cosubstraten

Cosubstrates, or cosubstraten in Dutch, are a class of cofactors in enzymology. They are substrate molecules that bind to an enzyme together with the primary substrate and participate directly in the catalytic transformation. Unlike prosthetic groups, cosubstrates are not permanently bound; they associate and dissociate during turnover and are regenerated after the reaction.

A canonical example is NAD+ in many dehydrogenase reactions. It accepts a hydride from the substrate to form NADH, and NAD+ is regenerated when NADH is oxidized back to NAD+ by the electron transport chain or other pathways. NADP+ plays a similar role in anabolic reactions.

Cosubstrates are typically small, diffusible molecules. Their cellular availability and the ratio of cosubstrate to substrate can influence reaction rate and metabolic flux. From a kinetic perspective, cosubstrate binding is treated as a separate aspect of enzyme activity, with its own affinity constants.

In relation to other cofactors, cosubstrates are distinguished from prosthetic groups, which remain tightly bound to the enzyme, and from permanently bound cofactors. They are central to many metabolic processes, including glycolysis and oxidative phosphorylation, where redox cosubstrates shuttle electrons between substrates and electron carriers.