Principles of Protein Structure - G. Schultz 1982

Covalent Protein Structure
Chain Assemblies
Oligomeric Proteins

Oligomers are simpler than single giant polypeptide chains. Oligomeric Proteins offer certain fundamental advantages over giant polypeptide chains composed of individual functional domains. This likely explains why most large proteins are oligomeric [81].

(a) If some or all subunits are identical, coding for an oligomeric protein requires significantly less genetic material than coding for a single giant polypeptide chain. Although the isolated subunits of many oligomeric proteins are non-functional [89, 90], such subunits carry all the information necessary for self-assembly and The formation of a functional unit. This makes it possible to reduce the length of the DNA (or RNA) region encoding a protein with a given function. This effect is well illustrated by medium-sized oligomers, such as tetrameric dehydrogenase [81, 91] and especially biologically vital structures like the tobacco mosaic virus coat protein, which consists of 2,130 self-assembling identical subunits with 158 amino acid residues each.

(b) The probability of a defect occurring is significantly higher in large polypeptide chains than in oligomeric Proteins of the same size. Assuming that a given Protein Synthesis mechanism produces 1 error per 1,000 amino acid residues [81]; then for a single polypeptide consisting of 1,000 residues, the probability of a defect is high [it follows from the Poisson distribution that the average number of correct copies is 37 %]. In contrast, the synthesis of four subunits, each consisting of 250 residues, yields a higher proportion of non-defective subunits, specifically

(c) Oligomeric assemblies are much better adapted for molecular evolution [92]. They adapt to environmental changes considerably faster than large single polypeptide chains.

(d) Oligomer formation is the simplest solution to the problems faced by primitive Cells. Since Cell membranes were highly imperfect, the leakage of molecules with M = 20,000 through membrane pores was substantial. Aggregation was presumably a simpler way to increase molecular size than the synthesis of large chains [93].