Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Cofactors

In certain cases, Enzymes require special mediators known as Cofactors to facilitate catalysis. Cofactors are non-protein substances that participate in the intermediate stages of an enzymatic reaction (or a cycle of reactions) without being consumed in the process. In the vast majority of cases, cofactors are regenerated in their original form upon completion of the catalytic act.

Owing to their diverse chemical nature, cofactors can be divided into two main groups: Coenzymes (which bind loosely to the enzyme and dissociate from it during catalysis) and prosthetic groups (which are firmly bound to the enzyme molecule).

The primary mechanisms by which cofactors participate in catalysis are as follows:

— acting as carriers between enzymes. By interacting with one enzyme, the carrier accepts a portion of the substrate, migrates to another enzyme, and transfers this moiety to the second substrate, after which it is released. Such a mechanism is typical for most coenzymes;

— functioning as an "intra-enzymatic" carrier, which is characteristic primarily of prosthetic groups. A prosthetic group attaches a portion of a substrate molecule and transfers it to a second substrate bound within the Active Site of the same enzyme. In this case, the prosthetic group can be viewed as an integral part of the enzyme's catalytic site;

— altering the conformation of the enzyme molecule by interacting with it outside the active site, which can induce the transition of the active site into a catalytically active configuration;

— stabilizing the conformation of the enzyme that favors the catalytically active state;

— acting as a template. For instance, nucleic acid polymerases require a "program"—a template—according to which a new molecule is synthesized;

— serving as intermediate compounds. Sometimes an enzyme may utilize a cofactor molecule in a reaction to form a product while simultaneously generating a new cofactor molecule at the expense of the substrate.

Among currently known enzymes, approximately 40% are capable of mediating catalysis only with the assistance of cofactors. The most widespread are cofactors that facilitate The transfer of reducing equivalents, as well as phosphate, acyl, and carboxyl groups.