P-pleated sheet conformation

Figure 15.1 DOS plots corresponding to optimum GA compositions of aperiodic poly (gly-ala-ser) in (a) a-helical conformation and (b) p-pleated sheet conformation.

Chothia, C. Conformation of twisted p-pleated sheets in proteins. /. Mol. Biol. 75 295-302, 1973. 

Folding of a peptide probably occurs coincident with its biosynthesis (see Chapter 38). The physiologically active conformation reflects the amino acid sequence, steric hindrance, and noncovalent interactions (eg, hydrogen bonding, hydrophobic interactions) between residues. Common conformations include a-helices and P pleated sheets (see Chapter 5). 

The second structural element to be proposed by Pauling and Corey was the P-pleated sheet (Figure 4.7). These sheets are made up of P-strands, typically from 5 to 10 residues long, in an almost fully extended conformation, aligned alongside one another with hydrogen bonds formed between the C=0 bonds of one strand and the NH of the other, and vice versa. The P-sheets are pleated (i.e. they undulate) with the Ca atoms alternatively a little above, or a little below the plane of the P-sheet, which means that the side chains project alternatively above and below the plane. P-Strands can interact to form two types of pleated sheets. 

Secondary structures are regions of the peptide chain with a defined conformation (see p. 68) that are stabilized by H-bonds. In insulin (2), the a-helical areas are predominant, making up 57% of the molecule 6% consists of p-pleated-sheet structures, and 10% of p-turns, while the remainder (27%) cannot be assigned to any of the secondary structures. 

The three-dimensional conformations of localized regions of a protein are called its secondary structure. These regions arise due to hydrogen bonding between the N-H proton of one amide and C=0 oxygen of another. Two arrangements that are particularly stable are called the a-helix and the P-pleated sheet. 

All enzyme molecules possess the primary, secondary, and tertiary structural characteristics of proteins (see Chapter 20). In addition, most enzymes also exhibit the quaternary level of structure. The primary structure, the linear sequence of amino adds linked through their a-carboxyl and a-amino groups by peptide bonds, is specific for each type of enzyme molecule. Each polypeptide cham is coiled up into three-dimensional secondary and tertiary levels of structure. Secondary structure refers to the conformation of limited segments of the polypeptide chain, namely a-helices, P-pleated sheets, random coils, and p-turns. The arrangement of secondary structural elements and amino acid side chain interactions that define the three-dimensional structure of the folded protein is referred to as its tertiary structure. In many cases biological activity, such as the catalytic activity of enzymes, requires two or more folded polypeptide chains (subunits) to associate to form a functional molecule. The arrangement of these subunits defines the quaternary structure. The subunits may be copies of the... 

Secondary structure (Section 27.19) The conformation with respect to nearest neighbor amino acids in a peptide or protein. The a helix and the P pleated sheet are examples of protein secondary structures. 

We have shown that the shape of a protein is absolutely essential to its function. We have also mentioned that life can exist only within a rather narrow range of temperature and pH. How are these two concepts related As we will see, extremes of pH or temperature have a drastic effect on protein conformation, causing the molecules to lose their characteristic three-dimensional shape. Denaturation occurs when the organized structures of a globular protein, the a-helix, the p-pleated sheet, and tertiary folds become completely disorganized. However, it does not alter the primary structure. Denaturation of an a-helical protein is shown in Figure 19.17. 

Fig. 20-2 -Pleated sheet conformation. (After W. H. Brown and E. P. Rogers, General, Organic, and Biochemistry, 3rd ed.. Brooks/Cole, Monterey, CA, 1987.)...

Secondary structures are regular elements such as a-helices and p-pleated sheets, which are formed between relatively small parts of the protein sequence. These structural domains are determined by the conformation of the peptide backbone, the influence of side-chains is not taken into account for secondary structures. 

The p-pleated sheet is found extensively only in the protein of silk. However, a number of proteins in which a single polypeptide chain forms sections of a p-pleated sheet structure by folding back on itself are known. I Figure 9.9 depicts a protein chain exhibiting the P-pleated sheet, the a-helix, and other structural conformations. Note the position of the hydrogen bonds in the two secondary structures. 

The fibers of the helical a-form of poly(L-leucine) are also wool-like but have not been commercialized. They have only been included in Table 38-3 to demonstrate the influence of macromolecular conformation on the fiber properties. For example, the P form or pleated-sheet conformation of poly(L-leucine) has, of course, quite different properties it is silklike. 

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