Q.46 The number of peptide bonds in a 20-residue linear peptide is ___.
A 20-residue linear peptide contains 19 peptide bonds. This follows the fundamental rule in biochemistry: peptide bonds equal the number of amino acid residues minus one.
Peptide Bond Basics
Peptide bonds form through dehydration synthesis, linking the carboxyl group of one amino acid to the amino group of another, releasing water. In a linear chain, each new residue adds one bond without closing a loop. For n residues, bonds = n – 1; thus, a dipeptide (2 residues) has 1 bond, tripeptide (3) has 2.
Correct Answer: 19
A 20-residue peptide has 20 amino acids connected sequentially. The first residue’s amino terminus and last residue’s carboxyl terminus remain free, yielding exactly 19 linkages. This applies strictly to linear, non-cyclic structures common in polypeptide chains.
Common Options Explained
Multiple-choice questions often test this concept. Here’s a breakdown:
| Option | Description | Correct? | Reason |
|---|---|---|---|
| 20 | Equals number of residues | No | Counts amino acids, not bonds; overestimates by 1 |
| | |||
| 19 | Residues minus 1 | Yes | Standard formula for linear peptides |
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| 18 | Assumes extra free ends | No | Underestimates; ignores full chain linkage |
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| 21 | Exceeds residues | No | Impossible in linear form; suggests branching error |
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Exam Relevance
This question appears in biology entrance exams like NEET, testing protein structure knowledge. Master it alongside tetrapeptide examples (4 residues = 3 bonds) for pattern recognition. Linear peptides differ from cyclic ones, where bonds equal residues.
