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From Proteopedia

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Introduction

Ribonuclease A is an enzyme involved in catalyzing RNA degradation. The structure of RNase A has been determined through crystallization and FABMS. There are several features of its structure that are pertinent to its folding and function. The catalytic residues of RNase A include His12, His 119, and Lys41. The active site lies within its cleft. RNase A has eight cysteine residues present that form four disulfide linkages which contribute to the ordered structure that has been observed as well as the speed of folding. Another important aspect to folding is the presence of two cis proline residues. There are specific parts of a protein that are required for the folding structure.

Folding

Proline Conformation Most amino acids found in nature 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 formation and two in the trans formation. RNase A mutants help illustrate the key components in its folding mechanism. 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. Although Disulfide Bonds In RNase A, there are four disulfide bonds present that contribute to the thermal stability and the rate of folding. 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 is the main contributor to the thermodynamic stability. The importance of the disulfide bonds were examined by oxidizing and reducing