Protoglobin from Methanosarcina acetivorans (MaPgb) 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 in vitro reaction of ferric MaPgb with nitrite. In contrast to other globins, a large excess of nitrite did not induce the formation of a nitriglobin form in MaPgb. Surprisingly, the addition of nitrite in mildly acidic pH led to the formation of a stable nitric-oxide ligated ferric form of the protein (MaPgb-NO). Furthermore, the 300-700 nm ECD spectrum of ferric MaPgb 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 MaPgb and the related quantum-mechanical admixture of the S = 5/2 and S = 3/2 state of its ferric form. The Soret and Q ellipticity change strongly upon formation of MaPgb-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 MaPgb.