BIOSYNTHESIS OF PTERIDINES - NMR-STUDIES ON THE REACTION-MECHANISMS OF GTP CYCLOHYDROLASE-I, PYRUVOYLTETRAHYDROPTERIN SYNTHASE, AND SEPIAPTERIN REDUCTASE

GTP cyclohydrolase I catalyzes a ring expansion affording dihydroneopt erin triphosphate from GTP. [1',2',3',4',5'-C-13(5),2'-H-2(1)]GTP was prepared enzymatically from [U-C-13(6)]glucose for use as enzyme subst rate. Multinuclear NMR experiments showed that the reaction catalyzed by GTP cyclohydrolase I involves the release of a proton from C-2' of GTP that is exchanged with the bulk solvent. Subsequently, a proton is reintroduced stereospecifically from the bulk solvent. This is in lin e with an Amadori rearrangement mechanism. The proton introduced from solvent occupies the pro-7R position in the enzyme product. The data a lso confirm that the reaction catalyzed by pyruvoyltetrahydropterin sy nthase results in the incorporation of solvent protons into positions C-6 and C-3' of the enzyme product. On the other hand, the reaction ca talyzed by sepiapterin reductase does not involve any detectable incor poration of solvent protons into tetrahydrobiopterin.