Q.Answer the questions with reference to the following figure [showing two polypeptide chains linked together by a bond]. i. Name the type of bond formed between two polypeptides. ii. Which amino acid is involved in the formation of such bond? iii. Amongst I, II, III and IV structural levels of protein, which level of structure includes such bond?

Step 1. When a figure shows two distinct polypeptide chains held together (rather than one chain folded onto itself), the bond most commonly responsible is a disulphide bond — a covalent S–S bond formed between the sulphur-bearing side chains of two cysteine residues.

Step 2. The amino acid that forms this bond is cysteine, since only cysteine among the 20 amino acids carries the –SH (thiol) side group needed to form a disulphide linkage on oxidation. …

Apply standard protein-chemistry facts about inter-chain disulphide bonds and th …

• Naming the peptide bond instead of the disulphide bond — the peptide bond links amino acids within one chain, not two separate polypeptide chains to each other.
• Calling the level 'tertiary' — tertiary structure describes the folding of a single chain; a bond joining two distinct chains belongs to quaternary structure. …