Parallel P-pleated sheets

The p-pleated sheet structure occurs in fibrous as well as globular proteins and is formed by intermolecular hydrogen bonds between a carboxyl group oxygen of one amino acid and an amine hydrogen of an adjacent polypeptide chain. Parallel p-pleated sheets form when the adjacent polypeptide chains are oriented in one direction (from N-terminal to C-terminal end or vice versa). Antiparallel p-pleated... 

Figure 11.2 Polypeptide chains held together by hydrogen bonds, indicated by dashed lines, in a configuration called a P sheet, (a) The antiparallel P pleated sheet, (b) The parallel P pleated sheet. (Illustration, Irving Geis/Geis Archive Trust. Copyright Howard Hughes Medical Institute. Reproduced with permission.)...

Crystal structure(s) of ACTH-(1-39) or 1-24 are not known. Suitable crystals for X-ray diffraction experiments could be obtained however, for the heptapeptide 4-10 (54, 55) and the smaller tetrapeptide 4-7 (54, 56). In the former case, an anti--parallel p-pleated sheet structure of the backbone was found with clustering of hydrophobic (Met, PheandTrp) and hydrophilic (Glu, His, Arg) side-chains as remarkable features. ACTH-(4-7)... 

In a parallel p-pleated sheet, the strands run in the same direction from the N- to C-terminal amino acid. 

Figure 28.11 The parallel and antiparallel forms of the p-pleated sheet Parallel p-pleated sheet...

Suggest a reason why antiparallel p-pleated sheets are generally more stable than parallel p-pleated sheets. 

Consider two molecules of a tetrapeptide composed only of alanine residues. Draw the hydrogen bonding interactions that result when these two peptides adopt a parallel p-pleated sheet arrangement. Answer this same question for the antiparallel p-pleated sheet arrangement. 

The second common secondary structure in proteins resembles the pleated folds of drapery and is known as p-pleated sheet (Figure 19.8). All of the carbonyl oxygens and amide hydrogens in a p-pleated sheet are involved in hydrogen bonds, and the polypeptide chain is nearly completely extended. The polypeptide chains in a P-pleated sheet can have two orientations. If the N-termini are head to head, the structure is known as a parallel p-pleated sheet. And if the N-terminus of one chain is aligned with the C-terminus of a second chain (head to tail), the structure is known as an antiparallel p-pleated sheet. 

A parallel p-pleated sheet is one in which the hydrogen bonded peptide chains have their amino-termini aligned head-to-head. 

In contrast to those who study metal binding sites within alpha-hehcal structures, another group has focused its efforts on the design of a metallopep-tide that adopts a parallel p-pleated sheet structure (33). The polypyiidyl complex [Ru(bpy)2L] is used, where bpy is 2,2 -bipyridine and L = 3,5-dicarboxy-2,2 -bipyridine. The carboxyl groups are used as points of attachment for the... 

Figure 7.37. Possible structural model of scrapie prion protein from electron crystallography studies, showing full sixfold symmetric top view with triangular arrangement of parallel P-pleated sheets at the center surrounded by additional partially a-helical sequences. (Reproduced with permission from Wille et al. Copyright 2002 National Academy of Sciences, U.S.A.)...

Figure 25.12 Hydrogen bonds (a) in a parallel p-pleated sheet structure, in which all the polypeptide chains are oriented in the same direction, and p) in an antiparaiiei fS-pleated sheet, in which adpcent polypeptide chains run in opposite directions. For color key, see Fig. 25.9.

Antiparallel P-pleated sheets are more stable then parallel P-pleated sheets because of geometry. The N-H and C=0 of one chain are directly aligned with the N-H and C=0 of an adjacent chain in the antiparallel P-pleated sheet, whereas they are not in the parallel P-pleated sheet. This makes the latter set of hydrogen bonds weaker. 

Figure 27.11 Hydrogen Bonding In Parallel p-Pleated Sheet... 

Helices and sheets can combine in many other ways. For example, an enzyme that catalyzes a reaction in the degradation of glucose (glycolysis) called triose phosphate isomerase, which has 248 amino acid residues, contains many strand-hehx-strand motifs p-a-P (Figure 27.16). The p strands form a parallel P-pleated sheet. Figure 27.17 shows the tertiary structure of triose phosphate... 

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