ABSTRACT
The structures of crystalline D-xylose isomerase (D-xylose ketol-isomerase; EC 5.3.1.5) from Streptomyces rubiginosus and of its complexes with substrate and with an active-site-directed inhibitor have been determined by x-ray diffraction techniques and refined to 1.9-A resolution. This study identifies the active site, as well as two metal-binding sites. The metal ions are important in maintaining the structure of the active-site region and one of them binds C3-O and C5-O of the substrate forming a six-membered ring. This study has revealed a very close contact between histidine and C1 of a substrate, suggesting that this is the active-site base that abstracts a proton from substrate. The mechanism-based inhibitor is a substrate analog and is turned over by the enzyme to give a product that alkylates this same histidine, reinforcing our interpretation. The changes in structure of the native enzyme, the enzyme with bound substrate, and the alkylated enzyme indicate that the mechanism involves an "open-chain" conformation of substrate and that the intermediate in the isomerization reaction is probably a cis-ene diol because the active-site histidine is correctly placed to abstract a proton from C1 or C2 of the substrate. A water molecule binds to C1O and C2O of the substrate and so may act as a proton donor or acceptor in the enolization of a ring-opened substrate.
Subject(s)
Aldose-Ketose Isomerases , Carbohydrate Epimerases/metabolism , Deoxy Sugars/analogs & derivatives , Deoxyglucose/analogs & derivatives , Binding Sites , Carbohydrate Epimerases/antagonists & inhibitors , Deoxyglucose/metabolism , Models, Molecular , Protein Binding , Protein Conformation , Streptomyces/enzymology , X-Ray Diffraction , Xylose/metabolismABSTRACT
Treatment of methyl 4-O-mesy-alpha-D-galactopyranoside with benzyl bromide in N,N-dimethylformamide in the presence of silver oxide yielded methyl 2,3,6-tri-O-benzyl-4-O-mesyl-alpha-D-galactopyranoside which with tert-butylammonium fluoride at reflux underwent nucleophilic displacement to give methyl 2,3,6-tri-O-benzyl-4-deoxy-4-fluoro-alpha-D-glucopyranoside. This compound on hydrogenolysis provided crystalline methyl 4-deoxy-4-fluoro-alpha-D-glucopyranoside (9). The structure of 9 was established by its conversion to the 2,3,6-tri-O-acetyl derivative and by n.m.r. and m.s. analysis. Acid hydrolysis of 9 gave 4-deoxy-4-fluoro-D-glucose (1). A modification of an established synthesis of 4-deoxy-D-xylo-hexose (2) from methyl 2,3,6-tri-O-benzoyl-alpha-D-galactopyranoside is described. A systematic comparison was made of the transport parameters (Kx and Vmax) of D-glucose, 2, and 1 in human erythrocytes. The Kx values observed for the above sugars are: 4.0mM, 4.5mM, and 4.6mM, respectively. These results indicate that O-4 in beta-D-glucopyranose is not involved in hydrogen bonding to the carrier protein associated with the transport of D-glucose in the erythrocyte.
Subject(s)
Deoxy Sugars/analogs & derivatives , Deoxyglucose/analogs & derivatives , Erythrocytes/metabolism , Biological Transport , Deoxyglucose/blood , Deoxyglucose/chemical synthesis , Gas Chromatography-Mass Spectrometry , Humans , Kinetics , Magnetic Resonance SpectroscopyABSTRACT
Both 2-deoxy-2-fluoro-D-glucose and 2-deoxy-2-fluoro-D-mannose were found to be potent inhibitors of the synthesis of infectious Semliki forest and fowl plague virus in chicken embryo cells and also of pseudorabies virus grown in rabbit kidney cells. It was found that the pseudorabies virus-mediated cell fusion and the synthesis of functional hemagglutinin of fowl plague virus were blocked. In all cases the 2-deoxy-2-fluoro-D-mannose-caused inhibition was stronger than the 2-deoxy-2-fluoro-D-glucose- or 2-deoxy-D-glucose-mediated blocks. Studies on the virus-specified proteins from Semiliki forest virus-infected cells grown in the presence of the inhibitors show that the target of the fluorosugar action, parallel to the well-studied effects of 2-deoxy-D-glucose, is the glycoprotein biosynthesis.
