Most enzymes involved in cell signaling, such as protein kinases, protein phosphatases, GTPases, and nucleotide cyclases catalyze nucleophilic substitutions at phosphorus. When possible, the mechanisms of such enzymes are most clearly described quantitatively in terms of how associative or dissociative they are. The mechanisms of cell signaling enzymes range from I8% associative (cAMPdependent protein kinase) to D50% associative (G protein Giโฃ1). Their catalytic powers range from 10 5.7 (p21 ras ) to 10 11.7 ( Ser-Thr protein phosphatase), usually comparable in magnitude with those of nonsignaling enzymes of the same mechanistic class. Exceptions are G proteins, which are 10 3 -to 10 5 -fold poorer catalysts than F1 and myosin ATPases. The lower catalytic powers of G proteins may be ascribed to the absence of general base catalysis, and additionally in the case of p21 ras , to the absence of a catalytic Arg residue, which interacts with the transition state. From kinetic studies of mutant and metal ion substituted enzymes, the catalytic powers of cell signaling and related enzymes can be rationalized quantitatively by factors contributed by metal ion catalysis (H10 5 ), general acid catalysis (D10 3ุ1 ), general base catalysis (D10 3ุ1 ), and transition-state stabilization by cationic and hydrogen bond donating residues (D10 3ุ1 ).