Figure 1. The hydrolysis of a phosphate ester, consisting of the transfer of a phosphoryl group from the ester to water. In biological systems the R group may be the nucleophilic side-chain of an amino acid, or a small molecule such as glucose.

Figure 2. Some of the components of the regulation processes governing glycogen metabolism. For a fuller picture of the regulation of this process, see Cohen (1999).

Figure 3. The operation of the two-component signal transduction system, in which a sensor His kinase phosphorylates a His residue in its dimer partner. Subsequently, the phosphoryl group is transferred to the Asp residue of a second protein, the response regulator.

Figure 4. Mechanistic possibilities for the chemical step of phosphoryl transfer from a phosphate ester to a nucleophile (Nu), which is water in this example. In a dissociative mechanism, the ester group (the leaving group) departs first and the nucleophile does not participate until the second step. In an associative mechanism, the phosphoryl transfer is an addition–elimination process, in which the nucleophile adds in the first step and the leaving group departs in a subsequent step. In a concerted mechanism, the nucleophile adds and the leaving group departs in the same step.

Figure 5. The overall reaction catalysed by kinases, showing the , , nomenclature for the phosphoryl groups of nucleoside triphosphates. For protein kinases, the nucleophile (Nu) is the side-chain of threonine, serine, tyrosine or histidine.

Figure 6. A representation of the residues at the active site of cAMP-dependent protein kinase that mechanistic studies indicate are involved in interactions with the substrate ATP and/or in the catalytic reaction.