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'Coenzymes' are small organic non-protein molecules that carry chemical groups between enzymes.

Contents

Terminology

Coenzymes as metabolic intermediates

Types

Vitamin and nucleotide derivatives

Not related to vitamins

See also

References

External links

Terminology

The term coenzymes is commonly used loosely, and coenzymes can also be defined as organic, non-protein cofactors. Lehninger Principles of Biochemistry, , David L., Nelson, W. H. Freeman and company, , Coenzymes are also sometimes referred to as cosubstrates, but this usage is unusual.
Coenzymes are consumed in the reactions in which they are substrates, for example: the coenzyme NADH is converted to NAD⁺ by oxidoreductases. Coenzymes are however regenerated and their concentration maintained at a steady level in the cell.
A special subset of coenzymes are prosthetic groups. These have more in common with cofactors since they are tightly bound to enzymes and are not released as part of the reaction. Prosthetic groups include molybdopterin, lipoamide and biotin.

Coenzymes as metabolic intermediates

Metabolism involves a vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of functional groups.^[1] This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions.^[2] These group-transfer intermediates are the coenzymes.
Each class of group-transfer reaction is carried out by a particular coenzyme, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. An example of this are the dehydrogenases that use nicotinamide adenine dinucleotide (NADH) as a cofactor. Here, hundreds of separate types of enzymes remove electrons from their substrates and reduce NADH and this reduced coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates.^[3]

Types

Many coenzymes are phosphorylated water-soluble vitamins. Coenzymes are also commonly made from nucleotides such as adenosine triphosphate, the biochemical carrier of phosphate groups, or coenzyme A, the coenzyme that carries acyl groups.

Vitamin and nucleotide derivatives

★ Ascorbic acid (Vitamin C)

★ Coenzyme A - Contains pantothenic acid (Vitamin B⁵) and ATP

★ Coenzyme B₁₂

★ Riboflavin (B₂) - FAD and FMN

★ Thiamine pyrophosphate (B₁)

★ NAD and NADP - Contain both a nucleotide and a Niacin (vitaimin B₃) moiety.

★ Folic acid

Not related to vitamins

★ Coenzyme Q. (Coenzyme Q is unusual as it carries electrons between enzymes by diffusing within cell membranes, as this coenzyme is not water soluble. )

★ Molybdopterin

See also

★ Cofactor

★ Enzymes

★ Adenosine triphosphate

References

1. The Ninth Sir Hans Krebs Lecture. Compartmentation and communication in living systems. Ligand conduction: a general catalytic principle in chemical, osmotic and chemiosmotic reaction systems, Mitchell P, , , Eur J Biochem, 1979
2. Mechanisms of enzyme-catalyzed group transfer reactions, Wimmer M, Rose I, , , Annu Rev Biochem,
3. The power to reduce: pyridine nucleotides--small molecules with a multitude of functions, Pollak N, Dölle C, Ziegler M, , , Biochem J, 2007

External links

★ Examples at City University of New York

★ Overview at National Institutes of Health

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