Open AccessThe Respiratory‐Chain NADPH Dehydrogenase of Azotobacter vinelandii
Author(s) -
Ackrell Brian A. C.,
Erickson Sandra K.,
Jones Colin W.
Publication year - 1972
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1972.tb01778.x
Subject(s) - azotobacter vinelandii , nad+ kinase , biochemistry , dehydrogenase , allosteric regulation , nadph dehydrogenase , chemistry , enzyme , nucleotide , nitrogenase , biology , nitrogen fixation , nitrogen , gene , nitric oxide synthase , organic chemistry
1 NADPH dehydrogenase activity of Azotobacter vinelandii respiratory membranes is inhibited by adenine nucleotides (pure competitive inhibition; AMP>ADP>ATP) and by NAD + (partially competitive, allosteric inhibition). No allosteric activators, other than NADPH, were found for this enzyme. 2 The dehydrogenase exhibits kinetic properties compatible with the presence of a catalytic site, which binds one molecule of NADPH (or one molecule of adenine nucleotide), and an apparently acid‐sensitive, site (or sites) which binds one molecule of either NADPH (positive homotropic effector) or NAD + (negative heterotropic effector). 3 These results suggest that in the presence of NAD + or low energy charge, NADPH may be converted to NADH via the NADPHNAD + transhydrogenase, and thence oxidised via the more efficiently phosphorylating NADH dehydrogenase. 4 The prime function of the NADPH dehydrogenase is probably to oxidise the excess NADPH which might be expected to accumulate when nitrogen‐fixation is switched off ( e.g. under highly aerobic conditions) and thus to contribute to the phenomenon of nitrogenase protection by enhanced respiration and also to the maintenance of the desired intracellular NADPH/NADP + ratio.