Consider the structure of an RNase A molecule pdb ID 5RSA If

Consider the structure of an RNase A molecule (pdb ID: 5RSA). If you make the mutation Cys26Ser, you decrease the activity of the wild type enzyme by 50%. Propose a reason for this decrease in the activity. You find that both mutants His12Ala and His119Ala are completely inactive. Explain this observation. If the Cys26Ser mutation is harbored in both His mutants (separately) and each of these double mutants are mixed together (1:1), you recover 25% of the wild-type activity. Propose a mechanism for this observation. Lastly, given

Solution

A

In native RNase A molecule, there are eight cysteine residues which carry out disulfide bonding in order to maintain the stable structure. Cys26 is one of them. So if one of them is mutated to serine which can not help in disulphide linkage there will be distortion in the native shape. This will affect the activity of the enzyme too.

B His 12 and His 119 are a part of the active site of the enzyme. They help in acid base catalysis action which is the mode by which this enzyme acts.