isozymes

Isozymes, or isoenzymes, are multiple molecular forms of an enzyme that catalyze the same chemical reaction but differ in amino acid sequence and often in kinetic properties, regulatory features, and tissue distribution. They can arise from different genes within a gene family or from alternative splicing and post-translational modifications of a single gene. These variations allow different tissues to regulate the same metabolic step in distinct ways.

Isozymes often show different kinetic parameters, such as Km and Vmax, as well as different regulatory properties, pH optima, temperature stability, and sensitivity to inhibitors or activators. They may also be localized to different cellular compartments, such as cytosol or mitochondria, contributing to localized control of metabolism and development. The diversity of isozymes reflects evolutionary processes like gene duplication and divergence, enabling tissue- and context-specific enzyme performance.

Common examples include lactate dehydrogenase (LDH), which has five isoforms formed from combinations of H and M subunits, yielding tissue-biased patterns (eg, heart-rich H forms vs muscle-rich M forms); creatine kinase isoenzymes CK-MM, CK-MB, and CK-BB; and various alkaline phosphatase isoenzymes arising from liver, bone, placenta, and intestine. Clinical use of isoenzyme analysis includes identifying tissue sources of injury or disease and monitoring organ-specific conditions. Separation and detection are typically accomplished by electrophoresis or isoelectric focusing, followed by activity staining or immunoassays that distinguish individual isoforms.