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 pro teins from Rhodobacter capsulatus; ferredoxin I, NifF, Fe-protein of n itrogenase and dithionite were studied using stopped-flow spectrophoto metry. Kinetic evidence was obtained for the formation of a tight (0.4 4 mu M) complex between NifF and Fe-protein. Under the same conditions , FdI interacted only weakly (K-d > 325 mu M) with Fe-protein. There w as no evidence for complex formation between NifF and FdI since the re action NifF(SQ)+FdI(red) had a bimolecular rate constant of 12.5 +/- 1 .2 x 10(3) M-1 s(-1). These results suggest that NifF, which is presen t in only small quantities in the cell, can make a significant contrib ution 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 redu ce Fe-protein and not in series. (C) 1998 Elsevier Science B.V. All ri ghts reserved.