Counting amino acids via peptide bonds in protein structure

Steps:

• Examine the protein backbone for repeating -NH-CH(R)-CO- units, each representing an amino acid residue.
• Identify peptide bonds (-CO-NH-) linking these units; each bond connects two residues.
• Count the bonds: the structure shows three peptide bonds.
• Calculate total amino acids as bonds plus one, yielding four residues.

Why C is correct:

• A polypeptide chain's length is defined as the number of amino acid monomers, which equals the number of peptide bonds plus one, confirming four monomers here.

Why the others are wrong:

• A: Ignores the full chain, undercounting by two residues.
• B: Counts only the bonds, missing the extra terminal residue.
• D: Mistakenly adds an extraneous group as a fifth residue.

Final answer: C