coenzyme

Small organic nonprotein compound that attaches to an enzyme and is necessary for its correct functioning. Tightly bound coenzymes are known as prosthetic groups; more loosely bound ones are called cofactors. The coenzyme itself is not usually changed during a reaction. If it is, it is usually converted rapidly back to its original form. Well-known coenzymes include NAD, ATP, and coenzyme A.

Most coenzymes are nucleotides, for example: ATP adenosine triphosphate; NAD nicotinamide adenine dinucleotide, and flavin nucleotides; uridine diphosphate, used in galactose metabolism; guanosine triphosphate, used in protein synthesis; cytidine nucleotides, which take part in the synthesis of fatty acids; acetyl coenzyme A, used in the synthesis of glucose, fatty acids, and steroids. Cytochromes are an important group of coenzymes used in the formation of ATP.

Vitamins are very important to living organisms because many are precursors of the coenzymes essential for normal activity. Normally, the more loosely bound cofactors are metal ions. Zinc ions are necessary for the activity of the enzymes alcohol dehydrogenase, carbonic anhydrase, and carboxypeptidase, while magnesium is required by enzymes such as ATPases, that transfer phosphate groups. Iron is used by cytochromes and peroxidase, as well as catalase and the photosynthetic electron carrier ferredoxin. Potassium is required by pyruvate phosphokinase, and sodium is essential for the working of plasma membrane ATPases. Metal cofactors may work in two ways. They may form links to bring the substrate and enzyme together, or they may possess catalytic ability. For example, the iron atom of catalase is the catalytic centre in the decomposition of hydrogen peroxide. Generally, however, cofactors are intermediate electron-carrying or functional group-carrying compounds.