Answer:
C.The β‑pleated sheet is held together by hydrogen bonds between adjacent segments.
Explanation:
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The most common secondary structures are α-helix and β-sheets. The structures are defined by regular hydrogen bonds formed between the N-H and C=O groups of the amino acids that form the chain. These structures form in segments of the protein as an intermediate before it folds into the 3D tertiary structure.
<u>α helix </u>
It is a cylindrical structure that comprehends a helical backbone, while the side chains extend outward in a helical distribution. The α-helix stabilizes through hydrogen bonds between the amines and carbonyls groups of the backbone. Each carbonyl group forms a hydrogen bond with the amine group four residues later in the main chain. Thus, except for the amino acids near the end of the α-helix, all the carbonyls and amines groups in the main chain are linked by hydrogen bonds. Each residue corresponds to a translation of 1,5Ǻ and a turn of 100º this equals 3,6 residues per turn.
From the observer point of view, if the rotation of the helix is clockwise or right-handed, it's called dextrorotation and if the rotation is counterclockwise or left-handed it's called levorotation. Dexorotation or dextrogyre is the most common rotation of α-helixes in proteins. Levorotation or levogyre is very rare but can be found in proteins with a large content of achiral glycine.
<u>β-sheets (β-pleated sheets)
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This structure consists of at least two β-strands (polypeptide chains), the strands conform a backbone of three to ten amino acids in an extended formation that connects laterally with hydrogen bonds. The distance between adjacent amino acids in a β-strand is approximately 3,5Ǻ in contrast to the 1,5Ǻ distance of an α helix. The chains that form a β-sheet have directionality conferred by their N-terminus and C-terminus. Adjacent β-strands can form hydrogen bonds in antiparallel, parallel or mixed arrangements. In the antiparallel arrangement, the adjacent strands are said to have opposite directions (N-C vs C-N) this allows the bonds to be established between the amines and carbonyls groups of each amino acid with the carbonyls and amines of the adjacent amino acid. This way the bonds between carbonyls and amines are planar, which allows strong interstrand stability.
In the parallel arrangement, the adjacent strands have the same direction (N-C vs N-C). In this type of arrangement, each amine forms a hydrogen bond with the carbonyl of the adjacent amino acid, but its carbonyl group forms a hydrogen bond with the amine group of the amino acid two residues later.
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