Fluorescent Analogue of Hexa-coordinate Globins to Monitor Accessibility of the Heme Pocket

Neuroglobin (Ngb), cytoglobin (Cygb) and globin X (GbX) belong to the family of vertebrate hexa-coordinate globins. Recent studies have proposed that these globins carry out various function including ROS/RNS scavenger and in vivo studies have indicated the role of Cygb and Ngb in carcinogenesis, cell proliferation, apoptosis and cell survival under hypoxic conditions. Globin X was found to be important for the reduction of nitrite to nitric oxide in fish red blood cells. It was shown that the internal disulfide bond that is unique to hexa-coordinate vertebrate globins modules the heme iron affinity for oxygen in Ngb and Cygb. To probe the mechanism of the ligand migration in Cygb, Ngb and GbX, we replaced the non-fluorescent Fe protoporphyrin IX (FePPIX) with the fluorescent analogue Zn protoporphyrin IX (ZnPPIX) and characterized the ligand migration by monitoring the quenching of the ZnPPIX fluorescent emission by methyl viologen and the quenching of the ZnPPIX triplet state by oxygen. We observed an increase in the Ksv value for methyl viologen quenching in protein variants that are missing the internal disulfide bond, suggesting that the presence of the intraprotein disulfide bond regulates the accessibility of the heme binding pocket. Interestingly the lifetime of the triplet state for ZnPPIX incorporated in hexacoordinate vertebrate globins is not sensitive to changes in the heme environment as the observed lifetime is comparable to that determined previously for myoglobin.