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4.4 Quaternary Structure: Multiple Polypeptide Chains Can Assemble into a Single Protein

Many proteins consist of more than one polypeptide chain in their functional states. Each polypeptide chain in such a protein is called a subunit. Quaternary structure refers to the arrangement of subunits and the nature of their interactions. The interactions among subunits of proteins displaying quaternary structure are usually the weak interactions discussed in Chapter 2: hydrogen bonds, ionic bonds, and van der Waals interactions.

Figure 4.29: Living Figure 4.29: Quaternary structure

Quaternary structure. The Cro protein of bacteriophage λ is a dimer of identical subunits.

The simplest sort of quaternary structure is a dimer consisting of two identical subunits. This organization is present in Cro, a DNA-binding protein found in a bacterial virus called lambda (λ) that regulates gene expression (Figure 4.29). Quaternary structure can be as simple as two identical subunits or as complex as dozens of different polypeptide chains. 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 4.30. Thus, the hemoglobin molecule exists as an α₂β₂ tetramer. Note that the hemoglobin subunits are called a and β for historical reasons, and the a and β designations have no relation to the α helix or the β strand.

Figure 4.30: Living Figure 4.30a: The α2β2 tetramer of human hemoglobin

Living Figure 4.30b: The α2β2 tetramer of human hemoglobin

The α₂β₂ tetramer of human hemoglobin. The structure of the two identical α subunits (red) and the two identical β subunits (yellow). (A) The ribbon diagram shows that they are composed mainly of α helices. (B) the space-filling model illustrates the close packing of the atoms and shows that the heme groups (gray) occupy crevices in the protein.