Which group is at the C-terminus of a polypeptide?

Which group is at the C-terminus of a polypeptide?

• A. Carboxyl group
• B. Amino group
• C. Sulfhydryl group
• D. Phosphate group

Answer: (A)

At the end of a polypeptide, the free carboxyl group marks the C-terminus. As amino acids bond, the amino group of one residue links to the carboxyl group of the next, creating a backbone that has two distinct ends: the N-terminus with a free amino group on the first amino acid, and the C-terminus with a free carboxyl group on the last amino acid. Thus, the C-terminus is defined by that terminal carboxyl group (often as -COO^- at physiological pH). The other groups aren’t standard termini: the sulfhydryl group sits on certain side chains (like cysteine), and phosphate groups are not the defining feature of the chain ends.

At the end of a polypeptide, the free carboxyl group marks the C-terminus. As amino acids bond, the amino group of one residue links to the carboxyl group of the next, creating a backbone that has two distinct ends: the N-terminus with a free amino group on the first amino acid, and the C-terminus with a free carboxyl group on the last amino acid. Thus, the C-terminus is defined by that terminal carboxyl group (often as -COO^- at physiological pH). The other groups aren’t standard termini: the sulfhydryl group sits on certain side chains (like cysteine), and phosphate groups are not the defining feature of the chain ends.