The predominant cellular currency for metabolic phosphoryl group transfers is the nucleoside triphosphate adenosine triphosphate (ATP). Its tetra-anionic triphosphate side chain with two phosphoric anhydride linkages (P–O–P, P–O–P) is the preferred phosphoryl group (PO₃²⁻) donor to an array of nucleophilic cosubstrates. Mostly those involve nucleophilic oxygen atoms, including alcohols, acids, enolate ions, phosphate anions, and water, generating phosphate monoesters, acyl phosphates, enol phosphates, phosphodiesters, and net ATP hydrolysis respectively. More than 100 low molecular weight metabolic products generated by enzymatic phosphotransferase (aka kinases) by terminal –PO₃²⁻ group transfer include glucose-6-phosphate, acetyl phosphate, and phosphoenolpyruvate. Correspondingly, there are ∼520 protein kinases in human proteomes.