Dissimilatory Nitrite and Nitric Oxide Reductases

Abstract

The inorganic nitrogen cycle (Figure 1) consists of several linked biological processes and one abiological process, the reaction of N2 and O2 in lightning discharges and internal combustion engines to produce NOx and, ultimately, nitrate. All of the processes except nitrogen fixation involve reduction or oxidation of species containing N-O bonds of order greater than 1, such as NO3, NO2, NO, and N2O. Denitrification is the anaerobic use by bacteria of nitrogen oxide species as terminal electron acceptors in place of O2. It is important because it constitutes the only process that returns large amounts of fixed nitrogen to the atmosphere, thereby completing the terrestrial nitrogen cycle (Figure 1). It is also important commercially in that denitrification, by itself or in combination with nitrification, can result in the loss of up to 30% of fixed nitrogen fertilizer.3 Further, denitrification is a “leaky” process under many conditions, resulting in the release of large amounts of N2O, a greenhouse gas that is also implicated in atmospheric ozone depletion,4 into the atmosphere. Denitrifiers are potentially of great importance in bioremediation efforts,5 since dissolved nitrate concentrations are often easier to control than are oxygen concentrations. In addition, denitrifiers have important potential applications in wastewater remediation, as evidenced by the use of immobilized nitrate, nitrite, and nitrous oxide reductase enzymes for electrochemical6 or biochemical7 removal of nitrate from water. Finally, the well-studied enzymes of denitrification provide potential structural and spectroscopic models for mammalian enzymes that produce and utilize NO in a variety of signal transduction pathways.8 Denitrification is a basic physiological process that is crucial for energy generation in the survival of a variety of bacteria.2 Denitrifying bacteria occupy a wide range of natural habitats, including soil, water, foods, and the digestive tract.9-11 Although these organisms prefer oxygen as an electron acceptor, in the absence of oxygen they can obtain energy from electron transport phosphorylation coupled to the reduction of nitrogen oxide species (NO3, NO2, NO, N2O). Before proceeding further, it is important to distinguish denitrification from two other physiological * Phone: 31-20-525-5045. Fax: 31-20-525-5124. E-mail: BAA@ CHEM.UVA.NL. Bruce A. Averill was born in Ohio in 1948 but grew up in New England. He received a B.S. in chemistry from Michigan State University in 1969. This was followed in 1973 by a Ph.D. in inorganic chemistry with Dick Holm at M.I.T., where he was an NSF predoctoral fellow. He subsequently spent 3 years learning biochemistry on NSF and NIH postdoctoral fellowships with Bob Abeles at Brandeis University and Bill Orme-Johnson at the University of Wisconsin, Madison. In 1976, he returned as an assistant professor of chemistry to Michigan State University, where he was named an Alfred P. Sloan Foundation Fellow and promoted to associate professor in 1981. The next year, he moved to the University of Virginia, where he was promoted to professor of chemistry in 1988. During his time at Virginia, the focus of his research shifted more toward studies of metalloenzymes. In 1994, he moved to the E. C. Slater Institute of the University of Amsterdam as a professor of biochemistry, specializing in biocatalysis. His research now focuses on the structure and mechanism of iron-containing phosphatases and the enzymes of denitrification.