Stopped-flow kinetic studies of low potential electron carriers of the photosynthetic bacterium, Rhodobacter capsulatus: ferredoxin I and NifF.
The kinetics of electron-transfer reactions involving nif-specific proteins from Rhodobacter capsulatus; ferredoxin I, NifF, Fe-protein of nitrogenase and dithionite were studied using stopped-flow spectrophotometry. Kinetic evidence was obtained for the formation of a tight (0.44 microM) complex between NifF and Fe-protein. Under the same conditions, FdI interacted only weakly (Kd > 325 microM) with Fe-protein. There was no evidence for complex formation between NifF and FdI since the reaction NifFSQ + FdIred had a bimolecular rate constant of 12.5 +/- 1.2 x 10(3) M-1 s-1. These results suggest that NifF, which is present in only small quantities in the cell, can make a significant contribution to the overall rate of nitrogen fixation due its high reactivity with Fe-protein. Moreover, the apparent lack of specific interaction between NifF and FdI suggest that they act in vivo in parallel to reduce Fe-protein and not in series.
Hallenbeck
P C
PC
Département de Microbiologie et Immunologie, Université de Montréal, Que., Canada. hallenbe@ere.umontreal.ca
Gennaro
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G
D004227:Dithionite / Q000737:chemistry; D004579:Electron Transport; D005288:Ferredoxins / Q000737:chemistry*; D005418:Flavodoxin / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D009591:Nitrogenase / Q000737:chemistry; D010084:Oxidation-Reduction; D016354:Rhodobacter capsulatus / Q000737:chemistry*
D005288:Ferredoxins; D005418:Flavodoxin; D004227:Dithionite; D009591:Nitrogenase