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Antiparallel fi-Sheet Proteins

Another important class of tertiary protein conformations is the antiparallel f-sheet structures. Antiparallel ^-sheets, which usually arrange hydrophobic residues on just one side of the sheet, can exist with one side exposed to solvent. The minimal structure for an antiparallel ^-sheet protein is thus a two-layered structure, with hydrophobic faces of the two sheets juxtaposed and the opposite faces exposed to solvent. Such domains consist of ^-sheets arranged in a cylinder or barrel shape. These structures are usually less symmetric than the singly wound parallel barrels and are not as efficiently hydrogen bonded, but they occur much more frequently in nature. Barrel structures tend to be either all parallel or all antiparallel and usually consist of even numbers of ^-strands. Good examples of antiparallel structures include soybean trypsin inhibitor, rubredoxin, and domain 2 of papain (Figure 6.32). Topology diagrams of antiparallel ^-sheet barrels reveal that many of them arrange the polypeptide sequence in an interlocking pattern reminiscent of patterns found on ancient Greek vases (Figure 6.33) and are thus referred to as a Greek key topology. Several of these, including concanavalin A and y-crystallin, contain an extra swirl in the Greek key pattern (see Figure 6.33). Antiparallel arrangements of ^-strands can also form sheets as well as barrels. Glyceraldehyde-3-phosphate dehydrogenase, Streptomyces subtilisin inhibitor, and glutathione reductase are examples of single-sheet, double-layered topology (Figure 6.34).

Metal- and Disulfide-Rich Proteins

Other than the structural classes just described and a few miscellaneous structures that do not fit nicely into these categories, there is only one other major class of protein tertiary structures—the small metal-rich and disulfide-rich structures. These proteins or fragments of proteins are usually small (<100 residues), and their conformations are heavily influenced by their high content of either liganded metals or disulfide bonds. The structures of disulfide-rich proteins are unstable if their disulfide bonds are broken. Figure 6.35 shows several representative disulfide-rich proteins, including insulin, phospholipase A2, and crambin (from the seeds of Crambe abyssinica), as well as several metal-rich proteins, including ferredoxin and high potential iron protein (HiPIP). The structures of some of these proteins bear a striking resemblance to structural classes that have already been discussed. For example, phospholipase A2

Soybean trypsin inhibitor

Rubredoxin

Papain domain 2

FIGURE 6.32 • Examples of antiparallel ß-sheet structures in proteins. (Jane Richardson)

Rubredoxin

Soybean trypsin inhibitor

Rubredoxin

Papain domain 2

Rubredoxin

FIGURE 6.32 • Examples of antiparallel ß-sheet structures in proteins. (Jane Richardson)

FIGURE 6.33 • Examples of the so-called Greek key antiparallel ^-barrel structure in proteins.

"Greek key" topology

Concanavalin A

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