Cytochrome cd(1) nitrite reductase catalyses the conversion of nitrite
to nitric oxide in the nitrogen cycle(1,2). The crystal structure of
the oxidized enzyme(3,4) shows that the d(1) haem iron of the active s
ite is ligated by His/Tyr side chains, and the c haem iron is ligated
by a His/His ligand pair. Here we show that both haems undergo re-liga
tion during catalysis, Upon reduction, the tyrosine ligand of the d(1)
haem is released to allow substrate binding, Concomitantly, a refoldi
ng of the cytochrome c domain takes place, resulting in an unexpected
change of the c haem iron coordination from His17/His69 to Met106/His6
9. This step is similar to the last steps in the folding of cytochrome
c(5-8). The changes must affect the redox potential of the haems, and
suggest a mechanism by which internal electron transfer is regulated,
Structures of reaction intermediates show how nitric oxide is formed
and expelled from the active-site iron, as well as how both haems retu
rn to their starting coordination. These results show how redox energy
can be switched into conformational energy within a haem protein.