Quaternary Structure Assemblies Of Protein Subunits

Many proteins associate to form multiple molecular structures that are held together tightly but not covalently by the same interactions that stabilize tertiary structures. For example, gap junction channels (page 55) are formed from six identical monomers. CAP is only active when it dimerizes (Fig. 9.18). Hemoglobin, the protein that carries oxygen in our red blood cells, is formed from four individual polypeptide chains, two a chains and two

Figure 9.20. Hemoglobin, a tetrameric protein with heme prosthetic groups. (Illustration: Irving Geis. Rights owned by Howard Hughes Medical Institute. Reproduction by permission only.)

ß chains (Fig. 9.20). In all these cases, we call the three-dimensional arrangement of the protein subunits as they fit together to form the multimolecular structure the quaternary structure of the protein.

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