Canonical RNase A family includes all vertebrate homologues to the bovine pancreatic ribonuclease A (RNase A) that contain the catalytic site, necessary for RNase activity. In the human genome 8 RNases , refered to as "canonical" RNases, have been identified, pancreatic RNase (RNase 1), Eosinophil Derived Neurotoxin (SEDN/RNASE 2), Eosinophil Cationic Protein (ECP/RNase 3), RNase 4, Angiogenin (RNase 5), RNase 6 or k6, the skin derived RNase (RNase 7) and RNase 8. The eight human genes are all located in a cluster on chromosome 14. Canonical RNase A enzymes have special biological activities; for example, some stimulate the development of vascular endothelial cells, dendritic cells, and neurons, are cytotoxic/anti-tumoral and/or anti-pathogenic. RNase A is involved in endonucleolytic cleavage of 3'-phosphomononucleotides and 3'-phosphooligonucleotides ending in C-P or U-P with 2',3'-cyclic phosphate intermediates. The catalytic mechanism is a transphosphorylation of P-O 5' bonds on the 3' side of pyrimidines and subsequent hydrolysis to generate 3' phosphate groups. The canonical RNase A family proteins have a conserved catalytic triad (two histidines and one lysine). They also share 6 to 8 cysteines that form three to four disulfide bonds. Two disulfide bonds that are close to the N and C termini contribute most significantly to conformational stability. Angiogenin or RNAse 5 has been implicated in tumor-associated angiogenesis. Comparative analysis in mammals and birds indicates that the whole family may have originated from a RNase 5-like gene. This hypothesis is supported by the fact that only RNase 5-like RNases have been reported outside the mammalian class. The RNase 5 group would therefore be the most ancient form of this family, and all other members would have arisen during mammalian evolution.
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