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Ribonuclease

This enzyme catalyzes the cleavage of RNA via a cyclic phosphate intermediate. DNA lacks the 2$^\prime$-hydroxyl group essential for formation of this cyclic form. In this enzyme two histidines are located near the bond to be broken. In the cyclization step one acts as a general-base catalyst and the other as a general acid catalyst (His-12 and His-119 in bovine pancreatic ribonuclease). These roles are reversed in the step involving hydrolysis of the cyclic phosphate. The molecule has a well-defined binding cleft. The mechanism seems to involve nucleophilic displacement on phosphorus with a pentacovalent intermediate, with the attacking nucleophile entering opposite the leaving group (`in-line').

Reference

Richards, F. M. and Wyckoff, H. W., in P. B. Boyer (ed.), The Enzymes , Vol. IV, 3rd edition, New York, London and San Francisco, Academic Press (1971).

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