Interaction of ligands with the distal glutamine in elephant myoglobin.

The effects of distal glutamine (E7) replacement in elephant myoglobin were studied by comparing the temperature-dependent nitrosyl electron spin resonance spectra, redox potentials, and the acid-alkaline equilibria of elephant and human myoglobins. For myoglobins containing a distal histidine, the nitrosyl ESR spectra do not exhibit superhyperfine splitting until near liquid helium temperatures (Yoshimura, T., Ozaki, T., Shintani, Y., and Watanabe, H. (1979) Arch. Biochem. Biophys. 193, 301-313). Studies presented here show that the ESR spectra of nitrosyl elephant myoglobin exhibit 9-line superhyperfine splitting well above liquid nitrogen temperatures, similar to the temperature profiles of isolated heme complexes (Morse, R.H. (1980) Fed. Proc. 39, 2006). It is concluded that the shift in the spectral equilibrium to higher temperature indicates a diminished interaction between NO and the distal position in elephant myoglobin. In addition, the redox potential of elephant myoglobin was found to be nearly 100 mV greater than that of human myoglobin, and the pKa of the acid-alkaline equilibrium (oxidized myoglobin) was 8.5, being 0.4 unit less than that of other vertebrate myoglobins. These different reactivities between elephant and human myoglobins are discussed based on the nature of charge interactions between polar ligands and distal glutamine and histidine.