Chapter 16
Redox Properties of the Nitrogenase Proteins from Azotobacter vinelandii 1
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G. D. Watt , H. Huang , and K. R. N . Reddy
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Department of Chemistry, Brigham Young University, Provo, UT 84602 Department of Biology, Xiamen University, Xiamen, People's Republic of China Department of Chemistry, University of Maryland, Baltimore, M D 21228 2
Downloaded by UNIV LAVAL on July 10, 2016 | http://pubs.acs.org Publication Date: July 26, 1993 | doi: 10.1021/bk-1993-0535.ch016
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Nitrogenase consists of two dissimilar, metalloproteins (the MoFe protein and the Fe protein) which together catalyze the six-electron reduction of dinitrogen (N ) to ammonia under mild biological conditions. Both protein components contain metal clusters which are redox active. When the two nitrogenase proteins are combined under turnover conditions, electrons are transferred among the various metal clusters and ultimately to the reducible substrate N . We are investigating the spectroscopic and electrochemical properties of various redox states in each protein component to determine their nature, their redox stoichiometry and how they are related to one another. Protein redox intermediates prepared by reaction with artificial redox reagents are being examined. Properties of the isolated FeMoco metal cluster component isolated from the MoFe protein are also being determined. A very low potential redox state of the Fe protein has been prepared which is more reduced than the dithionite-reduced state. This new species has redox reactivity in nitrogenase turnover and may represent an intermediate in the nitrogen reduction process. 2
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Biological nitrogen fixation is a reductive biochemical process (1-3) carried out by the nitrogenase enzyme complex found in several species of microorganisms, which is responsible for the input of ammonia into the earth's nitrogen cycle. The overall reaction, which can be represented as: N2 + 8H+ +8e = 2NH3 + H2, requires a considerable input of biochemical energy in the form of ATP and low potential reductant and is catalyzed by a protein complex composed (1-3) of the redox active iron protein (Fe protein) and the molybdenum iron protein (MoFe protein). The structure of the smaller and simpler Fe protein is known (4, Rees et al chapter 11, this volume) and consists of an
