19.60. Oxytocin, a nonapeptide hormone secreted by the pituitary gland, function

Question

19.60. Oxytocin, a nonapeptide hormone secreted by the pituitary gland, functions by stimulating uterine contraction and lactation during childbirth. Its sequence was determined from the following evidence: 1. Oxytocin is a cyclic compound containing a disulfide bridge between two cysteine residues. 2. When the disulfide bridge is reduced, oxytocin has the constitution N, C2, Q, G, I, L, P, Y 3. Partial hydrolysis of reduced oxytocin yields seven fragments: D-C, I-E, C-Y, L-G, Y-I-E, E-D-C, C-P-L 4. Gly is the C-terminal group. 5. Both E and D are present as their side-chain amides (Q and N) rather than as free side- chain acids. What is the amino acid sequence of reduced oxytocin? What is the structure of oxytocin itself?

Explanation / Answer

Ans.

The amino acid sequence of oxytocin is cysteine - tyrosine - isoleucine - glutamine - asparagine - cysteine - proline - leucine - glycine (CYIQNCPLG). cysteine is the N-terminal amino acid and glycine is the C-terminal amino acid.

The structure of oxytocin involves a cyclic six amino acid ring formed by the disulfide bond between cysteine residues at amino acid positions 1 and 6 with 3 amino acid residues hanging off.

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