2.5

Quaternary Structure: Polypeptide Chains Can Assemble into Multisubunit Structures

FIGURE 2.48 Quaternary structure. The Cro protein of bacteriophage λ is a dimer of identical subunits.
[Drawn from 5CRO.pdb.]

Four levels of structure are frequently cited in discussions of protein architecture. So far, we have considered three. Primary structure is the amino acid sequence. Secondary structure refers to the spatial arrangement of amino acid residues that are nearby in the sequence. Some of these arrangements give rise to periodic structures. The α helix and β strand are elements of secondary structure. Tertiary structure refers to the spatial arrangement of amino acid residues that are far apart in the sequence and to the pattern of disulfide bonds. We now turn to proteins containing more than one polypeptide chain. Such proteins exhibit a fourth level of structural organization. Each polypeptide chain in such a protein is called a subunit. Quaternary structure refers to the spatial arrangement of subunits and the nature of their interactions. The simplest sort of quaternary structure is a dimer, consisting of two identical subunits. This organization is present in the DNA-binding protein Cro, found in a bacterial virus called λ (Figure 2.48). More-complicated quaternary structures also are common. More than one type of subunit can be present, often in variable numbers. For example, human hemoglobin, the oxygen-carrying protein in blood, consists of two subunits of one type (designated α) and two subunits of another type (designated β), as illustrated in Figure 2.49. Thus, the hemoglobin molecule exists as an α2β2 tetramer. Subtle changes in the arrangement of subunits within the hemoglobin molecule allow it to carry oxygen from the lungs to tissues with great efficiency (Chapter 7).

FIGURE 2.50Complex quaternary structure. The coat of human rhinovirus, the cause of the common cold, comprises 60 copies of each of four subunits. The three most prominent subunits are shown as different colors.
FIGURE 2.49The α2β2 tetramer of human hemoglobin. The structure of the two α subunits (red) is similar to but not identical with that of the two β subunits (yellow). The molecule contains four heme groups (gray with the iron atom shown in purple). (A) The ribbon diagram highlights the similarity of the subunits and shows that they are composed mainly of α helices. (B) The space-filling model illustrates how the heme groups occupy crevices in the protein.
[Drawn from 1A N.pdb.]

49

Viruses make the most of a limited amount of genetic information by forming coats that use the same kind of subunit repetitively in a symmetric array. The coat of rhinovirus, the virus that causes the common cold, includes 60 copies of each of four subunits (Figure 2.50). The subunits come together to form a nearly spherical shell that encloses the viral genome.

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