The first glimpse of a complex of nitrogenase component proteins by solution X-ray scattering: Conformation of the electron transfer transition state complex of Klebsiella pneumoniae nitrogenase

An essential feature of the mechanism of nitrogenase, the enzyme responsible for biological nitrogen fixation, is the formation of a transient electron transfer complex between the MoFe protein containing the active site at which N-2 is reduced, and the Fe protein, which functions as a specific electron donor to the MoFe protein. We have obtained high quality solution X-ray scattering data using synchrotron X-rays of a stable putative electron transfer complex, (MoFe-protein)(Fe-protein . ADP . ALF(4))(2), of Klebsiella pneumoniae and used the model-independent approach based on the multipole expansion method to provide a stable and unique shape restoration at similar to 15 Angstrom resolution. The biological significance of this first molecular structure of a nitrogenase complex is discussed. (C) 1997 Academic Press Limited.