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How does ATP act as a coenzyme in cellular reactions?

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A multi-panel diagram illustrates how a substrate binds to an enzyme's active site through induced fit, leading to product formation, and shows how non-substrates and allosteric regulators can alter enzyme activity by binding to different sites on the enzyme. — Enzyme In the induced-fit theory of enzyme-substrate binding, a substrate approaches the surface of an enzyme (step 1 in box A, B, C) and causes a change in the enzyme shape that results in the correct alignment of the catalytic groups (triangles A and B; circles C and D represent substrate-binding groups on the enzyme that are essential for catalytic activity). The catalytic groups react with the substrate to form products (step 2). The products then separate from the enzyme, freeing it to repeat the sequence (step 3). Boxes D and E represent examples of molecules that are too large or too small for proper catalytic alignment. Boxes F and G demonstrate binding of an inhibitor molecule (I and I′) to an allosteric site, thereby preventing interaction of the enzyme with the substrate. Box H illustrates binding of an allosteric activator (X), a nonsubstrate molecule capable of reacting with the enzyme.

Adenosine triphosphate (ATP) acts as a coenzyme by serving as a chemical-group donor in enzyme-mediated reactions. Unlike catalysts that remain unchanged, ATP participates stoichiometrically in these reactions, meaning it is consumed and modified during the process.123

Specifically, ATP functions as a shuttle for chemical energy by donating a phosphoric acid group (a phosphoryl group) to a substrate molecule, a process known as phosphorylation. This transfer is facilitated by specific enzymes that couple the exergonic release of energy from ATP hydrolysis to endergonic cellular activities that would otherwise be energetically unfavorable.425

Once ATP has donated its phosphate group and been converted to ADP, it can be restored to its original state through subsequent enzyme-catalyzed reactions, such as those occurring during cellular respiration. By linking the activities of different enzymes, ATP enables the exchange of energy and functional groups necessary for essential metabolic pathways.123