8. Insulin is a peptide hormone that is central to regulating carbohydrate and f

Question

8. Insulin is a peptide hormone that is central to regulating carbohydrate and fat metabolism in the human body. It is composed of 51 amino acids, in two chains, held together by disulfide bridges. A disulfide bridge is formed between two amino acid side chains that have thiol (–SH) substituents (like an alcohol, but with sulfur).

(a) Which amino acid(s) have a thiol group? Which amino acid(s) have a thio-ether group? Draw these structures.

(b) When two thiols come together to form a disulfide, two hydrogen atoms have to be lost to some reagent: 2 RSH + X à RSSR + XH2 Is this an oxidation of RSH or a reduction? Does it resemble the conversion of an alcohol to an aldehyde, or an aldehyde to an alcohol?

(c) The formation of disulfides is reversible: The RSSR linkage can be cleaved to make 2 RSH. For insulin, this would destroy the structure and liberate the two separate chains. Would you predict that this would be more likely to happen under oxidizing or reducing conditions?

Explanation / Answer

a. amino acid cysteine has thiol group, where as methionine has thio ether group.

b. when two thiols come together to form disulphide bond, they show up oxidation of RSH by losing two hydrogen atoms.This resembles the conversion of an alcohol to an aldehyde.

c. disulphide bond in insulin could be reversibly reduced and also can be reoxidised.

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