Question

A small segment of a polypeptide gave on complete hydrolysis 3 molecules of alanine, 2 molecules of glycine, and 3 molecules of cysteine. What is the number of peptide linkages in the segment of the polypeptide?

• A. 7
• B. 8
• C. 5
• D. 6

Hint:

Think of amino acids as railroad cars. To couple 8 train cars together in a single straight line, you only need 7 couplers. Similarly, an 8-residue peptide chain contains exactly $8 - 1 = 7$ peptide bonds!

Answer 1

The Correct Option is A

Concept: A polypeptide chain is formed through condensation reactions between individual amino acid monomers. Each linkage formed between the carboxyl group ($-\text{COOH}$) of one amino acid and the amino group ($-\text{NH}_2$) of the next is called a peptide bond ($-\text{CO}-\text{NH}-$). For an unbranched linear polymer chain composed of $N$ amino acid residues, the total number of intermediate peptide bonds linking them together is always: \[ \text{Number of Peptide Linkages} = N - 1 \]

Step 1: Sum the total number of individual amino acid units ($N$).
From the complete hydrolysis data, the structural fragments consist of:
• Alanine units $= 3$
• Glycine units $= 2$
• Cysteine units $= 3$ Total number of amino acid monomers ($N$) in the segment: \[ N = 3 + 2 + 3 = 8\text{ amino acids} \]

Step 2: Calculate the total number of linking peptide bonds.
Using our polymer connection relationship: \[ \text{Peptide Linkages} = 8 - 1 = 7 \]