Biosynthesis of riboflavin - Studies on the mechanism of GTP cyclohydrolase II

GTP cyclohydrolase II catalyzes the first committed reaction in the biosynt hesis of the vitamin riboflavin, The recombinant enzyme from Escherichia co li is shown to pro duce 2,5 diamino-6-beta -ribosylamino 4(3H)-pyrimidinone 5'-phosphate and GMP at an approximate molar ratio of 10:1. The main produ ct is subject to spontaneous isomerization affording the alpha -anomer. O-1 8 from solvent water is incorporated by the enzyme into the phosphate group of the 5-aminopyrimidine derivative as well as GMP. These data are consist ent with the transient formation of a covalent phosphoguanosyl derivative o f the enzyme. Subsequent ring opening of the covalently bound nucleotide fo llowed by hydrolysis of the phosphodiester bond could then afford the pyrim idine type product. The hydrolysis of the phosphodiester bond without prior ring opening could afford GMP. The enzyme reaction is cooperative with a H ill coefficient of 1.3, Inhibition by pyrophosphate is competitive. Inhibit ion by orthophosphate is partially uncompetitive at low concentration and c ompetitive at concentrations above 6 mM.