noun, plural: subunits
(general) A subdivision or a distinct component of a larger unit
The word subunit when used in general sense pertains to a particular part of a larger unit. In structural biology, subunit may pertain to biochemical molecules comprised of distinct components. For instance, protein subunit pertains to a protein molecule that can assemble with another protein molecule to form a complex. Myosin, for example, is comprised of six subunits. Microtubules, which are tubular polymers of the cytoskeleton in the cytoplasm, are each built of tubulin subunits. Proteins that have a relatively small number of subunits are sometimes described as oligomeric. Those that have a large number of subunits are described as multimeric. Protein subunits may also be classified as regulatory subunit or catalytic subunit depending on the primary function that they serve. There are however certain complexes that have both regulatory and catalytic subunits and are termed holoenzymes.