Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

The Chemical Basis of Life
Amino Acids and Proteins
Quaternary Structure and Regulation of Biological Processes

Proteins can be composed of several polypeptide chains (subunits); Hemoglobin is perhaps the most widely known of these. The quaternary Structure of a protein refers to the way its subunits are arranged. As shown by the data in Table 2.10, many proteins, especially Enzymes (whose names typically end with the suffix -ase), are oligomers and must therefore possess a characteristic quaternary structure. Quaternary structure is believed to be stabilized by the same forces and bonds as tertiary structure. In some cases, such as Insulin, Disulfide Bonds participate in The formation of quaternary structures; however, in most of the Examples listed in Table 2.10, the subunits are held together in the oligomeric protein molecule by weaker interactions. Many Oligomeric Proteins are known to be capable of self-assembly; for example, separated α- and β-chains of hemoglobin in solution rapidly associate to form intact hemoglobin molecules. This property of proteins is highly illustrative, as it demonstrates (at least in some cases) the governing role of the one-dimensional biochemical DNA code, which determines not only the Introduction/19.html">Primary Structure of Proteins but also, through it, all higher levels of Cell/13.html">Protein Structure and, consequently, their specific biological Functions.

Current evidence indicates that the assembly of certain protein molecules from multiple subunits serves at least two important biological functions: first, it regulates the catalytic activity of enzymes, and second, it provides extensive opportunities for constructing related but non-identical molecules from the same set of subunits. This latter function is illustrated by proteins known as Isoenzymes or isozymes. Isoenzymes are distinct molecular forms of an enzyme that catalyze the same reaction within organisms of a single species. The existence of isoenzymes might seem redundant at first glance; however, the availability of parallel yet distinct catalytic processes is actually an essential element of several biochemical regulatory systems (Chaps. 3 and 5). Some isoenzymes are known to be oligomeric proteins; a case has been described of isoenzymes composed of five subunits of only two types. Such a design can be considered highly efficient and advantageous, as it enables the synthesis of five different proteins from just two polypeptide chains.

Class="center">Table 2.10. Characteristics of Some oligomeric proteins*

^a Reproduced with permission from: Loewy, A., Siekevitz, P., Cell Structure and function. — Moscow: Mir, 1971.