Homooligomers

Homooligomers are protein complexes composed of two or more identical polypeptide chains derived from the same gene product. They contrast with heterooligomers, which contain different subunits. Common assemblies are dimers, trimers, tetramers, and higher-order structures that often exhibit symmetry, such as cyclic (Cn) or dihedral (Dn) arrangements.

Interfaces between identical subunits create a stable core and enable communication across subunits. Interactions involve hydrophobic contacts, hydrogen bonds, and salt bridges. The identical nature of the subunits can support allosteric regulation, relaying ligand binding or chemical changes across the complex.

Functions: Homooligomerization can increase catalytic efficiency by providing multiple active sites, enhance structural stability, confine active sites within a defined geometry, or form channels and cages. Oligomerization can also promote cooperative binding or regulate activity through subunit coordination.

Examples: Ferritin, which assembles 24 identical subunits into a hollow sphere for iron storage. GroEL, a bacterial chaperonin, forms a homo-oligomer of 14 identical subunits arranged in two rings. Lumazine synthase can form 60-subunit icosahedral shells. Aquaporins often exist as homotetramers in membranes, with each subunit serving as a functional unit.

Biogenesis and regulation: Subunit concentration, pH, ionic strength, and the presence of ligands or chaperones influence assembly and disassembly. Some homooligomers are dynamic, equilibrating between monomeric and oligomeric states as cellular needs change.