Sandbox Reserved 197: Difference between revisions

There are features of every protein that directly or indirectly effect the folding of that protein.  Several of these features have been identified in RNase A by the use of mutants of the native form.  These mutations and the study of the kinetics and final structure show whether that particular feature is involved in the folding of the protein.  One particular feature of RNase A is the presence of ''cis'' proline residues.  In nature, most amino acids reside in a ''trans'' conformation. Due to their cyclic structure, prolines are more stable in a ''cis'' conformation. RNase A contains four proline residues, two reside in the "cis" conformation and two in the "trans" conformation. The Tyr92-Pro93 peptide group of RNase A in its native state is found in the ''cis'' conformation. Despite a P93A mutation, a cis conformation still forms; this is an unlikely conformation for an alanine residue.   Upon unfolding, Tyr92-Ala93 undergoes isomerization to form its favored "trans" conformation. This points to the fact that this "cis" bond formation is a key component to the protein structure of RNase A.  

 

Another important feature of the folding of RNase A is the presence of four disulfide bonds.  These bonds contribute to the thermal stability and the rate of folding of RNase A.  The residues involved in these linkages include Cys26-Cys84, Cys58-Cys110, Cys40-Cys95, and Cys65-Cys72.  Cys26-Cys84 and Cys58-Cys110 create an interaction between an alpha-helix and a beta sheet.  This connection is the main contributor to the thermodynamic stability.