Choline oxidase catalyzes the oxidation of choline to glycine betaine, a compatible solute that accumulates in pathogenic bacteria and plants so they can withstand osmotic and temperature stresses. The crystal structure of choline oxidase was determined and refined to a resolution of 1 ...
Choline oxidase catalyzes the oxidation of choline to glycine betaine, a compatible solute that accumulates in pathogenic bacteria and plants so they can withstand osmotic and temperature stresses. The crystal structure of choline oxidase was determined and refined to a resolution of 1.86 A with data collected at 100 K using synchrotron X-ray radiation. The structure reveals a covalent linkage between His99 Nepsilon2 and FAD C8M atoms, and a 123 A3 solvent-excluded cavity adjacent to the re face of the flavin. A hypothetical model for choline docked into the cavity suggests that several aromatic residues and Glu312 may orient the cationic substrate for efficient catalysis. The role of the negative charge on Glu312 was investigated by engineering variant enzymes in which Glu312 was replaced with alanine, glutamine, or aspartate. The Glu312Ala enzyme was inactive. The Glu312Gln enzyme exhibited a Kd value for choline at least 500 times larger than that of the wild-type enzyme. The Glu312Asp enzyme had a kcat/KO2 value similar to that of the wild-type enzyme but kcat and kcat/Km values that were 230 and 35 times lower, respectively, than in the wild-type enzyme. These data are consistent with the spatial location of the negative charge on residue 312 being important for the oxidation of the alcohol substrate. Solvent viscosity and substrate kinetic isotope effects suggest the presence of an internal equilibrium in the Glu312Asp enzyme prior to the hydride transfer reaction. Altogether, the crystallographic and mechanistic data suggest that Glu312 is important for binding and positioning of the substrate in the active site of choline oxidase.
Related Citations:
Cloning, Sequence Analysis, and Purification of Choline Oxidase from Arthrobacter Globiformis: A Bacterial Enzyme Involved in Osmotic Stress Tolerance Fan, F., Ghanem, M., Gadda, G. (2004) Arch Biochem Biophys 421: 149
Kinetic Mechanism of Choline Oxidase from Arthrobacter Globiformis Gadda, G. (2003) Biochim Biophys Acta 1646: 112
The Trimethylammonium Headgroup of Choline is a Major Determinant for Substrate Binding and Specificity in Choline Oxidase Gadda, G., Powell, N.L., Menon, P. (2004) Arch Biochem Biophys 430: 264
On the Catalytic Mechanism of Choline Oxidase Fan, F., Gadda, G. (2005) J Am Chem Soc 127: 2067
On the Catalytic Role of the Conserved Active Site Residue His466 of Choline Oxidase Ghanem, M., Gadda, G. (2005) Biochemistry 44: 893
Effects of Reversing the Protein Positive Charge in the Proximity of the Flavin N(1) Locus of Choline Oxidase Ghanem, M., Gadda, G. (2006) Biochemistry 45: 1979
Organizational Affiliation:
Department of Chemistry, Georgia State University, Atlanta, Georgia 30302-4098, USA.
Download Mendeley
