multimers

Multimers are molecular complexes formed by the association of two or more subunits into a single functional unit. They can include identical subunits (homomers) or different ones (heteromers). When the subunits are the same, the term homomultimer is used; when not, heteromultimer. Multimerization is common in proteins and in nucleic acid–protein assemblies, and it is distinct from polymers, which are long chains of repeating units.

Examples of multimers include hemoglobin, a heterotetramer consisting of two alpha and two beta chains; immunoglobulin G (IgG), a heterotetramer of two heavy and two light chains; and transthyretin, a homotetramer. Viral capsids and many metabolic enzymes also form multimers. In some cases, the basic functional unit is a dimer or tetramer that further assembles into larger complexes.

Subunit interactions in multimers are usually noncovalent, allowing reversible assembly, though covalent linkages such as disulfide bonds can stabilize certain multimers. The arrangement of subunits can be highly symmetric, yielding dimers, tetramers, or higher-order quaternary structures and enabling organized three-dimensional architectures.

Function and regulation: Multimerization can alter enzymatic activity, binding specificity, stability, and regulatory properties. It can enable cooperativity and allostery, where binding at one site affects others. Assembly can be responsive to ligand binding, pH, ionic strength, or post-translational modifications, and disassembly may occur when conditions change.

Relevance: Aberrant multimerization is linked to diseases and misfolding, while controlled multimerization is exploited in biotechnology and medicine, including engineered multisubunit enzymes and antibody-based therapeutics.