Research
The research interests of the Rees group have centered on the structures and mechanisms of complex metalloproteins and integral membrane proteins, particularly those involved in ATP-dependent transduction processes. The metalloprotein work defined the unusual structures of the nitrogenase FeMo-cofactor and the more widespread Mo-cofactor that participate in basic reactions of the biological nitrogen and sulfur cycles, while the membrane protein studies addressed the structural basis of photosynthesis, mechanosensation and ATP dependent transport processes.
Nitrogenase
The enzyme nitrogenase catalyzes the conversion of atmospheric dinitrogen to the metabolically usable form of ammonia during the process of biological nitrogen fixation. We want to understand how nitrogenase achieves this transformation under physiological conditions, which contrast sharply with the high temperatures and pressures of the industrial Haber-Bosch process. A central focus of our studies has been the structural and functional characterization of the nitrogenase component proteins (the iron (Fe-) protein and the molybdenum-iron (MoFe-) protein) and their constituent metalloclusters, particularly the iron molybdenum cofactor (FeMo-cofactor) of the MoFe-protein that provides the active site for dinitrogen reduction. We have elucidated the structure of the FeMo-cofactor and solved the first structures of a liganded form of the FeMo-cofactor with bound carbon monoxide. We subsequently generated a selenated form of the FeMo-cofactor and established that rearrangements of the cofactor can occur during turnover. Using single particle cryoEM and chemical analysis, we established from analysis of two forms of the Azotobacter vinelandii MoFe-protein – a high pH turnover inactivated species and a ΔNifV variant that cannot synthesize homocitrate – that changes at the homocitrate site may be coupled to α-subunit domain and FeMo-cofactor disordering. Our results point towards a dynamic active site in which homocitrate plays a role in enabling nitrogenase catalysis by facilitating activation of the FeMo-cofactor from a relatively stable form to a state capable of reducing dinitrogen under ambient conditions.
