Isoenzymes and Multiple Forms

A single catalytic activity is often carried out by several distinct molecular forms of an enzyme. These isoenzymes catalyse the same reaction but differ in amino-acid sequence, structure, or subunit composition, and they can be expressed differently across tissues and stages of development, giving the same metabolic step locally tuned properties.

Definition

Isoenzymes (isozymes) are multiple molecular forms of an enzyme that catalyse the same reaction but differ in primary structure, subunit composition, or physical properties, often arising from distinct genes or from different combinations of subunits.

Scope

The entry covers what isoenzymes are and how they arise, the lactate dehydrogenase family as the classic illustration, the tissue- and development-specific expression of isoenzyme patterns, and how such patterns are detected. It is a reference treatment of enzyme multiplicity and is not a source of diagnostic interpretation or clinical guidance.

Core questions

• What are isoenzymes and how do they differ from one another?
• How do isoenzymes arise?
• Why does the same reaction need tissue-specific forms?
• How are isoenzyme patterns detected and described?

Key concepts

• Multiple molecular forms of an enzyme
• Subunit combination (e.g. LDH heterotetramers)
• Distinct genes versus post-translational variants
• Tissue- and development-specific expression
• Kinetic and regulatory differences between isoenzymes
• Electrophoretic separation of isoenzymes

Mechanisms

Isoenzymes can arise from separate genes, from alternative processing, or from different combinations of subunits assembled into the active enzyme. The textbook example is lactate dehydrogenase, whose active form is a tetramer built from two kinds of subunit; the five possible combinations yield five isoenzymes with characteristic tissue distributions and kinetic properties. Because the forms differ in charge and structure, they can be separated by electrophoresis, revealing tissue- and stage-specific patterns. These differences let the same chemical step be tuned to local metabolic demands while sharing one catalytic identity, which is why isoenzymes still receive a common EC number.

Clinical relevance

Because isoenzyme patterns differ between tissues, the relative abundance of particular forms can reflect tissue origin, a principle that underlies the use of enzyme patterns in laboratory medicine. This entry explains the biochemistry of isoenzymes for reference and is not a basis for diagnostic interpretation or treatment decisions.

History

The recognition that one enzyme activity could exist as several separable forms crystallised in the late 1950s, when Markert and Moller (1959) described tissue-, developmental-, and species-specific multiple forms and introduced the term isozyme. Markert's subsequent dissociation-and-recombination experiments (1963) showed that lactate dehydrogenase isoenzymes are built from interchangeable subunits, providing a structural explanation for enzyme multiplicity that became a model for the field.

Key figures

• Clement L. Markert

Seminal works

• markert-moller-1959
• markert-1963

Frequently asked questions

If isoenzymes are different proteins, why do they share an EC number?

Because the EC system classifies enzymes by the reaction they catalyse, and isoenzymes catalyse the same reaction; their structural differences do not change the catalytic identity that the EC number records.

How do isoenzymes of one enzyme end up in different tissues?

Different genes or subunit combinations are expressed in different tissues and developmental stages, so each tissue assembles the isoenzyme combination suited to its metabolic needs.