Octameric

Octameric is an adjective describing a molecule, protein complex, or assembly that contains eight subunits or units, or that displays eightfold symmetry. It is used across chemistry, biochemistry, and structural biology to denote a specific stoichiometry or architecture of a macromolecular complex.

A well-known example is the histone octamer that forms the core of the nucleosome in eukaryotic chromatin. This octamer comprises two copies each of the core histones H2A, H2B, H3, and H4, around which about 147 base pairs of DNA are wound. This eight-subunit core provides the foundation for higher-order chromatin organization and regulation of gene expression.

Octameric assemblies are found in a variety of contexts beyond chromatin, including different protein complexes, enzyme rings, and viral capsid components. The exact arrangement of eight subunits—whether arranged in a ring, a compact cluster, or a more complex geometry—affects properties such as stability, cooperativity, and allosteric communication among subunits.

Determining whether a complex is octameric and elucidating its structure typically involves techniques such as X-ray crystallography, cryo-electron microscopy, native mass spectrometry, analytical ultracentrifugation, and size-exclusion chromatography. The term octameric emphasizes the stoichiometry and symmetry of the assembly, and is distinct from the noun octamer, which denotes the unit itself, or from octamerism, referring to the state of being an octamer.