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Answer:
D. all of these
Explanation:
Proteins are chains of amino acids (polypeptides) that adopt a specific conformation in space. To adopt this conformation, the amino acid chains fold locally and regularly, forming what is known as secondary structures. Mainly, there are two types of secondary structures: the α-helix and the β-pleated sheet.
In the α-helix, the polypeptide chain is spirally wound on itself. This structure is maintained thanks to hydrogen bonds between the carboxyl group of an amino acid and the amino group of the fourth amino acid that follows in the chain.
In the β-pleated sheet the polypeptide skeleton is almost extended in a zig zag shape and stability is achieved by the association of two or more peptide chains next to each other by hydrogen bonds between the amino and carboxyl groups of the peptide bond between adjacent chains. For this reason, hydrogen bonds are almost perpendicular to amino acid chains. In the image Bsheet this phenomenon can be observed.
Usually, the chains are drawn with an arrow to indicate the direction of the polypeptide chain from the amino group to the terminal carboxyl. Thus, adjacent chains of a beta sheet can be oriented in the same direction (parallel beta sheets), or in opposite directions (antiparallel beta sheets). The same folded beta sheet can have parallel and antiparallel chains. The image Bsheet 2 shows parallel and antiparallel chains.
