detection &amp; Identification Using Electrochemical Sensors

Electrochemical sensors are widely used for rapid and accurate measurement of blood glucose and can be adapted for detection of a wide variety of analytes. Electrochemical sensors operate by transducing a biological recognition event into a useful electrical signal. Signal transduction occurs by coupling the activity of a redox enzyme to an amperometric electrode. Sensor specificity is either an inherent characteristic of the enzyme, glucose oxidase in the case of a glucose sensor, or a product of linkage between the enzyme and an antibody or probe.
Here, we describe an electrochemical sensor assay method to directly detect and identify bacteria. In every case, the probes described here are DNA oligonucleotides. This method is based on sandwich hybridization of capture and detector probes with target ribosomal RNA (rRNA). The capture probe is anchored to the sensor surface, while the detector probe is linked to horseradish peroxidase (HRP). When a substrate such as 3,3',5,5'-tetramethylbenzidine (TMB) is added to an electrode with capture-target-detector complexes bound to its surface, the substrate is oxidized by HRP and reduced by the working electrode. This redox cycle results in shuttling of electrons by the substrate from the electrode to HRP, producing current flow in the electrode.

Bacterial Detection & Identification Using Electrochemical Sensors

We describe an electrochemical sensor assay method for rapid bacterial detection and identification. The assay involves a sensor array functionalized with DNA oligonucleotide capture probes for ribosomal RNA (rRNA) species-specific sequences. Sandwich hybridization of target rRNA with the capture probe and a horseradish peroxidase-linked DNA oligonucleotide detector probe produces a measurable amperometric current.

Electrochemical  sensors are widely used for rapid and accurate measurement of blood glucose and  can be adapted for detection of a wide variety of ...

Bioengineering

Measuring Trans-Plasma Membrane Electron Transport by C2C12 Myotubes

Trans-plasma membrane electron transport (tPMET) plays a role in protection of cells from intracellular reductive stress as well as protection from damage by extracellular oxidants. This process of transporting electrons from intracellular reductants to extracellular oxidants is not well defined. Here we present spectrophotometric assays by C2C12 myotubes to monitor tPMET utilizing the extracellular electron acceptors: water-soluble tetrazolium salt-1 (WST-1) and 2,6-dichlorophenolindophenol (DPIP or DCIP). Through reduction of these electron acceptors, we are able to monitor this process in a real-time analysis. With the addition of enzymes such as ascorbate oxidase (AO) and superoxide dismutase (SOD) to the assays, we can determine which portion of tPMET is due to ascorbate export or superoxide production, respectively. While WST-1 was shown to produce stable results with low background, DPIP was able to be re-oxidized after the addition of AO and SOD, which was demonstrated with spectrophotometric analysis. This method demonstrates a real-time, multi-well, quick spectrophotometric assay with advantages over other methods used to monitor tPMET, such as ferricyanide (FeCN) and ferricytochrome c reduction.

The goal of this protocol is to spectrophotometrically monitor trans-plasma membrane electron transport utilizing extracellular electron acceptors and to analyze enzymatic interactions that may occur with these extracellular electron acceptors.

Trans-plasma membrane electron transport (tPMET) plays a role in protection of cells from intracellular reductive stress as well as protection from damage ...

Self-standing Electrochemical Set-up to Enrich Anode-respiring Bacteria On-site

Anaerobic respiration coupled with electron transport to insoluble minerals (referred to as extracellular electron transport [EET]) is thought to be critical for microbial energy production and persistence in many subsurface environments, especially those lacking soluble terminal electron acceptors. While EET-capable microbes have been successfully isolated from various environments, the diversity of bacteria capable of EET is still poorly understood, especially in difficult-to-sample, low energy or extreme environments, such as many subsurface ecosystems. Here, we describe an on-site electrochemical system to enrich EET-capable bacteria using an anode as a respiratory terminal electron acceptor. This anode is connected&#160;to a cathode capable of&#160;catalyzing abiotic oxygen reduction. Comparing this approach with electrocultivation methods that use a potentiostat for poising the electrode potential, the two-electrode system does not require an external power source. We present an example of our on-site enrichment utilized in an alkaline pond at the Cedars, a terrestrial serpentinization site in Northern California. Prior attempts to cultivate mineral reducing bacteria were unsuccessful, which is likely due to the low-biomass nature of this site and/or the low relative abundance of metal reducing microbes. Prior to implementing our two-electrode enrichment, we measured the vertical profile of dissolved oxygen concentration. This allowed us to place the carbon felt anode and platinum-electroplated carbon felt cathode at depths that would support anaerobic and aerobic processes, respectively. Following on-site incubation, we further enriched the anodic electrode in the laboratory and confirmed&#160;a distinct microbial community compared to the surface-attached or biofilm communities normally observed at the Cedars. This enrichment subsequently led to the isolation of the first electrogenic microbe from the Cedars. This method of on-site microbial enrichment has the potential to greatly enhance the isolation of EET-capable bacteria from low biomass or difficult to sample habitats.

On-site microbial enrichment or in situ cultivation techniques can facilitate the isolation of difficult-to-culture microbial taxa, especially from low-biomass or geochemically extreme environments. Here, we describe an electrochemical set-up without using an external power source to enrich microbial strains that are capable of extracellular electron transport (EET).

Anaerobic respiration coupled with electron transport to insoluble minerals (referred to as extracellular electron transport [EET]) is thought to be ...

Characterizing Electron Transport through Living Biofilms

Here we demonstrate the method of electrochemical gating used to characterize electrical conductivity of electrode-grown microbial biofilms under physiologically relevant conditions.1 These measurements are performed on living biofilms in aqueous medium using source and drain electrodes patterned on a glass surface in a specialized configuration referred to as an interdigitated electrode (IDA) array. A biofilm is grown that extends across the gap connecting the source and drain. Potentials are applied to the electrodes (ES and ED) generating a source-drain current (ISD) through the biofilm between the electrodes. The dependency of electrical conductivity on gate potential (the average of the source and drain potentials, EG = [ED + ES]/2) is determined by systematically changing the gate potential and measuring the resulting source-drain current. The dependency of conductivity on gate potential provides mechanistic information about the extracellular electron transport process underlying the electrical conductivity of the specific biofilm under investigation. The electrochemical gating measurement method described here is based directly on that used by M. S. Wrighton2,3 and colleagues and R. W. Murray4,5,6 and colleagues in the 1980&#39;s to investigate thin film conductive polymers.

A protocol for measuring electrical conductivity of living microbial biofilms under physiologically relevant conditions is presented.

Here we demonstrate the method of electrochemical gating used to characterize electrical conductivity of electrode-grown microbial biofilms under ...

Chemistry

In Situ Characterization of Shewanella oneidensis MR1 Biofilms by SALVI and ToF-SIMS

Bacterial biofilms are surface-associated communities that are vastly studied to understand their self-produced extracellular polymeric substances (EPS) and their roles in environmental microbiology. This study outlines a method to cultivate biofilm attachment to the System for Analysis at the Liquid Vacuum Interface (SALVI) and achieve in situ chemical mapping of a living biofilm by time-of-flight secondary ion mass spectrometry (ToF-SIMS). This is done through the culturing of bacteria both outside and within the SALVI channel with our specialized setup, as well as through optical imaging techniques to detect the biofilm presence and thickness before ToF-SIMS analysis. Our results show the characteristic peaks of the Shewanella biofilm in its natural hydrated state, highlighting upon its localized water cluster environment, as well as EPS fragments, which are drastically different from the same biofilm's dehydrated state. These results demonstrate the breakthrough capability of SALVI that allows for in situ biofilm imaging with a vacuum-based chemical imaging instrument.

In Situ Characterization of Shewanella oneidensis MR1 Biofilms by SALVI and ToF-SIMS

This article presents a method for growing a biofilm for in situ time-of-flight secondary ion mass spectrometry for chemical mapping in its hydrated state, enabled by a microfluidic reactor, System for Analysis at the liquid Vacuum Interface. The Shewanella oneidensis MR-1 with green fluorescence protein was used as a model.

Bacterial biofilms are surface-associated communities that are vastly studied to understand their self-produced extracellular polymeric substances (EPS) ...

A Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS) Platform for Investigating Peptide Biosynthetic Enzymes

Hydrogen-deuterium exchange mass spectrometry (HDX-MS) is a powerful method for the biophysical characterization of enzyme conformational changes and enzyme-substrate interactions. Among its many benefits, HDX-MS consumes only small amounts of material, can be performed under near native conditions without the need for enzyme/substrate labeling, and can provide spatially resolved information on enzyme conformational dynamics&#8722;even for large enzymes and multiprotein complexes. The method is initiated by the dilution of the enzyme of interest into buffer prepared in D2O. This triggers the exchange of protium in peptide bond amides (N-H) with deuterium (N-D). At the desired exchange time points, reaction aliquots are quenched, the enzyme is proteolyzed into peptides, the peptides are separated by ultra-performance liquid chromatography (UPLC), and the change in mass of each peptide (due to the exchange of hydrogen for deuterium) is recorded by MS. The amount of deuterium uptake by each peptide is strongly dependent on the local hydrogen bonding environment of that peptide. Peptides present in very dynamic regions of the enzyme exchange deuterium very rapidly, whereas peptides derived from well-ordered regions undergo exchange much more slowly. In this manner, the HDX rate reports on local enzyme conformational dynamics. Perturbations to deuterium uptake levels in the presence of different ligands can then be used to map ligand binding sites, identify allosteric networks, and to understand the role of conformational dynamics in enzyme function. Here, we illustrate how we have used HDX-MS to better understand the biosynthesis of a type of peptide natural products called lanthipeptides. Lanthipeptides are genetically encoded peptides that are post-translationally modified by large, multifunctional, conformationally dynamic enzymes that are difficult to study with traditional structural biology approaches. HDX-MS provides an ideal and adaptable platform for investigating the mechanistic properties of these types of enzymes.

A Hydrogen-Deuterium Exchange Mass Spectrometry HDX-MS Platform for Investigating Peptide Biosynthetic Enzymes

Lanthipeptide synthetases catalyze multistep reactions during the biosynthesis of peptide natural products. Here, we describe a continuous, bottom-up, hydrogen-deuterium exchange mass spectrometry (HDX-MS) workflow that can be employed to study the conformational dynamics of lanthipeptide synthetases, as well as other similar enzymes involved in peptide natural product biosynthesis.

Hydrogen-deuterium exchange mass spectrometry (HDX-MS) is a powerful method for the biophysical characterization of enzyme conformational changes and ...

Chronoamperometry Analysis of EET in Lactiplantibacillus plantarum with DHNA

Characterizing Mediated Extracellular Electron Transfer in Lactic Acid Bacteria with a Three-Electrode, Two-Chamber Bioelectrochemical System

Many bacteria perform extracellular electron transfer (EET), whereby electrons are transferred from the cell to an extracellular terminal electron acceptor. This electron acceptor can be an electrode and electrons can be delivered indirectly via a redox-active mediator molecule. Here, we present a protocol to study mediated EET in Lactiplantibacillus plantarum, a probiotic lactic acid bacterium widely used in the food industry, using a bioelectrochemical system. We detail how to assemble a three-electrode, two-chambered bioelectrochemical system and provide guidance on characterizing EET in the presence of a soluble mediator using chronoamperometry and cyclic voltammetry techniques. We use representative data from 1,4-dihydroxy-2-naphthoic acid (DHNA)-mediated EET experiments with L. plantarum to demonstrate data analysis and interpretation. The techniques described in this protocol can open new opportunities for electro-fermentation and bioelectrocatalysis. Recent applications of this electrochemical technique with L. plantarum demonstrated an acceleration of metabolic flux towards producing fermentation end-products, which are critical flavor components in food fermentation. As such, this system has the potential to be further developed to alter flavors in food production or produce valuable chemicals.

Here we present a protocol for characterizing mediated extracellular electron transfer (EET) in lactic acid bacteria using a three-electrode, two-chamber bioelectrochemical system. We illustrate this method with Lactiplantibacillus plantarum and the redox mediator 1,4-dihydroxy-2-naphthoic acid and provide a thorough description of the electrochemical techniques used to evaluate mediated EET.

Many bacteria perform extracellular electron transfer (EET), whereby electrons are transferred from the cell to an extracellular terminal electron ...

Measuring Electron Transfer Activity in Shewanella oneidensis Using OECTs

Translating Extracellular Electron Transfer Activities with Organic Electrochemical Transistors

Extracellular electron transfer (EET) is a process through which certain microorganisms can transfer electrons across their cell membranes to external electron acceptors, linking cellular metabolism to their environment. While Geobacter and Shewanella have been the primary models for EET research, emerging studies reveal that EET-active species are also associated with fermentation and the human gut microbiome. Leveraging the ability of EET to bridge biological and electronic systems, we present a protocol for using organic electrochemical transistors (OECTs) to translate microbial EET activity into easily detectable electrical signals. This system enables the use of cellular responses to external stimuli for biosensing and biocomputing applications. Specifically, we demonstrated the de-doping of the p-type poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) channel in the OECT is driven by cellular EET from Shewanella oneidensis. By transcriptionally controlling EET flux by genetic circuits, we establish the biosensing capability of this hybrid OECT system to detect chemical stimuli, such as inducer molecules. Furthermore, we introduce plasmid-based Boolean logic gates within the cells, allowing them to process environmental signals and drive current changes in the OECTs, further demonstrating the biocomputing potential of these devices. This method provides a novel interface between biological systems and electronics, enabling future high-throughput screening, biosensing, and biocomputing applications.

Here, we present a protocol for using organic electrochemical transistors (OECTs) to translate extracellular electron transfer (EET) activity in Shewanella oneidensis into electrical signals. The hybrid OECT system provides enhanced robustness, sensitivity, and potential for rapid, high-throughput testing, making it an effective tool for EET measurements.

Extracellular electron transfer (EET) is a process through which certain microorganisms can transfer electrons across their cell membranes to external ...

Electrochemical Detection of Deuterium Kinetic Isotope Effect on Extracellular Electron Transport in Shewanella oneidensis MR-1

Summary

Explore More Videos

Chapters in this video

Waste Water Derived Electroactive Microbial Biofilms: Growth, Maintenance, and Basic Characterization

Time-resolved ElectroSpray Ionization Hydrogen-deuterium Exchange Mass Spectrometry for Studying Protein Structure and Dynamics

Bacterial Detection & Identification Using Electrochemical Sensors

Measuring Trans-Plasma Membrane Electron Transport by C2C12 Myotubes

Self-standing Electrochemical Set-up to Enrich Anode-respiring Bacteria On-site

Characterizing Electron Transport through Living Biofilms

In Situ Characterization of Shewanella oneidensis MR1 Biofilms by SALVI and ToF-SIMS

A Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS) Platform for Investigating Peptide Biosynthetic Enzymes

Characterizing Mediated Extracellular Electron Transfer in Lactic Acid Bacteria with a Three-Electrode, Two-Chamber Bioelectrochemical System

Translating Extracellular Electron Transfer Activities with Organic Electrochemical Transistors