Principles of Protein Structure - G. Schultz 1982

Covalent Structure of Proteins
Chain Assemblies
Environmental Effects

As shown in the previous section, the domain is the fundamental unit. Associations of such domains are of secondary importance. The following Structure/133.html">Discussion examines how the protein's environment influences The Nature of domain assembly.

Unlike monomers, oligomers can dissociate. Proteins are generally classified into monomers and oligomers. According to the definition by Klotz et al. [81], a protein is considered a "monomer" if it consists of a single polypeptide chain or is built from multiple chains linked covalently (e.g., by disulfide bridges). Under this nomenclature, proteins such as Insulin, a-Chymotrypsin, and IMMUNOGLOBULINS, which are assemblies of covalently linked chains, are classified as monomers. A distinctive feature of "oligomeric" proteins is that they are constructed from so-called subunits—smaller entities held together by non-covalent forces (Figs. 4.1 and 5.18). As noted above, monomers may consist of several functional domains or an even greater number of Structural domains. This also applies to oligomeric subunits, although a subunit is frequently equivalent to a functional domain.

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Fig. 4.2. Proteins composed of Ig domains.

a — superoxide dismutase subunit (detailed structure) [800]. The chain folding is shown as a stereo view of the C_α backbone (Sec. 7.2); the N- and C-termini of both chains are indicated. The subunit includes an Ig domain and a copper-containing catalytic center with a Zn ion nearby; b — histocompatibility antigen (human leukocyte antigen = HL-A protein). The molecule is symmetrical; each half consists of a heavy H chain and a light L chain. The light chain, which represents a single Ig domain, is also known as β₂-microglobulin. Both heavy chains are linked by a disulfide bridge, which is apparently located in the membrane-anchoring region of the molecule; c — immunoglobulin G. The molecule is symmetrical; each half consists of a heavy H chain and a light L chain. Each chain is connected to at least one other chain by one or more disulfide bridges. Standard designations are used for individual domains along the L chain and the H chain.

Each large circle in b and c represents a single Ig domain. Carbohydrate moieties of the proteins are not shown.