Electron transfer and half-reactivity in nitrogenase.
Nitrogenase is a globally important enzyme that catalyses the reduction of atmospheric dinitrogen into ammonia and is thus an important part of the nitrogen cycle. The nitrogenase enzyme is composed of a catalytic molybdenum-iron protein (MoFe protein) and a protein containing an [Fe4-S4] cluster (Fe protein) that functions as a dedicated ATP-dependent reductase. The current understanding of electron transfer between these two proteins is based on stopped-flow spectrophotometry, which has allowed the rates of complex formation and electron transfer to be accurately determined. Surprisingly, a total of four Fe protein molecules are required to saturate one MoFe protein molecule, despite there being only two well-characterized Fe-protein-binding sites. This has led to the conclusion that the purified Fe protein is only half-active with respect to electron transfer to the MoFe protein. Studies on the electron transfer between both proteins using rapid-quench EPR confirmed that, during pre-steady-state electron transfer, the Fe protein only becomes half-oxidized. However, stopped-flow spectrophotometry on MoFe protein that had only one active site occupied was saturated by approximately three Fe protein equivalents. These results imply that the Fe protein has a second interaction during the initial stages of mixing that is not involved in electron transfer.
Centre for Molecular and Structural Biochemistry, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK. tom.clarke@uea.ac.uk
Thomas A
TA
Clarke
Shirley
S
Fairhurst
David J
DJ
Lowe
Nicholas J
NJ
Watmough
Robert R
RR
Eady
D000255:Adenosine Triphosphate / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D020134:Catalytic Domain; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport / Q000502:physiology*; D007711:Klebsiella pneumoniae / Q000378:metabolism; D008983:Molybdoferredoxin / Q000737:chemistry / Q000378:metabolism; D058458:Nitrogen Cycle / Q000502:physiology*; D009591:Nitrogenase / Q000737:chemistry* / Q000378:metabolism*; D010088:Oxidoreductases / Q000378:metabolism; D011487:Protein Conformation; D013053:Spectrophotometry / Q000379:methods
D001426:Bacterial Proteins; D008983:Molybdoferredoxin; D000255:Adenosine Triphosphate; D010088:Oxidoreductases; D009591:Nitrogenase