We performed a set of experiments with solutions made of 0.13 mmol imidazole phosphate and 1.30 mmol of glycerol and with different equivalents of histidine catalyst (1.00, 0.75, 0.50 and 0.25 eq.). In all reactions the phosphorylation of glycerol was accelerated (SI Figure ). Importantly, at lower equivalents of histidine the yield of phosphorylated glycerol was greater than would be expected if a histidine only participated in one catalytic cycle. For example, in SI Figure after 48.8 h the yield of glycerol phosphate is 29.0 % for 1.00 eq. of histidine and 26.5 % for 0.5 eq. of histidine (the uncatalysed reaction has a yield of 12.2 %). If the histidine only participated in a single catalytic cycle then it would be expected that the yield for 0.50 eq. would be half that of 1.00 eq.. This confirms that the histidyls turnover and participate in multiple catalytic cycles.