FIGURE 7.07 The Beta Sheet—Hydrogen Bonding

A) A polypeptide chain is shown folded back and forth three times to form a flattened sheet. B) The polypeptide backbone of the sheet is depicted demonstrating how the p-sheet forms a zigzag in three dimensions. C) A ball and stick model shows the hydrogen bonding between the strands of the p-sheet.

with a mixture of d- and l- amino acids, although a stable left-handed helix could theoretically be formed from d- amino acids).

The R-groups extend outwards from the tightly packed helical polypeptide backbone. Of the 20 amino acids, Ala, Glu, Leu and Met are good a-helix formers but Tyr, Ser, Gly and Pro are not. Proline is totally incompatible with the a-helix, due to its rigid ring structure. Furthermore, when proline resides are incorporated, no hydrogen atoms remain on the nitrogen atom that takes part in peptide bonding. Consequently, proline residues interrupt hydrogen-bonding patterns. In addition, two bulky residues or two residues with the same charge that lie next to each other in the polypeptide chain will not fit properly into an a-helix. Overall, the a-helix forms a solid cylindrical rod.

The p-sheet is also held together by hydrogen bonding between peptide groups but in this case the polypeptide chain is folded back on itself to give a flattish zigzag structure (Fig. 7.07). Like the a-helix, the p-sheet is very stable because all of the peptide groups (except for those on the edge of the sheet) take part in two hydrogen bonds. In the p-sheet, these go sideways from each peptide group, one to each side. The sections of the polypeptide chain which lie side by side are usually, although not always, antiparallel and the R-groups lie alternately above and below the zigzagging plane of the p-sheet.

A variety of p-sheet conformations is known (Fig. 7.08). Although some p-sheets are flat, most known p-sheets are twisted (in a right-handed manner) so that the sheet structure as a whole is not flat but curved. In some cases, p-sheets may curve around so that the last strand bonds to the first, forming a barrel structure.

A reverse turn (also known as a p-turn or p-bend) is where the polypeptide chain turns back upon itself. Beta-sheets have reverse turns at the ends of each segment, but such turns are also found frequently in other places. Pro and Gly are often found in

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