Question

It is possible to convert the cysteine that is a part of Cys-tRNACys to alanine 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

If cysteine which is a ppart of the cys-tRNAcys is reduced to Ala-tRNAcys, the tRNA will carry the anticodon for cysteine but will add alanine instead of cysteine in the primary structure. So alanine will be present instead of cysteine residues in the primary structure of insulin. Cysteine residues are involved in the formation interchain and intrachain disulphide bonds in the tertiary structure of insulin. In the absence of cysteine residues the disulphide bonds are also absent, so the A and B chains of insulin remain separately and cannot form the tertiary structure that can interact with the receptor of insulin and hence the function of insulin cannot be carried out.