Which of the following statements is true?
(a) Peptide bonds are the only covalent bonds that can link together two amino acids in proteins.
(b) The polypeptide backbone is free to rotate about each peptide bond.
(c) Nonpolar amino acids tend to be found in the interior of proteins.
(d) The sequence of the atoms in the polypeptide backbone varies between different proteins.
Choice (c) is the correct answer. Choice (a) is untrue, because some proteins also contain covalent disulfide bonds (–S–S– bonds) linking two amino acids. Choice (b) is untrue, because the peptide bond is rigid. Choice (d) is untrue, because the sequence of atoms in the polypeptide backbone itself is always the same from protein to protein; it is the order of the amino acid side chains that differs.
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