Sandbox Reserved 197

From Proteopedia

Revision as of 12:13, 1 April 2011

Introduction

Ribonuclease A is an enzyme involved in catalyzing RNA degradation. The structure of RNase A has been determined through crystallography [1] and FABMS [2]. RNase A is composed of four anti-parallel B-sheets and 3 a-helixes. The lies within the cleft and houses three residues important for catalysis: His12, His119, and Lys41. Presence of eight cysteine residues that form four disulfide bonds and four cis proline residues greatly effect the structure and folding kinetics of RNase A.

Folding

There are features of every protein that directly or indirectly effect the folding of that protein [3]. Several of these features have been identified in RNase A by the use of site directed mutagenesis to wildtype RNase A. These mutations and the study of the kinetics and final structure in comparison to the native form show whether that particular feature is involved in the folding of the protein.

Proline Conformation

One particular feature of RNase A is the presence of cis proline residues. In nature, most amino acids reside in a trans conformation [4]. 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 peptide group of RNase A in its native state is found in the cis conformation. Despite a 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.

Disulfide Bonds

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 , , , and . Cys26-Cys84 and Cys58-Cys110 create an interaction between an α-helix and a β-sheet. This connection is the main contributor to the thermodynamic stability. RNase A actually has a rate-determining three-disulfide intermediate. An analog of this, , shows RNase A, missing the disulfide bond, Cys40-Cys95, that would normally occur here. As you can see in the variant, there are only 3 disulfide bonds present, shown in red.

Medical Importance

Protein folding, along with its inhibitions, is immensely important to the human. Such diseases as ALS, Alzheimer's Disease, and Parkinson's Disease can all be traced back to the protein. Proteins can form aberrant aggregates when they do not fold correctly. This abnormaility can be fatally toxic to the human nerve cells. During folding, proteins sometimes make a mistake. Each protein contains . The hydrophilic residues lie on the outer part of the protein and the hydrophobic residues bury themselves due to the hydrophobic effect [5]. In the case of these aggregates, the mistake exposes "sticky" of the interior that can cause several proteins to stick to one another. In the future researchers hope to design drugs that combat this mistake in the protein folding. The use of ribonuclease A in protein folding research has been an instrumental feature in designing experiments to determine these "misfolding" snapshots and in developing therapies to prevent this problem in the future.

References

External Links

http://en.wikipedia.org/wiki/Ribonuclease_A