Enzyme-catalyzed phosphoryl transfer phosphodiesters

AH fungal RNases (T, T , Ni, Ui, and U2) treated in this section catalyze the reaction shown in Fig. 1. The first step (phosphate transfer) is the cleavage of the phosphodiester bond between the 3 and 5 positions of the ribose moities in the RNA chain with the formation of nucleoside 2, 3 -cyclic phosphates and oligonucleotides with 2, 3 -cyclic phosphate at 3 terminal. The nature of the phosphodiester bonds to be cleaved depends on the base specificity of the enzyme. This phosphoryl transfer step is reversible. In the second step (hydrolysis), these terminal cyclic phosphate groups are hydrolyzed with the formation of corresponding 3 -phosphates. Because the first-step is usually faster than the second step, more or less accumulation of the cyclic phosphate may be observed. 

Mechanistically related enzymes that are ubiquitous in their requirement for a metal ion are those that catalyze phosphoryl group transfer reactions. These enzymes include the (phospho)kinases and phosphatases, which catalyze the transfer of a phosphoryl group (PO3) from the y-position of a nucleotide to an acceptor molecule (the phosphatases are a special case wherein the acceptor is the solvent water) nucleotidyltransferases and nucleases, which catalyze the transfer of a nucleoside phosphodiester to an acceptor molecule (the nucleases and phosphodiesterases are special cases wherein the acceptor is the solvent water) and phosphomutases. In all of these cases, the mechanism of phosphoryl group transfer can occur through one of several postulated pathways. The possible mechanistic extremes are those described as dissociative, analogous to an Sn 1 reaction. 

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