cosubstraty

Cosubstraty, or co-substrates, are a class of cofactors in enzymology that participate in enzyme-catalyzed reactions together with a primary substrate. They bind reversibly to the enzyme and are modified during the catalytic cycle, but are regenerated in subsequent steps, so their overall quantity remains constant.

In a typical reaction, a cosubstrate accepts or donates a chemical group or electrons and is converted to a different form. The enzyme then relies on the regeneration of the original cosubstrate form by separate cellular processes, allowing the cosubstrate to be used repeatedly. This distinguishes cosubstrates from permanently bound cofactors, or prosthetic groups, which stay attached to the enzyme throughout catalysis.

Common examples include NAD+ and NADP+, which act as redox cosubstrates in dehydrogenases and other oxidoreductases, cycling between oxidized and reduced forms. Coenzyme A can function as a cosubstrate in acyl-transfer reactions, carrying acyl groups and being regenerated in related pathways. In many flavoproteins, flavin adenine dinucleotide (FAD) serves as a prosthetic group rather than a cosubstrate, but in some systems it can act more loosely associated and participate cyclically.

Cosubstrates are conceptually a subset of coenzymes: they are organic or inorganic cofactors required for catalytic activity that are not permanently bound to the enzyme. Their availability and recycling are important for metabolic regulation and the efficiency of enzymatic pathways.

See also: coenzyme, prosthetic group, redox cofactor, electron carrier.