Revue Roumaine de Chimie, 2009, 54(6), 513–522 NITRIC OXIDE AND NITRITE REDUCTION BY METALLOENZYMES Radu SILAGHI-DUMITRESCU “Babeş-Bolyai” University, Department of Chemistry and Chemical Engineering, Arany Janos street 11, Cluj-Napoca RO-400028, Roumania, Fax: +40-264-590818, e-mail: rsilaghi@chem.ubbcluj.ro Received December 17, 2008 Recent computational results from our group are reviewed on the mechanisms of enzymatic nitrite reduction to nitric oxide or ammonia, as well as nitric oxide reduction to nitrous oxide or ammonia, with some consideration for the electronic structures of electromeric species such as metal-nitric oxide adducts. Metal centers discussed include heme b, heme d 1 , heme c, non-heme diiron, and copper. Radu Silaghi-Dumitrescu was born in 1974 in Braşov, Roumania. He graduated from the “Babes-Bolyai” University in Cluj-Napoca, Roumania in 1997, with a B. Sc. Degree in chemistry, followed by an M. Sc. degree in heterocycle chemistry in 1998. He joined the faculty of the Department of Chemistry and Chemical Engineering in 1998, as assistant. He obtained a PhD degree in bioinorganic chemistry at the University of Georgia at Athens, GA, USA (2000-2004) under the direction of Prof. Donald M. Kurtz, Jr., working on non-heme iron proteins involved in oxidative and nitrosative stress. In 2005 he obtained a second PhD degree in chemistry under the direction of Prof. Ionel Haiduc, focused on theoretical studies of small molecule activation by hemes. In 2004 he took on a postdoctoral position in England at the University of Essex under the direction of Profs. Chris Cooper and Michael Wilson, working on hemoglobin-based blood substitutes. In 2006 he returned to the Department of Chemistry in Cluj-Napoca, where he became an associate professor in 2007, teaching bioinorganic chemistry and biochemistry. His current research interests are small molecule activation by metalloproteins and related metal centers, as studied by multidisciplinary approaches. The ISI database indexes 36 of his publications, cited a total of 158 times (self-citations excluded). INTRODUCTION Under anaerobic conditions, many microorgan- isms can sustain growth by using nitrate as respiratory terminal electron acceptor. Bacteria in fact exhibit a wide range of metabolic reactions with various oxides of nitrogen, sometimes referred to as the bacterial nitrogen cycle. 1 Nitrate and nitrite may be reduced to ammonia for the purpose of nitrogen assimilation (incorporation into organic matter, non-energy conserving) or dissimilation (using nitrate as respiratory electron acceptor, i.e., energy-conserving, but without incorporating the final reduced product into organic matter). Reduction of dinitrogen to ammonia is termed nitrogen fixation. All the known enzymes catalyzing the reactions of the nitrogen cycle are metalloenzymes. Nitrate reduction to nitrite is catalyzed by nitrate reductases, which are molybdopterin enzymes. The subsequent reduction of nitrite is catalyzed by two types of nitrite reductases: those reducing nitrite to ammonia (cytochrome c nitrite reductase, siroheme- containing nitrite reductase) and those reducing nitrite to nitric oxide (copper-containing nitrite reductase, cytochrome cd 1 nitrite reductase). 2-5 When produced by an NO-forming nitrite reductase, nitric oxide is further reduced to N 2 O by nitric oxide reductases, which contain either cytochrome bd-or P450-type active sites; 3 non-heme