Glucose-6-phosphate dehydrogenase from the cyanobacterium, Anabaena sp. PCC 7120: purification and kinetics of redox modulation.

Glucose-6-phosphate dehydrogenase is a particularly important enzyme in carbon catabolism in the chloroplasts of higher plants and in cyanobacteria. It catalyzes the first reaction in the oxidative pentose phosphate pathway which supplies reduced NADP for a variety of biosynthetic processes. The enzyme is known to be regulated by light. However, the dehydrogenase from plants has been difficult to purify and there is little information on kinetics and mechanism of deactivation. The glucose-6-phosphate dehydrogenase from the heterocystous cyanobacterium, Anabaena sp. PCC 7120, was purified to near homogeneity by chromatography on 2',5'-ADP Sepharose chromatography. The cyanobacterial enzyme apparently has different aggregation states or conformations depending on its concentration in solution and the pH. At a pH of 8.0 and low ionic strength, the enzyme has relatively low activity and exhibits sigmoidal kinetics on binding substrate and cofactor. Activity increases and the enzyme exhibits the more classical hyperbolic kinetics at pH 7.0. At the lower pH, glucose-6-phosphate dehydrogenase is inhibited by catalytic amounts of reduced thioredoxin-1 from Anabaena sp. The second thioredoxin from the cyanobacterium is much less effective, although its inhibitory effect is still greater than that of small molecule reducing agents such as glutathione. Glutamine was reported to stabilize the isolated enzyme, but actually is an activator at pH 8.0. The results suggest that cellular demand for reduced cofactor under nitrogen-fixing conditions overrides the pH-induced deactivation.