Structure of cytochrome c nitrite reductase

The enzyme cytochrome c nitrite reductase catalyses the six-electron reduct ion of nitrite to ammonia as one of the key steps in the biological nitroge n cycle(1), where it participates in the anaerobic energy metabolism of dis similatory nitrate ammonification(2). Here we report on the crystal structu re of this enzyme from the microorganism Sulfurospirillum deleyianum, which we solved by multiwavelength anomalous dispersion methods. We propose a re action scheme for the transformation of nitrite based on structural and spe ctroscopic information. Cytochrome c nitrite reductase is a functional dime r, with 10 dose-packed haem groups of type c and an unusual lysine-coordina ted high-spin haem at the active site. By comparing the haem arrangement of this nitrite reductase with that of other multihaem cytochromes, we have b een able to identify a family of proteins in which the orientation of haem groups is conserved whereas structure and function are not.