Specific subunits three-dimensional space They are arranged and connected by non covalent bonds. The spatial arrangement of subunits in protein molecules and the layout and interaction of subunit contact sites become Four level structure （quaternary structure）。 A compound formally eliminates two univalent or one bivalent atoms or groups, and the rest is called subunit. The subunit has two different structures:
Subunit
Subunit also called Subunit （subunit）。 Refers to a series of particles or smaller structures that constitute an entire unit or structure. For example, many enzyme products are composed of several polypeptide subunits, which form a catalytic whole through specific combination. A large number of such as microtubule , flagellum and cilium are all composed of some subunits. One Peptide chain By alpha folding and β angle Having a specific conformation protein (called Tertiary structure ）。 Consisting of two or more, having a three-level structure Globular protein adopt Noncovalent bond , which are combined with each other to form a specific conformational name Four level structure 。 Each peptide chain (i.e., each globular protein) is called subunit or subunit. Generally, each subunit has corresponding biochemistry Function. It should also be noted that subunits are peptide chains, but peptide chains are not necessarily subunits. as insulin A-chain and B-chain, via Covalent Of Disulfide bond It is linked into insulin molecule, but A-chain and B-chain are not subunits. Only when subunits polymerize into a complete quaternary structure can the resulting protein have biological activity 。 as hemoglobin It is composed of four subunits, RNA polymerase It is composed of five subunits (α 2 β β 'δ). The subunit is different from the enzyme complex. Definition:
Subunit: composition Protein quaternary structure The smallest covalent unit refers to the protein with tertiary structure Globulin Subunit refers to a protein molecule with a quaternary structure Polypeptide chain A globulin folded into a tertiary structure. 
