This work was supported by the Canadian Institutes of Health Research (CIHR), National Science and Engineering Research Council (NSERC), and a CIHR training grant scholarship (to L.F.). We thank Prof. Gerard D. Wright (McMaster University, Canada) for the AAC(6)-Ii expression plasmid.

1. Preparing stock solutions
<ol>
	<li>Purify the macromolecule of interest. (In this case, aminoglycoside N-6'-acetyltransferase-Ii (AAC6'-Ii), is isolated as reported elsewhere.13)</li>
	<li>Prepare 4 litres of dialysis buffer. (In this case we used 25 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES, MW 238.3 g/mol), containing 2 mM ethylenediaminetetraacetic acid (EDTA, MW 292.2), at pH 7.5.)</li>
	<li>Dialyze the protein The AAC(6')-Ii sample (5 mL at 400 &mu;M) used must be dialyzed in ...

<ol>
	<li>Cliff, M. J., Ladbury, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+survey+of+the+year+2002+literature+on+applications+of+isothermal+titration+calorimetry.">A survey of the year 2002 literature on applications of isothermal titration calorimetry.</a> Journal of Molecular Recognition. 16, 383-391 (2003).</li><li>Cliff, M. J., Gutierrez, A., Ladbury, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+survey+of+the+year+2003+literature+on+applications+of+isothermal+titration+calorimetry.">A survey of the year 2003 literature on applications of isothermal titration calorimetry.</a> Journal of Molecular Recognition. 17, 513-523 (2004).</li><li>Ababou, A., Ladbury, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Survey+of+the+year+2004:+literature+on+applications+of+isothermal+titration+calorimetry.">Survey of the year 2004: literature on applications of isothermal titration calorimetry.</a> Journal of Molecular Recognition. 19, 79-89 (2006).</li><li>Ababou, A., Ladbury, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Survey+of+the+year+2005:+literature+on+applications+of+isothermal+titration+calorimetry.">Survey of the year 2005: literature on applications of isothermal titration calorimetry.</a> Journal of Molecular Recognition. 20, 4-14 (2007).</li><li>Okhrimenko, O. k. s. a. n. a., J, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+survey+of+the+year+2006+literature+on+applications+of+isothermal+titration+calorimetry.">A survey of the year 2006 literature on applications of isothermal titration calorimetry.</a> Journal of Molecular Recognition. 21, 1-19 (2008).</li><li>Bjelic, S., Jelesarov, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+survey+of+the+year+2007+literature+on+applications+of+isothermal+titration+calorimetry.">A survey of the year 2007 literature on applications of isothermal titration calorimetry.</a> Journal of Molecular Recognition. 21, 289-312 (2008).</li><li>Leavitt, S., Freire, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+measurement+of+protein+binding+energetics+by+isothermal+titration+calorimetry.">Direct measurement of protein binding energetics by isothermal titration calorimetry.</a> Current Opinion in Structural Biology. 11, 560-566 (2001).</li><li>Wiseman, T., Williston, S., Brandts, J. F., Lin, L. -. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+measurement+of+binding+constants+and+heats+of+binding+using+a+new+titration+calorimeter.">Rapid measurement of binding constants and heats of binding using a new titration calorimeter.</a> Analytical Biochemistry. 179, 131-137 (1989).</li><li>Capaldi, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+X-Ray+Structure+of+Zebrafish+(Danio+rerio)+Ileal+Bile+Acid-Binding+Protein+Reveals+the+Presence+of+Binding+Sites+on+the+Surface+of+the+Protein+Molecule.">The X-Ray Structure of Zebrafish (Danio rerio) Ileal Bile Acid-Binding Protein Reveals the Presence of Binding Sites on the Surface of the Protein Molecule.</a> Journal of Molecular Biology. 385, 99-116 (2009).</li><li>Freiburger, L. A., Auclair, K., Mittermaier, A. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Elucidating+Protein+Binding+Mechanisms+by+Variable-c+ITC.">Elucidating Protein Binding Mechanisms by Variable-c ITC.</a> ChemBioChem. 10, 2871-2873 (2009).</li><li>Wybenga-Groot, L. E., Draker, K. -. a., Wright, G. D., Berghuis, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Crystal+structure+of+an+aminoglycoside+6'-N-acetyltransferase:+defining+the+GCN5-related+N-acetyltransferase+superfamily+fold.">Crystal structure of an aminoglycoside 6'-N-acetyltransferase: defining the GCN5-related N-acetyltransferase superfamily fold.</a> Structure. 7, 497-507 (1999).</li><li>Draker, K., Northrop, D. B., Wright, G. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Kinetic+Mechanism+of+the+GCN5-Related+Chromosomal+Aminoglycoside+Acetyltransferase+AAC(6')-Ii+from+Enterococcus+faecium:+Evidence+of+Dimer+Subunit+Cooperativity.">Kinetic Mechanism of the GCN5-Related Chromosomal Aminoglycoside Acetyltransferase AAC(6')-Ii from Enterococcus faecium: Evidence of Dimer Subunit Cooperativity.</a> Biochemistry. 42, 6565-6574 (2003).</li><li>Wright, G. D., Ladak, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Overexpression+and+characterization+of+the+chromosomal+aminoglycoside+6'-N-acetyltransferase+from+Enterococcus+faecium.">Overexpression and characterization of the chromosomal aminoglycoside 6'-N-acetyltransferase from Enterococcus faecium.</a> Antimicrob. Agents Chemother. 41, 956-960 (1997).</li><li>MicroCal. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> ITC Data Analysis in Origin. , (2004).</li></ol>

This analytical portion of variable-c fitting has been previously described in detail10. Here we report practical aspects of collecting variable-c datasets suitable for this approach. It is essential that all protein and ligand samples are drawn from the same stock solutions. Therefore it is important that sufficient stock solution is prepared initially to complete the entire series of experiments. This ensures the ratio of AAC(6')-Ii and AcCoA is constant among all experiments, and reduces random fluctuation...

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		<div>
		<table border="1">
		<thead>
		<tr>
		<th>Material Name</th>
		<th>Type</th>
		<th>Company</th>
		<th>Catalogue Number</th>
		<th>Comment</th>
		</tr>
		</thead>
		<tbody>
		
<tr>
<td>Acetyl coenzyme A (AcCoA)</td>
<td>&nbsp;</td>
<td>Sigma-Aldrich</td>
<td>A2056</td>
<td>&nbsp;</td>
<tr>
<td>4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)</td>
<td>&nbsp;</td>
<td>Fisher</td>
<td>7365-45-9</td>
<td>&nbsp;</td>
<tr>
<td>ethylenediaminetetraacetic acid (EDTA)</td>
<td>&nbsp;</td>
<td>Sigma-Aldrich</td>
<td>431788</td>
<td>&nbsp;</td>
<tr>
<td>Spectra/Por 2 Dialysis Tubing</td>
<td>&nbsp;</td>
<td>Spectrum Labs</td>
<td>132678</td>
<td>&nbsp;</td>
<tr>
<td>Sterile Syringe Filter (0.2 &mu;m)</td>
<td>&nbsp;</td>
<td>VWR</td>
<td>281445-477</td>
<td>&nbsp;</td>
<tr>
<td>Cellulos Nitrate Membrane Filters (0.45 &mu;m)</td>
<td>&nbsp;</td>
<td>Whatman</td>
<td>7184-004</td>
<td>&nbsp;</td>
<tr>
<td>VP-ITC</td>
<td>&nbsp;</td>
<td>MicroCal</td>
<td>VP-ITC</td>
<td>Microcalorimeter used for measurements</td>
</tr>
<tr>
<td>ThermoVac</td>
<td>&nbsp;</td>
<td>MicroCal</td>
<td>USB Thermo Vac</td>
<td>Temperature Controlled Degassing Station</td>
</tr>		</tbody>
		</table>
		</div>

collecting variable-concentration isothermal titration calorimetry datasets in order to determine binding mechanisms

Isothermal titration calorimetry (ITC) is commonly used to determine the thermodynamic parameters associated with the binding of a ligand to a host macromolecule. ITC has some advantages over common spectroscopic approaches for studying host/ligand interactions. For example, the heat released or absorbed when the two components interact is directly measured and does not require any exogenous reporters. Thus the binding enthalpy and the association constant (Ka) are directly obtained from ITC data, and can be used to compute the entropic contribution. Moreover, the shape of the isotherm is dependent on the c-value and the mechanistic model involved. The c-value is defined as c = n[P]tKa, where [P]t is the protein concentration, and n is the number of ligand binding sites within the host. In many cases, multiple binding sites for a given ligand are non-equivalent and ITC allows the characterization of the thermodynamic binding parameters for each individual binding site. This however requires that the correct binding model be used. This choice can be problematic if different models can fit the same experimental data. We have previously shown that this problem can be circumvented by performing experiments at several c-values. The multiple isotherms obtained at different c-values are fit simultaneously to separate models. The correct model is next identified based on the goodness of fit across the entire variable-c dataset. This process is applied here to the aminoglycoside resistance-causing enzyme aminoglycoside N-6'-acetyltransferase-Ii (AAC(6')-Ii). Although our methodology is applicable to any system, the necessity of this strategy is better demonstrated with a macromolecule-ligand system showing allostery or cooperativity, and when different binding models provide essentially identical fits to the same data. To our knowledge, there are no such systems commercially available. AAC(6')-Ii, is a homo-dimer containing two active sites, showing cooperativity between the two subunits. However ITC data obtained at a single c-value can be fit equally well to at least two different models a two-sets-of-sites independent model and a two-site sequential (cooperative) model. Through varying the c-value as explained above, it was established that the correct binding model for AAC(6')-Ii is a two-site sequential binding model. Herein, we describe the steps that must be taken when performing ITC experiments in order to obtain datasets suitable for variable-c analyses.

Watch this Scientific Journal Video about Collecting Variable-concentration Isothermal Titration Calorimetry Datasets in Order to Determine Binding Mechanisms at JoVE.com

Collecting Variable-concentration Isothermal Titration Calorimetry Datasets in Order to Determine Binding Mechanisms

ITC is a powerful tool for studying the binding of a ligand to its host. In complex systems however, several models may fit the data equally well. The method described here provides a means to elucidate the appropriate binding model for complex systems and extract the corresponding thermodynamic parameters.

Isothermal titration calorimetry (ITC) is commonly used to determine the thermodynamic parameters associated with the binding of a ligand to a host ...

collecting-variable-concentration-isothermal-titration-calorimetry

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Biology

12.7K visualizações.  McGill University. ITC é uma ferramenta poderosa para estudar a ligação de um ligante para seu hospedeiro. Em sistemas complexos no entanto, vários modelos podem ajustar os dados igualmente bem. O método descrito aqui fornece um meio para elucidar o modelo de encadernação adequada para sistemas complexos e extrair os parâmetros termodinâmicos correspondentes.