The reaction cycle combines the phosphorylation of an amino acid (activation) and its subsequent dephosphorylation (deactivation). Phosphorylation requires a phosphorylating agent that we refer to as fuel. Deactivation through hydrolysis can be spontaneous and sped up by catalysis. Thus, a dynamically phosphorylated amino acid appears in response to fuel, which can only be sustained by the cycle's continuous fuel consumption. We chose monoamidophosphate (MAP) as a phosphorylation agent because it is known to phosphorylate imidazole, 28 yet, it is relatively inert towards hydrolysis. Indeed, we found MAP's half-life of hydrolysis to be 16.5 ± 0.2 hours by 31 P nuclear magnetic resonance ( 31 P-NMR) in 500 mM 2-(N-morpholino)ethanesulfonic acid (MES) buffer at pH 6.5 and room temperature (Fig. , Fig. , and Table ). In contrast, in the presence of 75 mM of histidine, MAP's half-life for observed hydrolysis decreased to 3.0 ± 1 hours (Table ). Thus, histidine accelerated the conversion of 80 mM MAP, likely through its phosphorylation. We tested six further amino acids with phosphorylatable side groups-arginine, lysine, cysteine, serine, tyrosine, and aspartic acid (Fig. ). For all amino acids, MAP's half-life for observed hydrolysis remained similar to the half-life in the absence of amino acids (Table ). Thus, none of the other amino acids observably catalyzed the fuel conversion. Indeed, no phosphorylated products were made in observable amounts by 31 P-NMR (Fig. ). We focused on histidine as a catalyst and tested two more prebioticically relevant fuels, i.e., diamidophosphate (DAP) and trimetaphosphate (TMP, Fig. and Fig. ). With 75 mM histidine, no significant amount of TMP was converted over 80 hours. In contrast, DAP was converted with an observable half-life of 12 ± 0.3 hours, which is significantly faster than without histidine at an observable half-life of 190 ± 50 hours.