3.21 : ATP and Macromolecule Synthesis

Macromolecules such as proteins, polynucleotides, carbohydrates, and lipids are polymers of amino acids, nucleotides, monosaccharides, and fatty acids.

Since the synthesis of these macromolecules is energetically unfavorable, the energy released by ATP hydrolysis, a favorable exergonic process, is used to power these reactions.

For example, during polynucleotide chain synthesis, the terminal phosphates from two ATP molecules are released by hydrolysis. These phosphates are then transferred to nucleoside monophosphate, converting it into a high-energy intermediate called nucleoside triphosphate.

This intermediate attaches to the growing end of the polynucleotide chain by releasing pyrophosphate.

Synthesis of polymers can be oriented in two ways. In the case of lipids and proteins, synthesis occurs by head polymerization, where the reactive bond required for the condensation reaction is carried to the end of the growing polymer. Each monomer brings the reactive bond, required for the addition of the following monomer.

In tail polymerization, as seen in the synthesis of polynucleotides and carbohydrates, the reactive bond is carried by the incoming monomer, and is used immediately for its own addition.

Biological macromolecules are organic compounds, predominantly composed of carbon atoms. The carbon atoms are covalently bonded with hydrogen, oxygen, nitrogen, and other minor elements. There are four major biological macromolecule classes: carbohydrates, lipids, proteins, and nucleic acids.

Most macromolecules are composed of single subunits, or building blocks, called monomers. The monomers combine with each other using covalent bonds to form larger molecules known as polymers.

Conversion of monomers into polymers is an energetically intensive process. Therefore, cells rely on ATP, the energy currency of the cell, to power these biochemical processes. For example, during the conversion of glucose into glycogen, ATP is hydrolyzed into ADP and inorganic phosphate (Pi), and releases free energy. The released inorganic phosphate binds to glucose and converts it into glucose 6-phosphate which is required for glycogen synthesis.

This cycle of ATP continuously breaking down into ADP repeats to power all life's processes. Like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group.

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