Interaction of nitrite with ferric protoglobin from Methanosarcina acetivorans - an interesting model for spectroscopic studies of the haem-ligand interaction.

Protoglobin from (Pgb) is a dimeric globin belonging to the same lineage of the globin superfamily as globin-coupled sensors. A putative role in the scavenging of reactive nitrogen and oxygen species has been suggested as a possible adaptation mechanism of the host organism to different gaseous environments in the course of evolution. A combination of optical absorption, electronic circular dichroism (ECD), resonance Raman (rRaman), and electron paramagnetic resonance (EPR) reveal the unusual reaction of ferric Pgb with nitrite. In contrast to other globins, a large excess of nitrite did not induce the formation of a nitriglobin form in Pgb. Surprisingly, the addition of nitrite in mildly acidic pH led to the formation of a stable nitric-oxide ligated ferric form of the protein (Pgb-NO). Furthermore, the 300-700 nm ECD spectrum of ferric Pgb is for the first time reported and discussed, showing strong differences in the Soret and Q ellipticity compared to ferric myoglobin, in line with the unusually strongly ruffled haem group of Pgb and the related quantum-mechanical admixture of the = 5/2 and = 3/2 state of its ferric form. The Soret and Q ellipticity change strongly upon formation of Pgb-NO, revealing a significant effect of the nitric-oxide ligation on the haem group and pocket. The related changes in the asymmetric pyrrole half-ring stretching vibration modes observed in the rRaman spectra give experimental support to earlier theoretical models, in which an important role of the in-plane breathing modes of the haem was predicted for the stabilization of the binding of diatomic gases to Pgb.