Stereospecificity of hydrogen transfer catalyzed by D-galactose dehydrogenase from Pseudomonas saccharophila and Pseudomonas fluorescens

D-glucose-6-phosphate dehydrogenase (Entner-Doudoroff enzyme) from Pseudomonas fluorescens. Purification, properties and regulation

1. The existence of two different D-glucose-6-phosphate dehydrogenases in Pseudomonas fluorescens has been demonstrated. Based on their different specificity and their different metabolic regulation one enzyme is appointed to the Entner-Doudoroff pathway and the other to the hexose monophosphate pathway. 2. A procedure is described for the isolation of that D-glucose-6-phosphate dehydrogenase which forms part of the Entner-Doudoroff pathway (Entner-Doudoroff enzyme). A 950-fold purification was achieved with an overall yield of 44%. The final preparation, having a specific activity of about 300 mumol NADH formed per min per mg protein, was shown to be homogeneous. 3. The molecular weight of the Entner-Doudoroff enzyme has been determined to be 220000 by gel permeation chromatography, and that of the other enzyme (Zwischenferment) has been shown to be 265000. 4. The pI of the Entner-Doudoroff enzyme has been shown to be 5.24 and that of the Zwischenferment 4.27. The Entner-Doudoroff enzyme is stable in the range of pH 6 to 10.5 and shows its maximal activity at pH 8.9. 5. The Entner-Doudoroff enzyme showed specificity for NAD+ as well as for NADP+ and exhibited homotropic effects for D-glucose 6-phosphate. It is inhibited by ATP which acts as a negative allosteric effector. Other nucleoside triphosphates as well as ADP are also inhibitory. 6. The enzyme catalyzes the transfer of the axial hydrogen at carbon-1 of beta-D-glucopyranose 6-phosphate to the si face of carbon-4 of the nicotinamide ring and must be classified as B-side stereospecific dehydrogenase.