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For some chemical reactions a substance other than an enzyme and substrates are necessary. These substances are called coenzymes or cofactors. Coenzymes are therefore necessary if a particular enzyme is to be active. Like enzymes, coenzymes also remain unchanged at the end of a reaction.

Conenzymes, however, differ from enzymes in that they are not proteins. They are of low molecular weight and are heat stable. Enzymes, which are proteins, have a comparatively higher molecular weight and undergo denaturation at temperatures of 50 -60C. Coenzymes also differ in another respect. An enzyme may function independently of a coenzyme but a coenzyme cannot function without an enzyme.

An enzyme and a coenzyme together form an enzymatically active conjugated protein called the holozyme. The enzyme (protein) component of the holozyme is called an apoenzyme. The non-protein coenzyme component is called the prosthetic group (Holozyme = Apoenzyme + Prosthetic Group). The major coenzymes of dehydrogenases are the two dinucleotides, NAD and NADP.

The conenzyme (prosthetic group) may be tightly bound to the apoenzyme or may easily dissociate from it. In the above description the terms ‘prosthetic group’ and ‘conezyme’ have been used synonymously. Some authors, however, distinguish between a prosthetic group and a coenzyme. The prosthetic group remains attached to the apoenzyme while undergoing oxidation and reduction. The coenzyme on the other hand may undergo reduction while attached to one apoenzyme, and then migrate to another apoenzyme where it can be oxidized.

Many coenzymes are closely related to vitamins and are the derivation of vitamins. The B group vitamins (except biotin and lipoic acid) function as parts of some coenzyme, e.g. CoA is a derivative of pantothenic acid. When there is vitamin deficiency the coenzyme concentration decreases. Consequently enzyme function is depressed. Nucleotides may also function as coenzymes in certain metabolic reactions.

It some cases enzymes may be activated by simple substance like metal ions which are then called activators. The substrate forms, a complex with the metal ion and then reacts with the enzyme.

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