BIOSYNTHESIS OF PTERIDINES - NMR-STUDIES ON THE REACTION-MECHANISMS OF GTP CYCLOHYDROLASE-I, PYRUVOYLTETRAHYDROPTERIN SYNTHASE, AND SEPIAPTERIN REDUCTASE
A. Bracher et al., BIOSYNTHESIS OF PTERIDINES - NMR-STUDIES ON THE REACTION-MECHANISMS OF GTP CYCLOHYDROLASE-I, PYRUVOYLTETRAHYDROPTERIN SYNTHASE, AND SEPIAPTERIN REDUCTASE, The Journal of biological chemistry, 273(43), 1998, pp. 28132-28141
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.