An Examination of the Role of Asp-177 in the His-Asp Catalytic Dyad of Leuconostoc Mesenteroides Glucose 6-Phosphate Dehydrogenase: X-Ray Structure and Ph Dependence of Kinetic Parameters of the D177N Mutant Enzyme
Primary Citation of Related Structures: 1E77, 1E7M, 1E7Y
PubMed Abstract:
The role of Asp-177 in the His-Asp catalytic dyad of glucose 6-phosphate dehydrogenase from Leuconostoc mesenteroides has been investigated by a structural and functional characterization of the D177N mutant enzyme. Its three-dimensional structure has been determined by X-ray cryocrystallography in the presence of NAD(+) and in the presence of glucose 6-phosphate plus NADPH ...
The role of Asp-177 in the His-Asp catalytic dyad of glucose 6-phosphate dehydrogenase from Leuconostoc mesenteroides has been investigated by a structural and functional characterization of the D177N mutant enzyme. Its three-dimensional structure has been determined by X-ray cryocrystallography in the presence of NAD(+) and in the presence of glucose 6-phosphate plus NADPH. The structure of a glucose 6-phosphate complex of a mutant (Q365C) with normal enzyme activity has also been determined and substrate binding compared. To understand the effect of Asp-177 on the ionization properties of the catalytic base His-240, the pH dependence of kinetic parameters has been determined for the D177N mutant and compared to that of the wild-type enzyme. The structures give details of glucose 6-phosphate binding and show that replacement of the Asp-177 of the catalytic dyad with asparagine does not affect the overall structure of glucose 6-phosphate dehydrogenase. Additionally, the evidence suggests that the productive tautomer of His-240 in the D177N mutant enzyme is stabilized by a hydrogen bond with Asn-177; hence, the mutation does not affect tautomer stabilization. We conclude, therefore, that the absence of a negatively charged aspartate at 177 accounts for the decrease in catalytic activity at pH 7.8. Structural analysis suggests that the pH dependence of the kinetic parameters of D177N glucose 6-phosphate dehydrogenase results from an ionized water molecule replacing the missing negative charge of the mutated Asp-177 at high pH. Glucose 6-phosphate binding orders and orients His-178 in the D177N-glucose 6-phosphate-NADPH ternary complex and appears to be necessary to form this water-binding site.
Related Citations:
The Three-Dimensional Structure of Glucose 6-Phosphate Dehydrogenase from Leuconostoc Mesenteroides Refined at 2 Angstroms Resolution Rowland, P., Basak, A.K., Gover, S., Levy, H.R., Adams, M.J. (1994) Structure 2: 1073
Site-Directed Mutagenesis to Facilitate X-Ray Structural Studies of Leuconostoc Mesenteroides Glucose 6-Phosphate Dehydrogenase Adams, M.J., Basak, A.K., Gover, S., Rowland, P., Levy, H.R. (1993) Protein Sci 2: 859
Organizational Affiliation:
Department of Biology, Syracuse University, Syracuse, New York 13244, USA.
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