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.