multimer

A multimer is a molecule composed of multiple subunits that associate to form a single functional complex. Subunits may be identical, yielding homomultimers, or different, yielding heteromultimers. The term is commonly used for proteins and nucleic acid–protein assemblies, and it contrasts with monomers (single subunits) and polymers (long chains of repeating units).

Multimerization often occurs through non-covalent interactions such as hydrogen bonds, ionic contacts, and hydrophobic effects, though covalent disulfide bonds can also stabilize assemblies. The arrangement of subunits gives rise to a quaternary structure that can be essential for activity, regulation, and stability. Multimerization can be constitutive or induced by ligands, changes in concentration, pH, or cellular conditions.

Examples include hemoglobin, a heterotetramer (two alpha and two beta chains) whose cooperative binding of oxygen depends on quaternary structure; tubulin forming a heterodimer that polymerizes into microtubules; collagen triple helices as structural proteins; and viral capsids assembled from multiple copies of a protein subunit. Enzymes such as the pyruvate dehydrogenase complex also exist as multimers.

Understanding multimerization is important for biology and medicine. Cooperative binding, allostery, and regulation often rely on subunit interfaces. Conversely, abnormal multimerization can disrupt function or promote aggregation, contributing to disease and affecting viral assembly.

Analytical methods used to study multimers include structural approaches such as X-ray crystallography, cryo-electron microscopy, and nuclear magnetic resonance, and biophysical techniques like size-exclusion chromatography and analytical ultracentrifugation, as well as cross-linking and native mass spectrometry.