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How does NAD act as a coenzyme during enzyme-controlled 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.

Nicotinamide adenine dinucleotide (NAD) functions as a vital coenzyme in numerous dehydrogenase enzyme-controlled reactions. Its primary role is to act as an oxidizing or reducing agent during metabolism.

In these reactions, a dehydrogenase enzyme facilitates the transfer of a hydride ion (consisting of a proton and two electrons) from a substrate to the oxidized form of the coenzyme, NAD⁺. This process converts the substrate into an oxidized product and reduces NAD⁺ to NADH + H⁺.1

Once reduced, NADH acts as a carrier of reducing equivalents. It typically diffuses to the respiratory chain, where it is oxidized by specific enzymes, such as NADH dehydrogenase, to release energy that the cell can use for processes like ATP synthesis. Because each form of the coenzyme has a tendency to either accept or donate electrons, NAD⁺ and NADH constitute a redox couple that enables the stepwise transfer of energy within the cell.12