Protein Tertiary and Quaternary Structures: -Structural Diversity Reflects Functional Diversity in Globular Proteins
In a globular protein, different segments of a polypep tide chain (or multiple polypeptide chains) fold back on each other. As illustrated in Figure 4–15, this folding generates a compact form relative to polypeptides in a fully extended conformation. The folding also provides the structural diversity necessary for proteins to carry out a wide array of biological functions. Globular proteins include enzymes, transport proteins, motor proteins, regulatory proteins, immunoglobulins, and proteins with many other functions. As a new millennium begins, the number of known three-dimensional protein structures is in the thousands and more than doubles every two years. This wealth of structural information is revolutionizing our understanding of protein structure, the relation of structure
FIGURE 4–15 Globular protein structures are compact and varied. Human serum albumin (Mr 64,500) has 585 residues in a single chain. Given here are the approximate dimensions its single polypeptide chain would have if it occurred entirely in extended β conformation or as an α helix. Also shown is the size of the protein in its native globular form, as determined by X-ray crystallography; the polypeptide chain must be very compactly folded to fit into these dimensions.
to function, and even the evolutionary paths by which proteins arrived at their present state, which can be glimpsed in the family resemblances that are revealed as protein databases are sifted and sorted. The sheer variety of structures can seem daunting. Yet as new protein structures become available it is becoming increasingly clear that they are manifestations of a finite set of recognizable, stable folding patterns. Our discussion of globular protein structure begins with the principles gleaned from the earliest protein structures to be elucidated. This is followed by a de tailed description of protein substructure and comparative categorization. Such discussions are possible only because of the vast amount of information available over the Internet from resources such as the Protein Data Bank (PDB; www.rcsb.org/pdb), an archive of experi mentally determined three-dimensional structures of biological macromolecules.
