The nitrogenase from the facultative anaerobe

The effects of MgADP and MgATP on the kinetics of a pre-steady-state electron-transfer reaction and on the steady-state kinetics of H2 evolution for nitrogenase proteins of K. pneumoniae were studied. MgADP was a competitive inhibitor ofMgATP in the MgATPinduced electron transfer from the Fe-protein to the Mo-Fe-protein. A dissociation constant K' = 20,UM was determined for MgADP. The release of MgADP or a coupled conformation change in the Fe-protein ofK. pneumoniae occurred with a rate comparable with that of electron transfer, k2 x 102 s-. Neither homotropic nor heterotropic interactions involving MgATP and MgADP were observed for this reaction. Steadystate kinetic data for H2 evolution exhibited heterotropic effects between MgADP and MgATP. The data have been fitted to symmetry and sequential-type models involving conformation changes in two identical subunits. The data suggest that the enzyme can bind up to two molecules of either MgATP or MgADP, but is unable to bind both nucleotides simultaneously. The control of H2 evolution by the MgATP/MgADP ratio is not at the level of electron transfer between the Feand Mo-Fe-proteins.