The Production of Ammonia by Multiheme Cytochromes c - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 220

Figure 4 Arrangement of the ten heme c centers in the cytochrome c nitrite reductase (NrfA)
dimer from Sulfurospirillum deleyianum . The overall orientation of the hemes corresponds to that
in Figure 3 , with the active site located at heme 1 and the line indicating the NrfA dimer interface.
Hemes in the left monomer are numbered according to their attachment to the protein chain. In the
right monomer, the Fe-Fe distances (

) between the hemes are given. Taken by permission from
[ 58 ]; copyright 1999 Nature Publishing Group. PDB code: 1QDB.

Å

Actinobacteria, Verrucomicrobia, and Acidobacteria. All other clades had a
Cys-X-X-Cys-Lys motif in this location. In all NrfA protein hemes 2-4 serve as
electron transfer modules, whereas heme center 1 is the catalytic site which binds
nitrite and also sulfite [ 42 , 59 ] (Figures 5 , 6 , and 7 ).

Figure 5 Structures of substrate complexes of cytochrome c nitrite reductase (NrfA) from
Wolinella succinogenes. (a) Nitrite adduct, PDB code: 2E80, with the NO 2 anion binding to the
heme iron through the electron pair at the nitrogen atom. (b) Sulfite adduct, PDB code: 3BNF. The
binding mode is highly similar to nitrite but the bond distance to iron is slightly longer due to the
larger van der Waals radius of sulfur [ 72 ]. Taken by permission from [ 25 ]; copyright 2013
Elsevier.