Cytochrome c Nitrite Reductase from Wolinella succinogenes

Abstract

Cytochrome c nitrite reductase catalyzes the 6-elec-tron reduction of nitrite to ammonia. This second part of the respiratory pathway of nitrate ammonification is a key step in the biological nitrogen cycle. The x-ray struc-ture of the enzyme from the ⑀-proteobacterium Wolinella succinogenes has been solved to a resolution of 1.6 Å. It is a pentaheme c-type cytochrome whose heme groups are packed in characteristic motifs that also occur in other multiheme cytochromes. Structures of W. succinogenes nitrite reductase have been obtained with water bound to the active site heme iron as well as complexes with two inhibitors, sulfate and azide, whose binding modes and inhibitory functions differ signifi-cantly. Cytochrome c nitrite reductase is part of a highly optimized respiratory system found in a wide range of Gram-negative bacteria. It reduces both anionic and neutral substrates at the distal side of a lysine-coordi-nated high-spin heme group, which is accessible through two different channels, allowing for a guided flow of reaction educt and product. Based on sequence comparison and secondary structure prediction, we have demonstrated that cytochrome c nitrite reductases constitute a protein family of high structural similarity. The biogeochemical nitrogen cycle represents a network of reactions catalyzed by enzymes with different metal cofactors, which allows for redox transitions of nitrogen between its oxi-dation state (ϩ5), as in nitrate, and (Ϫ3), as in ammonia and its most abundant form dinitrogen. The one respiratory pathway that covers the whole range between nitrate and ammonia is the dissimilatory nitrate reduction to ammonia (1). Hereby, nitrate is first reduced to nitrite in a 2-electron step by a nitrate reductase, and subsequently the product nitrite is converted to