Phosphoryl transfer mechanistic possibilities

Mechanistic possibilities for phosphoryl transfer 51 Nomenclature issues 53... 

Figure 6.52 Mechanistic possibilities for transfer of a monophosphate, (a) 5 1 (P) involving a solvent-equilibrated metaphosphate intermediate. Protonation states are not shown, but different fonts for the phosphorus oxygens, representing different oxygen isotopes, indicate stereochemistry. (b) Canonical structures for metaphosphate and alkyl phosphate dianion, (c) 5n2(P) involving an intermediate which can pseudorotate, (d) Exploded 5Nl(P)-like >Sn2(P) transition states, and cartoon of calculated movements of phosphorus and non-bridge oxygens during phosphoryl transfer through such a transition state, where the leaving to nucleophile atom distance is constant.

Fig. 3. Two mechanistic possibilities for phosphoryl transfer of the monoanionic species of a phosphate monoester (a) stepwise transfer of the proton to the bridge oxygen atom in a preequilibrium step (b) proton transfer concerted with P-0 bond cleavage.

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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