Molecular biology, multiple isoenzymes

The four forms of hexokinase found in mammalian tissues are but one example of a common biological situation the same reaction catalyzed by two or more different molecular forms of an enzyme. These multiple forms, called isozymes or isoenzymes, may occur in the same species, in the same tissue, or even in the same cell. The different forms of the enzyme generally differ in kinetic or regulatory properties, in the cofactor they use (NADH or NADPH for dehydrogenase isozymes, for example), or in their subcellular distribution (soluble or membrane-bound). Isozymes may have similar, but not identical, amino acid sequences, and in many cases they clearly share a common evolutionary origin. 

Isoenzymes caused by the existence of multiple-gene loci usually differ quantitatively in catalytic properties. These differences may be manifested in such characteristics as molecular activity, K, values for substrate(s), sensitivity to various inhibitors, and relative rates of activity with substrate analogues (when the specificity of the isoenzymes allows the substrate to be varied), underscoring the biological importance of isoenzymatic variation. In contrast, multiple enzyme forms that arise by such posttranslational modifications as aggregation usually have similar catalytic properties. 

Enzymes commonly exist in multiple molecular forms, even in a single organism a series of four volumes has dealt with the molecular structures, physiological functions, developmental biology, and genetic aspects of isoenzymes. ... 

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