ribonuclease

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A group of enzymes, widely distributed in nature, which catalyze hydrolysis of the internucleotide phosphodiester bonds in ribonucleic acid (RNA). The sites of hydrolysis may vary considerably, depending upon the specificity of the particular enzyme. Differences in specificity for the site of cleavage have led to the use of these various ribonucleases as tools in determining the structure and chemistry of RNA. See also Enzyme; Nucleic acid.

Research on ribonuclease has played a prime role in advancing the understanding of protein structure and function; also, it was the first protein to be totally synthesized from its component amino acids. Since the elucidation of the amino acid sequence of ribonuclease, much information has been compiled with regard to the three-dimensional structure of the enzyme and to specific regions of the molecule which are catalytically important. See also Protein.

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IN BRIEF: n. - A transferase that catalyzes the hydrolysis of ribonucleic acid.

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abbr.: RNase or (sometimes) RNAase; any of a group of nuclease enzymes that cleave phosphodiester bonds in RNA. With the exception of RNase P, which is a ribozyme, they are all proteins. RNAses are classified into exoribonucleases, catalysing sequential cleavage of mononucleotides from either a free 5′ or 3′ terminus, or both, or endonucleases, cleaving bonds within the polyribonucleotide chain. Endonucleases, which can also cleave circular molecules of RNA, fall into further categories according to their specificity: EC 3.1.13, exoribonucleases producing 5′-phosphomonoesters; EC 3.1.14, exoribonucleases producing other than 5′-phosphomonoesters; EC 3.1.15, exonucleases active with either ribo- or deoxyribonucleic acids and producing 5′-phosphomonoesters; EC 3.1.16, exonucleases active with either ribo- or deoxyribonucleic acids and producing other than 5′-phosphomonoesters; EC 3.1.26, endoribonucleases producing 5′-phosphomonoesters; EC 3.1.27, endoribonucleases producing other than 5′-phosphomonoesters; EC 3.1.30, endonucleases active with either ribo- or deoxyribonucleic acids and producing 5′-phosphomonoesters; and EC 3.1.31, endonucleases active with either ribo- or deoxyribonucleic acids and producing other than 5′-phosphomonoesters. Ribonucleases include enzymes having mechanisms of (A) phosphotransferases, (B) phosphodiesterases, or (C) phosphorylases. In group C are polynucleotide phosphorylase and polynucleotide pyrophosphorylase. The feature of group A is the involvement of the 2′-OH group in an intramolecular attack at the adjacent phosphodiester bond, in addition to conferring specificity for one or more of the four bases. The nucleoside 2′:3′-cyclic phosphates are obligate intermediate products. In group B, a direct attack of water on the 3′:5′-phosphodiester bond is catalysed, so that this group includes the enzymes that hydrolyse both RNA and DNA and those that cleave diester bridges to form only 5′-nucleotide products. Thus group A may be classed as cyclizing and group B as non-cyclizing enzymes. The group A enzymes do not depend for their activity on divalent cations whereas the group B enzymes require Mg²⁺ or Ca²⁺. Within each group there are many types, as described below.

Exoribonuclease II EC 3.1.13.1; other name: ribonuclease II; reaction: exonucleolytic cleavage in the 3′- to 5′-direction to yield 5′-phosphomononucleotides. Exoribonuclease H EC 3.1.13.2; reaction: exonucleolytic cleavage to 5′-phosphomonoester oligonucleotides in both 5′- to 3′- and 3′- to 5′-directions. Poly(A)-specific ribonuclease EC 3.1.13.4; reaction: exonucleolytic cleavage of poly(A) to 5′-AMP. Yeast ribonuclease EC 3.1.14.1; reaction: exonucleolytic cleavage to 3′-phosphomononucleotides.

Endoribonucleases cleave the polynucleotide internally. This first group (sub-subclass EC 3.1.26) produce exclusively 5′-phosphomonoesters. Physarum polycephalum ribonuclease EC 3.1.26.1; reaction: endonucleolytic cleavage to 5′-phosphomonoester. Ribonuclease alpha EC 3.1.26.2; reaction: endonucleolytic cleavage to 5′-phosphomonoester. ribonuclease III EC 3.1.26.3; other names RNase O; RNase D; reaction: endonucleolytic cleavage to 5′-phosphomonoester. Ribonuclease H EC 3.1.26.4; other names endoribonuclease H (calf thymus); calf thymus ribonuclease H; reaction: endonucleolytic cleavage to 5′-phosphomonoester; it acts on RNA— DNA hybrids (hence the H in the name) and is required for removing RNA from Okazaki fragments; see also RNA-directed DNA polymerase. Ribonuclease P EC 3 1.26.5; reaction: endonucleolytic cleavage of RNA, removing 5′-sequence of extra nucleotides from tRNA precursor; it is a ribozyme; see RNase P. Ribonuclease IV EC 3.1.26.6; other names endoribonuclease IV; poly(A)-specific ribonuclease; reaction: endonucleolytic cleavage of poly(A) to fragments terminated by 3′-hydroxyl and 5′-phosphate groups. Ribonuclease P4 EC 3.1.26.7; reaction: endonucleolytic cleavage of RNA, removing 3′-sequence of extra nucleotides from tRNA precursor. Ribonuclease M5 EC 3.1.26.8; other name: 5S rRNA maturation nuclease; reaction: endonucleolytic cleavage of RNA, removing 21 and 42 nucleotides, respectively, from the 5′- and 3′-termini of a 5S rRNA precursor. Ribonuclease M16 cleaves the 5′ and 3′ termini of a 16S rRNA precursor. Ribonuclease M23 cleaves the 5′ and 3′ termini of a 23S rRNA precursor. Ribonuclease (poly(U)-specific) EC 3.1.26.9; reaction: endonucleolytic cleavage of poly(U) to fragments terminated by 3′-hydroxyl and 5′-phosphate groups. Ribonuclease IX EC 3.1.26.10; reaction: endonucleolytic cleavage of poly(U) or poly(C) to fragments terminated by 3′-hydroxyl and 5′-phosphate groups (see Rrp).

The second group of endoribonucleases (sub-subclass EC 3.1.27) yield products other than 5′-phosphomonoesters. Ribonuclease T2 EC 3.1.27.1; other name: ribonuclease II; reaction: two-stage endonucleolytic cleavage to 3′-phosphomononucleotides and 3′-phosphooligonucleotides with 2′,3′-cyclic phosphate intermediates. Bacillus subtilis ribonuclease EC 3.1.27.2; reaction: endonucleolytic cleavage to 2′,3′-cyclic nucleotides. Ribonuclease T1 EC 3.1.27.3; other names guanyloribonuclease; Aspergillus oryzae ribonuclease; ribonuclease N1; ribonuclease N2; reaction: two-stage endonucleolytic cleavage to 3′-phosphomononucleotides and 3′-phosphooligonucleotides ending in G-p with 2′,3′-cyclic phosphate intermediates. Ribonuclease U2 EC 3.1.27.4; reaction: two-stage endonucleolytic cleavage to 3′-phosphomononucleotides and 3′-phosphooligonucleotides ending in Ap or Gp with 2′,3′-cyclic phosphate intermediates. Pancreatic ribonuclease EC 3.1.27.5; other names: ribonuclease; ribonuclease I; ribonuclease A; a notably thermostable enzyme that catalyses the reaction: endonucleolytic cleavage to 3′-phosphomononucleotides and 3′-phosphooligonucleotides ending in Cp or Up with 2′,3′-cyclic phosphate intermediates. A glycosylated form, ribonuclease B, has the same specificity. Enterobacter ribonuclease EC 3.1.27.6; reaction: endonucleolytic cleavage to 3′-phosphomononucleotides and 3′-phosphooligonucleotides with 2′,3′-cyclic phosphate intermediates. Ribonuclease F EC 3.1.27.7; reaction: endonucleolytic cleavage of RNA precursor into two, leaving 5′-hydroxyl and 3′-phosphate groups. Ribonuclease V EC 3.1.27.8; other name: endoribonuclease V; reaction: hydrolysis of poly(A), forming oligoribonucleotides and ultimately 3′-AMP. Compare deoxyribonuclease.

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An enzyme that catalyzes the breakdown of ribonucleic acid.

(rī′bō-nōō′klē-ās)
n

An enzyme that acts as a catalyst for ribonucleic acid hydrolysis. It may also be called RNase.

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