Protein Tertiary and Quaternary Structures: -Globular Proteins Have a Variety of Tertiary Structures
With elucidation of the tertiary structures of hundreds of other globular proteins by x-ray analysis, it became clear that myoglobin illustrates only one of many ways in which a polypeptide chain can be folded. In Figure 4–18 the structures of cytochrome c, lysozyme, and ribonuclease are compared. These proteins have different amino acid sequences and different tertiary structures, reflecting differences in function. All are relatively small and easy to work with, facilitating structural analysis. Cytochrome c is a component of the respiratory chain of mitochondria (Chapter 19). Like myoglobin, cytochrome c is a heme protein. It contains a single polypeptide chain of about 100 residues (Mr12,400) and a single heme group. In this case, the protoporphyrin of the heme group is covalently attached to the polypep tide. Only about 40% of the polypeptide is in -helical segments, compared with 70% of the myoglobin chain. The rest of the cytochrome c chain contains turns and irregularly coiled and extended segments. Lysozyme (Mr14,600) is an enzyme abundant in egg white and human tears that catalyzes the hydrolytic cleavage of polysaccharides in the protective cell walls of some families of bacteria. Lysozyme, because it can lyse, or degrade, bacterial cell walls, serves as a bactericidal agent. As in cytochrome c, about 40% of its 129 amino acid residues are in -helical segments, but the arrangement is different and some -sheet structure is also present (Fig. 4–18). Four disulfide bonds con tribute stability to this structure. The helices line a long crevice in the side of the molecule, called the active site, which is the site of substrate binding and catalysis. The bacterial polysaccharide that is the substrate for lysozyme fits into this crevice.
Ribonuclease, another small globular protein (Mr 13,700), is an enzyme secreted by the pancreas into the small intestine, where it catalyzes the hydrolysis of certain bonds in the ribonucleic acids present in ingested food.
Its tertiary structure, determined by x-ray analysis, shows that little of its 124 amino acid polypeptide chain is in an α -helical conformation, but it contains many segments in the
