SMALL-ANGLE X-RAY-SCATTERING STUDIES OF THE IRON-MOLYBDENUM COFACTOR FROM AZOTOBACTER-VINELANDII NITROGENASE

The nitrogenase enzyme complex, consisting of the molybdenum-iron prot ein and the iron protein, plays a critical role in the biological redu ction of dinitrogen to ammonia (nitrogen fixation). The nitrogen-fixin g site within the molybdenum-iron protein is an iron-molybdenum-sulfur cofactor (FeMoco) of roughly 1000-2000 Dalton mass. Structural aspect s of FeMoco have been determined by spectroscopic and more recently by crystallographic studies. In order to determine the radius of gyratio n (R(g)) of isolated FeMoco, we have performed small-angle x-ray scatt ering studies of FeMoco in N-methylformamide solution, in the absence of the molybdenum-iron protein. Model compounds of known structure hav e also been examined in similar solvents, N,N-dimethylformamide and ac etonitrile, as controls and for calibration purposes. The R(g) values obtained for the models are in good agreement with calculations based upon their respective crystal structures. However, the R(g) obtained f or FeMoco clearly indicates that the cofactor is not monomeric in solu tion, but rather aggregated and possibly polydisperse. Further, R(g) v alues were also measured after addition of thiol, dithionite, and thio l and dithionite, to the FeMoco samples. The results indicate, surpris ingly, that oxidation state and putative thiol coordination have no de tectable effect on the aggregation behavior of FeMoco in solution, as determined by these measurements.