[Relationship of transhydrogenase and diaphorase activity of ferredoxin-NADP reductase with photosynthetic NADP reduction].

Abstract

Photosynthetic NADP reduction, diaphorase and transhydrogenase activity of ferredoxin-NADP reductase (EC 1.6.99.4 or 1.6.1.1, from the alga Bumilleriopsis filiformis) are compared by studying the influence of pyridine nucleotides and 2'-AMP. Together with previous findings dealing with the role of ferredoxin in the two latter activities 7 the results can be compiled as follows: 1. Ferredoxin-NADP reductase has two binding sites: one specific site for ferredoxin and one for pyridine nucleotides whether they are reduced or oxidized. 2. There is no substantial competition between ferredoxin and pyridine nucleotides for their respective binding sites. 3. Diaphorase substrates like dichlorophenolindophenol or methylviologen do not bind at the pyridine nucleotide site. It is suggested that they bind at the ferredoxin site. In vivo, therefore, the diaphorase site of the reductase is occupied by ferredoxin and represents the electron accepting part of the reductase. 4. During transhydrogenase reaction ferredoxin-NADP reductase is reduced by NADPH via the pyridine nucleotide binding site; during NADP reductase reaction the enzyme is reduced by ferredoxin at the ferredoxin site. In both reactions, however, NADP (and other nucleotides) are reduced at the same pyridine nucleotide binding site. Transhydrogenase activity, therefore, appears to be an artefact reaction, which can be found when the isolated reductase is no more reduced by its natural substrate ferredoxin.