Hydrogen-bonded protein structures pleated sheet

Not all proteins, however, form helical structures. If the substituent groups on the amino acids are small, as found in silk fibroin, then the polypeptide chains can line up side by side and form sheet-like arrangements. The chains tend to contract to acconunodate hydrogen bonding and form pleated sheets. This is called a -arrangement. Such an arrangement can be parallel and antiparallel. The identity period of the parallel one is 6.5 A and that of the antiparallel, 7.0 A. 

The secondary structure describes the overall conformation or shape of the protein molecule. Typical types of secondary structure are helices (cf Sections 4.2 and 4.6) and pleated sheets (j5 structures). Secondary structure results from main-chain hydrogen-bonded interactions. In pleated sheets, the a-amino acid chains can be arranged parallel or antiparallel to each other, with the antiparallel structure being thermodynamically more stable. a-Amino acids that yield helical homopolymers usually (but not inevitably) form helical sequences in proteins and polypeptides. The random coil that results from the rupture or lack of stabilizing hydrogens is not considered a secondary structure. Segments of a-helix, pleated sheet, and random coil are possible in the same molecule. 

Spider web is composed mostly of fibroin, a protein with pleated-sheet secondary structure. The pleated-sheet arrangement allows for multiple hydrogen bonds between molecules, conferring great strength. 

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. 

Silk fibers, which are obtained from the secretion of the silkworm, are double filaments that are enclosed by a coating of a gum (sericin) as they are secreted. The amino acid sequence of the silk protein was shown to be (glycine-serine-glycine-alanine-glycine-alanine). The polypeptide chains are bound into antiparallel pleated )ff-sheet structures by hydrogen bonding. The structures are also held together by van der Waal forces. ... 

The secondary structure of a protein is determined by the spatial arrangement of the polypeptide chain. Evidence obtained mainly from X-ray diffraction patterns (Linus Pauling, 1951, and others) has shown that the chain is typically wound into a helix. The helical form is maintained by hydrogen bonds located at spaced intervals, as shown in Fig. 26.8. The entire structure is called the ALPHA-HELIX. Other secondary structures of proteins include pleated sheets and random coils. 

Section 27 19 Two secondary structures of proteins are particularly prominent The pleated sheet is stabilized by hydrogen bonds between N—H and C=0 groups of adjacent chains The a helix is stabilized by hydrogen bonds within a single polypeptide chain... 

Pleated p sheet (Section 27.19) Type of protein secondary structure characterized by hydrogen bonds between NH and C=0 groups of adjacent parallel peptide chains. The individual chains are in an extended zigzag conformation. 

Figure 26.6 (a) The /3-pleated sheet secondary structure of proteins is stabilized by hydrogen bonds between parallel or antiparallel chains, (b) The structure of concanavalin A, a protein with extensive regions of antiparallel / sheets, shown as flat ribbons. 

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... 

Two common examples are the a-helix and the P-pleated sheet. These shapes are reinforced by hydrogen bonds. An individual protein may contain both types of secondary structures. Some proteins, like collagen, contain neither but have their own more characteristic secondary structures. 

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