Electron spin resonance studies of ferrihaemoglobin MOsaka

Vector electron paramagnetic resonance spectroscopy with first and second harmonic displays of ferrihemoglobin

Superimposed plots of electron paramagnetic resonance spectra with the first and second harmonic displays of ferrihemoglobin at pH 9.1 and 90 K were measured at 20 degree intervals of phase angle using a phase-sensitive detector. The high spin signal in the g = 6 region was observed in both displays, and a small splitting of the signal was found in the calculated amplitude spectrum of the second harmonic display, with g values of 5.95 and 6.05. Low spin signals were observed at g = 2.55, 2.25 and 1.82 in both harmonic displays. A signal in the g = 2.05 region was observed only in the second harmonic display. The signal is probably associated with the low spin spectrum; however, its origin is obscure.

Differential effects of pH and inositol hexaphosphate on the spectroscopic properties of the alpha and beta subunits in methemoglobins M Milwaukee and A

The effect of pH and inositol hexaphosphate on the electron spin resonance spectra of the alpha-hemes (g = 6.0) and the beta-hemes (g = 6.7) has been measured in methemoglobin M Milwaukee and compared with that of methemoglobin A (g = 6.0). The beta-hemes are found to be comparatively insensitive to both effectors while the alpha-hemes behave in a manner similar to the heme groups of methemoglobin A. Binding of inositol hexaphosphate enhances the high spin ESR signal of the alpha-hemes in both methemoglobins. Comparison of the optical properties of methemoglobins A and M Milwaukee over the pH range from 5.0 to 8.1 shows that inositol hexaphosphate has a differential effect on the subunit types in these two methemoglobins. At low pH the spectral changes observed upon inositol hexaphosphate binding arise primarily from the beta-hemes, while at neutral and alkaline pH these changes arise from both subunit types. The beta-heme spectral changes appear to be pH independent while those arising from the alpha-hemes are strongly pH dependent. It is concluded that it is the hydroxymet form of the alpha-hemes which undergoes spectral change upon inositol hexaphosphate binding to the beta-subunits. In methemoglobin A the spin state and paramagnetic susceptibility increase only in the neutral and alkaline pH ranges upon inositol hexaphosphate binding (Gupta, R.K. and Mildvan, R.S. (1975) J. Biol. Chem. 250, 246; Perutz, M.F., Sanders, J.K.M., Chenery, D.H., Noble, R.W., Penelly, R.R., Fung, L.W.-M., Ho, C., Giannini, I., Porschke, D. and Winkler, H. (1978) Biochemistry 17, 3640). Therefore the hydroxymet form of the alpha-hemes which is responsible for the observed spectral changes must also be responsible for these increases in the magnetic properties of methemoglobin A. Inositol hexaphosphate can bind to methemoglobin at alkaline pH if the beta-hemes are in the high spin form.

Hemoglobin A: an electron paramagnetic resonance study of the effects of interchain contacts on the heme symmetry of high-spin and low-spin derivatives of ferric alpha chains

Hemoglobin, the oxygen-binding heme protein of blood is composed of four subunits of two types, alpha and beta. Although the protein when isolated is diamagnetic, by oxidation it can be converted to various paramagnetic ferric forms exhibiting specific electron paramagnetic resonance (EPR) spectra. Using electron paramagnetic resonance, we have shown that the symmetry of the heme of isolated ferric alpha chains is different from that of the heme of these same subunits when associated with ferric beta chains in the parent tetramer. The high-spin form of the isolated ferric alpha chains shows a clearly resolved departure from tetragonal symmetry in contrast to ferrihemoglobin A, where the environment of the iron of all four chains is nearly tetragonal. This difference in heme symmetry suggests that the maintenance of tetragonal symmetry in the hemoglobin tetramer must be brought about by the configuration the alpha chains assume due to their contact with the beta chains. When isolated from the tetramer, some parts of the alpha chain must undergo physical displacement from a configuration that had been stabilized by the beta chains.Although differences in symmetry between monomer and tetramer are seen with high-spin forms, no such difference exists with two low-spin ferric forms. Here the symmetries of iron are primarily governed by the immediate ligand environment of the iron and not by the state of association of the protein.