The 2'-phosphate of NADP is critical for optimum productive binding to 6-phosphogluconate dehydrogenase from Candida utilis.
Initial velocity studies obtained with alternative dinucleotide substrates for the 6-phosphogluconate dehydrogenase reaction suggest that the 2'-phosphate is critical for the optimum productive binding of the dinucleotide substrate. Initial velocity patterns obtained by varying 6-phosphogluconate at different fixed levels of NAD are nearly parallel with apparent competitive substrate inhibition by 6-phosphogluconate at pH 7 and below but intersect to the left of the ordinate at pH 8 and above. Dead-end inhibition studies indicate that the mechanism is random at all pH values. Data are interpreted in terms of a random mechanism with marked antagonism in the binding of NAD and 6-phosphogluconate at low pH. Deuterium isotope effects on V and V/K for either substrate are equal at pH 8, indicating that the kinetic mechanism is rapid equilibrium random. A decrease in the pH and the subsequent protonation of the active site general base or some other enzyme residue with a similar pK apparently results in the ineffective binding of NAD. The latter suggests either a link between the protonation state of this group and the conformation of the dinucleotide binding site or an interaction between the two.
Department of Biochemistry and Molecular Biology, Texas College of Osteopathic Medicine, Fort Worth 76107.
A J
AJ
Berdis
P F
PF
Cook
D002175:Candida / Q000201:enzymology* / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009249:NADP / Q000737:chemistry* / Q000378:metabolism; D010734:Phosphogluconate Dehydrogenase / Q000378:metabolism*; D013329:Structure-Activity Relationship
D005656:Fungal Proteins; D009249:NADP; D010734:Phosphogluconate Dehydrogenase