Nicotinamide nucleotide transhydrogenase from Rhodobacter capsulatus; the H+/H- ratio and the activation state of the enzyme during reduction of acetyl pyridine adenine dinucleotide.
Chromatophores from Rhodobacter capsulatus were incubated in the dark with NADPH and acetylpyridineadenine dinucleotide (AcPdAD+) in the presence of different concentrations of myxothiazol. The transhydrogenase activity was monitored until an appropriate mass action ratio, [AcPdAD+][NADPH]/[AcPdADH][NADP+], was reached. The sample was then illuminated and the initial rate of either AcPdAD+ reduction by NADPH or AcPdADH oxidation by NADP+ was recorded. The ratio of H+ translocated per H- equivalent transferred by transhydrogenase was calculated from the value of the membrane potential (delta pH = 0) at which illumination caused no net reaction in either direction. The mean value for the H+/H- ratio was 0.55. At greater values of [AcPdAD+][NADPH]/[AcPdADH][NADP+] than were employed in the above experiments and over a wider range of concentrations of myxothiazol, it was found that incremental increases in the membrane potential always gave rise to a decrease, never an increase in the rate of AcPdAD+ reduction. In contrast to the H(+)-ATP synthase, there is no evidence of any activation/deactivation of H(+)-transhydrogenase by the protonmotive force.
Palmer
T
T
School of Biochemistry, University of Birmingham, UK.
Jackson
J B
JB
D008564:Membrane Potentials; D009243:NAD / Q000031:analogs & derivatives* / Q000737:chemistry; D009250:NADP Transhydrogenases / Q000737:chemistry / Q000302:isolation & purification*; D010084:Oxidation-Reduction; D011487:Protein Conformation; D016354:Rhodobacter capsulatus / Q000201:enzymology*
D009243:NAD; C005941:3-acetylpyridine adenine dinucleotide; D009250:NADP Transhydrogenases