文摘
Several members of a widespread class of bacterial and archaeal metalloflavoproteins, calledFprA, likely function as scavenging nitric oxide reductases (S-NORs). However, the only published X-raycrystal structure of an FprA is for a protein characterized as a rubredoxin:dioxygen oxidoreductase (ROO)from Desulfovibrio gigas. Therefore, the crystal structure of Moorella thermoacetica FprA, which hasbeen established to function as an S-NOR, was solved in three different states: as isolated, reduced, andreduced, NO-reacted. As is the case for D. gigas ROO, the M. thermoacetica FprA contains a solvent-bridged non-heme, non-sulfur diiron site with five-coordinate iron centers bridged by an aspartate, andterminal glutamate, aspartate, and histidine ligands. However, the M. thermoacetica FprA diiron site showedfour His ligands, two to each iron, in all three states, whereas the D. gigas ROO diiron site was reportedto contain only three His ligands, even though the fourth His residue is conserved. The Fe1-Fe2 distancewithin the diiron site of M. thermoacetica FprA remained at 3.2-3.4 Å with little or no movement of theprotein ligands in the three different states and with conservation of the two proximal open coordinationsites. Molecular modeling indicated that each open coordination site can accommodate an end-on NO.This relatively rigid and symmetrical diiron site structure is consistent with formation of a diferrousdinitrosyl as the committed catalytic intermediate leading to formation of N2O. These results providenew insight into the structural features that fine-tune biological non-heme diiron sites for dioxygen activationvs nitric oxide reduction.
