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From Proteopedia

Revision as of 02:36, 31 March 2011

Shown here is an image of RNase A with its active site residues highlighted in blue.

Introduction

Ribonucleases or RNA depolymerases are ribosomes that catalyze RNA degradation in cells. They are most commonly found in the pancreas because they digest large amounts of RNA excreted by the stomach. The pancreas in ruminants, such as cows, have especially high amounts of ribonucleases in order to process nutrients from cellulose. One such ribonuclease, ribonuclease A or RNase A from cows, has been thoroughly studied due to its prevalence and structure.

Structure

RNase A is made up of a single polypeptide chain of 124 residues. Of the 20 natural amino acids, RNase A possesses 19 of them, excluding tryptophan. This single polypeptide chain is cross-linked internally by four , which contribute to the stability of RNase A. RNase A is in the shape of a kidney, with the active-site residues located within the cleft of the kidney. Active site residues and focus on it?? A long four-stranded anti-parallel and three short make up the secondary structure of RNase A. The amino acid sequence determines the three-dimensional structure of RNase A based on side-chain interactions.

History

RNase A has been used as a foundation enzyme for study. The 1972 Nobel Prize in Chemistry was awarded to three researchers for their work with RNase A on the folding of chains in RNase A and the stability of RNase A. By mutating the residues of RNase A by site-directed mutagenesis, the effects of these mutations were more visibly analyzed with advances in analytical chemistry instrumentation and techniques. RNase A was the first enzyme and third protein for which its amino acid sequence was correctly determined and the third enzyme and fourth protein whose three-dimensional structure was determined by X-ray diffraction analysis. NMR spectroscopy and Fourier transform infrared (FTIR) spectroscopy also were used with RNase A to describe protein structure and protein folding.

Medical Implications

Inhibition of Metastasis: A recent study published in 2010 discusses the possibility of using the degradation capabilities of RNase A and a similar protein DNase I to treat tumors. Tumor propagation is associated with an imbalance in nucleic acid degradation, which display themselves as increased levels of nucleic acids and decreased levels of nuclease activity in the blood of patients. The high levels of nucleic acids are caused by the unregulated expression and the secretion of a specific tumor-derived miRNA and DNA. It was found that the combined treatment of the RNase A and the DNase produced the best results by slowing the growth rate of the tumor. But both the ribonucleases are toxic at high levels; thus, only low levels can be administered to prevent adverse effects.

Radicalization of RNase A: Tandem radical damage is a degenerative process where the radicalization of one molecule leads to a number of adverse biological effects. Methionine (Met) residues are transformed to alpha-aminobutyric acid (Aba) with a methanethiyl radical byproduct when bombarded with H*. When cyctine residues are radicalized, they are transformed to Alanine releasing sulfur radicals. This byproduct radical interact with the cis double bond in phospholipid fatty acids causing the conformational shift to the trans isomer. This switch from cis to trans causes many serious unfavorable biological diseases. Further studies into radical stresses on RNase A will help to better understand cellular degredation associated with aging and such degenerative pathologies.

Further Research

Replacement of Phe46: The phenylalanine-46 (Phe46) residue located within the hydrophobic core of RNase A was experimentally replaced with other hydrophoblic residues; leucine, valine and alanine. The x-ray crystallographic structures were determined in an attempt to conclude how the change would affect the conformational stability. It was concluded that the replacement of Phe46, which is key to the formation of the hydrophoblic core, causes the destabilization of the RNase A by preventing the core from being tightly packed. But this has no effect on the susbstrates ability to bind. The core itself is very limited to adjusting to changes because of its limited mobility from the study di-sulfide bonds