title: Mechanism and active site residues of GDP-fucose synthase.
abstract: L-fucose, 6-deoxy-L-galactose, is a key component of many important glycoconjugates including the blood group antigens and the Lewis(X) ligands. The biosynthesis of GDP-L-fucose begins with the action of a dehydratase that converts GDP-D-mannose into GDP-4-keto-6-deoxy-mannose. The enzyme GDP-fucose synthase, GFS, (also known as GDP-4-keto-6-deoxy-D-mannose epimerase/reductase, GMER) then converts GDP-4-keto-6-deoxy-D-mannose into GDP-L-fucose. The GFS reaction involves epimerizations at both C-3'' and C-5'' followed by an NADPH-dependent reduction of the carbonyl at C-4. This manuscript describes studies that elucidate the order of the epimerization steps and the roles of the active site acid/base residues responsible for the epimerizations. An active site mutant, Cys109Ser, produces GDP-6-deoxy-D-altrose as its major product indicating that C-3'' epimerization occurs first and premature reduction of the GDP-4-keto-6-deoxy-D-altrose intermediate becomes competitive with GDP-L-fucose production. The same mutation results in the appearance of a kinetic isotope effect when [3''-(2)H]-GDP-6-deoxy-4-keto-mannose is used as a substrate. This indicates that Cys109 is the base responsible for the deprotonation of the substrate at C-3''. The Cys109Ser mutant also catalyzes a rapid wash-in of solvent derived deuterium into the C-5'' position of GDP-fucose in the presence of NADP(+). This confirms the order of epimerizations and the role of Cys109. Finally, the inactive His179Gln mutant readily catalyzes the wash-out of deuterium from the C-3'' position of [3''-(2)H]-GDP-6-deoxy-4-keto-mannose. Together these results strongly implicate an ordered sequence of epimerizations (C-3'' followed by C-5'') and suggest that Cys109 acts as a base and His179 acts as an acid in both epimerization steps.
authors.affiliation: Department of Chemistry, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 Canada.
authors.forename: Stephen T B
authors.initials: ST
authors.lastname: Lau
authors.forename: Martin E
authors.initials: ME
authors.lastname: Tanner
mesh_terms: D002238:Carbohydrate Epimerases / Q000737:chemistry*; D002384:Catalysis; D020134:Catalytic Domain; D003001:Cloning, Molecular; D003903:Deuterium / Q000737:chemistry; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry*; D006153:Guanosine Diphosphate / Q000737:chemistry; D006154:Guanosine Diphosphate Fucose / Q000737:chemistry*; D007658:Ketone Oxidoreductases / Q000737:chemistry*; D007700:Kinetics; D009097:Multienzyme Complexes / Q000737:chemistry*; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D011994:Recombinant Proteins / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization; D013379:Substrate Specificity
chemical_list: D029968:Escherichia coli Proteins; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D012997:Solvents; C110682:wcaG protein, E coli; D006153:Guanosine Diphosphate; D006154:Guanosine Diphosphate Fucose; D003903:Deuterium; D007658:Ketone Oxidoreductases; D002238:Carbohydrate Epimerases