Wmk and Strong iioriffe Determine Mocromalecular Structure

FiCINt 5-8 A polypeptide chain folded into a helical configuration called the a helix. (Source: Molecular structure adapted frnrn Pauling I 1960. The nature nf the chemical bond and the structure of mafecuks and crystals: An intmduction to modem structural chemistry, 3rd edition, p. 500. Copyright © 1960 Cornell University. Used by permission of the publisher.)

conformation such that hydrogen bonds can form between carbonyl groups of one p strand and NM groups on the adjacent strand. Typically, a region of [5 sheet is composed of four to six separate stretches of polypeptide (each forming an individual p strand), each eight to ten amino acids in length. In the 0 sheet, adjacent amino acids are related by a rotation of 180[l and thus their respective side groups emerge from opposite sides of the p sheet (see Figure 5-9b).

[3 sheets come in predominantly one of two forms, These differ in the relative orientations of their chains (Figure 5-10). In one, the adjacent chains run in the same amino-to-carboxyl direction to produce a parallel fi sheet. In the other, the adjacent chains run in opposite directions to yield an antiparallel p sheet. Although less common, there are also 0 sheets that have both parallel and antiparallel components. In both parallel and antiparallel p sheets, alt the peptide groups lie approximately in the plane of the sheet. Structural studies have revealed that in most cases the individual strands of fi sheets tend to be twisted along their length in a right-handed manner (Figure 5-11). Thus, instead of flat sheets of protein, regions of (J sheet tend to curve to generate a compact protein module.

For a protein to fold properly, both the backbone and the side chains must adopt conformations that maximize favorable interactions. The a helix and p sheet are both very stable conformations of the polypeptide backbone. But for each side chain lo make the maximum number of weak bonds, proteins have to adopt more varied

Higher-Order Structures Are Determined by intra- and Intermolecular Interactions

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