The enzyme adenosine kinase (AK) exhibits a nearly complete dependency on the presence of pentavalent ions (PVI) such as phosphate, arsenate, and vanadate. To understand its basis, the effect of a large number of phosphorylated compounds on AK activity was examined. Several compounds, such as phosphoribosyl pyrophosphate, phosphoenol pyruvate, creatine phosphate, phosphorous acid, phosphonoformic acid, and inorganic pyrophosphate, were found to substitute for PVI in stimulating AK activity. Similar to PVI, these compounds lowered the Km of AK for adenosine. In contrast, many other structurally related compounds (i.e., phosphonoacetic acid, 2-carboxyethyl phosphonic acid, N-phosphonomethyl glycine, N-phosphonomethyl iminodiacetic acid) inhibited AK activity. These compounds seemed to compete with the activators for binding to AK. Structural comparisons of different compounds indicate that all activating compounds contain a net positive charge on the pentavalent atom (e.g., phosphorous), which should enable it to act as an acceptor for a nucleophilic group. We suggest that a phosphate (or other activator) bound near the active site participates in AK catalysis by forming a transient pentavalent intermediate with a nonbridging oxygen of the β-phosphate in ATP. This interaction likely facilitates the transfer of γ-phosphate from ATP to adenosine, thus accounting for the stimulating role of PVI in AK catalysis. The insight provided by these studies concerning the structural features of activators and inhibitors should also prove helpful in the design of more potent inhibitors of AK.