Regulation of the reduced nicotinamide adenine dinucleotide phosphate-ferredoxin reductase system in Clostridium kluyveri.

The mechanism of regulation of NADPH-ferredoxin reductase was studied in cell-free lysates of Clostridium kluyveri. The following activities, which are assumed to be linked to the enzyme, were investigated: ferredoxin reduction by NADPH, NADPf reduction by reduced ferredoxin, transhydrogenation from NADPH to NAD+, and methyl viologen reduction by NADPH. Ferredoxin reduction by NADPH is controlled by the oxidation-reduction state of the NAD+-NADH couple. NAD+ is an obligatory activator (Michaelis activation constant, & = 0.9 X 10e4M), which increases V,,,, while the K, of the substrate NADPH (Km = 2.25 X 1O-5 x) remains unaffected. &NAD+ analogues can substitute to varying degrees for ,& NAD+, while a(-NAD+ and NMN or AMP analogues are totally inactive. NADH is an inhibitor, competitive to NADf rather than to NADPH. NADP’ reduction by reduced ferredoxin is subject to product inhibition; NADPH is competitive to NADP+ (K, = 1.52 X 10-4M). The transhydrogenation from NADPH to NAD+ is stimulated by oxidized and inhibited by reduced ferredoxin; ferredoxin is not involved in the electron flow. Methyl viologen reduction by NADPH is not controlled by either NAD+ or NADH.