XAS study of the active site of a bacterial heme-sensor

Denitrifying bacteria control NO and NO2 cytosolic levels by regulating the expression of denitrification gene clusters via REDOX signalling of specific transcriptional factors that may act as NO sensors in vivo. A protein belonging to the subclass DNR (dissimilative nitrate respiration regulator) from Pseudomonas aeruginosa has been recently suggested to be a heme containing protein. Very recently the three dimensional structure of the apo-form of DNR (in the absence of heme) has been determined by X-Ray crystallography, whereas the holo-form (in the presence of heme) has not yet been crystallized. We have investigated the heme local structure in solution of ferric and ferrous holo-DNR by XAS. The Fe K-edge XANES spectrum of the ferric adduct displays typical features of a low-spin hexacoordinate Fe-heme complex, having two histidines ligated. After chemical reduction, relevant changes of the XANES fingerprints suggest a repositioning of the heme inside the hydrophobic core of the protein in agreement with previously reported structural and spectroscopic evidence. Partial release of the axial ligands leaves the Fe(II)heme available, and very reactive, to bind exogenous ligands like NO, thus supporting its role as the cofactor involved in NO sensing activity.