Phosphotransfer and Nucleotidyltransfer
Phosphotransferases, phosphatases, and nucleotidyltransferases catalyze nucleophilic substitution at phosphorus. They constitute a dominant class of enzymes in intermediary metabolism, energy transduction, nucleic acid biosynthesis and processing, and regulation of many cellular processes, including replication, cellular development, and apoptosis. The mechanisms of the action of these enzymes have been studied intensively at several levels, ranging from the biosynthesis of metabolites and nucleic acids to unmasking signaling networks to elucidating the molecular mechanisms of catalysis. We focus on the chemical mechanisms of the reactions of biological phosphates. More than 40 years of research on this chemistry reveals that the mechanisms can be grouped into two classes: the phosphoryl group (PO3−) transfer mechanisms and the nucleotidyl or alkylphosphoryl group (ROPO2−) transfer mechanisms. Because the fundamental chemical mechanisms of these reactions are not treated in textbooks, we begin by considering this chemistry and then move on to the enzymatic reaction mechanisms. Phosphomonoesters, phosphoanhydrides, and phosphoramidates undergo substitution at phosphorus by transfer of the phosphoryl (PO3–) group, that is, by P—O and P—N cleavage. The current description of a typical phosphoryl group transfer mechanism is one in which the phosphoryl donor and acceptor interact weakly with the phosphoryl group in flight in a transition state in which the total bonding to phosphorus is decreased relative to the ground state. The bonding is weak between phosphorus and the leaving group R–X and between phosphorus and the accepting group Y in the transition state of. Because of decreased bonding to phosphorus, this is a loose transition state that has been described as dissociative. The latter should not be confused with the dissociative mechanism, which is considered later. To avoid confusion, we use the term loose transition state. Detailed studies indicate that the bonding denoted by the dashed lines in represents partial covalency on the order of 10% to 20% of the strength of a full covalent bond, or a bond order of 0.1 to 0.2.