Question

1. It is possible to convert the cysteine (Cys) that is a part of Cys-tRNACys to alanine (Ala) by a catalytic reduction. If the resulting Ala-tRNACys were added to a mixture of ribosomes along with tRNAs correctly charged with the other 19 amino acids, all the other cofactors and proteins needed to make proteins in vitro, and mRNA for Insulin, what effect on the primary structure, tertiary structure and function of the insulin thus made would you expect?

Answer 1

Insulin is a polypeptide hormone consisting of 2 chains A and B. A chains has 21 amino acids while B chain has 30 amino acids.

The 2 xhains are bound together in tertiary structure by 3 disulfide bonds

A chain Bchain

Cys7 Cys7

Cys20 Cys19

Cys4 - Cys 11

When synthesized in the preproinsulin form it also contains a C peptide between the A and B chains which is later cleaved before secretion.

In the above question, Cys-tRNACys was converted to Ala-tRNACys. Since, the tRNA has anti codon specific for Cys but the aminoa cid in the amino acid acceptor stem is converted to Ala, it would still recognize Cys codon on mRNA but would load Ala instead of Cys. Thus all the Cys residues in insulin would be replaced by Ala.

Since, there are no Cys ressidues there can be no disulphide bond formation. Thus the A and B chain would remain isolated and not in contact i.e. tertiary and quarternary structure of insulin would not form.

Thus in primary structure all Cys would be replaced by Ala while tertiary structure and so the quarternary structyre would not form.

This insulin molecule formed would be non-functional