tailieunhanh - Báo cáo Y học: Biosynthesis of vitamin B2 An essential zinc ion at the catalytic site of GTP cyclohydrolase II

GTP cyclohydrolase II catalyzes the hydrolytic release of formate and pyrophosphate from GTP producing 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5¢-phos-phate, the first committed intermediate in the biosynthesis of riboflavin. The enzyme was shown to contain one zinc ion per subunit. Replacement of cysteine residue 54, 65 or 67 with serine resulted in proteins devoid of bound zinc and unable to release formate fromthe imidazole ring ofGTPor from the intermediate analog, 2-amino-5-formylamino-6-ribosylamino-4(3H)-pyrimidinone 5¢-triphosphate. . | Eur. J. Biochem. 269 5264-5270 2002 FEBS 2002 doi Biosynthesis of vitamin B2 An essential zinc ion at the catalytic site of GTP cyclohydrolase II Johannes Kaiser1 Nicholas Schramek1 Sabine Eberhardt1 Stefanie Puttmer2. Michael Schuster2 and Adelbert Bacher1 1Lehrstuhl fur Organische Chemie und Biochemie Technische Universitat MUnchen Garching Germany 2Lehrstuhl fur Anorganische und Analytische Chemie Technische Universitdt Munchen Garching Germany GTP cyclohydrolase II catalyzes the hydrolytic release of formate and pyrophosphate from GTP producing 2 5-diamino-6-ribosylamino-4 3H -pyrimidinone 5 -phos-phate the first committed intermediate in the biosynthesis of riboflavin. The enzyme was shown to contain one zinc ion per subunit. Replacement of cysteine residue 54 65 or 67 with serine resulted in proteins devoid of bound zinc and unable to release formate from the imidazole ring of GTP or from the intermediate analog 2-amino-5-formylamino-6-ribosylamino-4 3H -pyrimidinone 5 -triphosphate. However the mutant proteins retained the capacity to release pyrophosphate from GTP and from the formamide-type intermediate analog. The data suggest that the enzyme catalyzes an ordered reaction in which the hydrolytic release of pyrophosphate precedes the hydrolytic attack of the imidazole ring. Ring opening and formate release are both dependent on a zinc ion acting as a Lewis acid which activates the two water molecules involved in the sequential hydrolysis of two carbon-nitrogen bonds. Keywords formate GTP cyclohydrolase imidazole ring pyrophosphate zinc ion. GTP cyclohydrolases catalyze the first steps in the biosynthetic pathways of riboflavin tetrahydrofolate and tetrahydrobiopterin. More specifically GTP cyclohydrolase I catalyzes the release of C8 of GTP Compound 1 Fig. 1 followed by the formation of a novel pyrazine ring with inclusion of carbon atoms 1 and 2 of the ribose side chain 1 2 . The reaction product dihydroneopterin .