High pressure NMR studies of hemoproteins. The effect of pressure on the tertiary and quaternary structures of human adult ferrous hemoglobin

Reversible Hexacoordination of α-Hemoglobin-stabilizing Protein (AHSP)/α-Hemoglobin Versus Pressure

Using high hydrostatic pressure or hydrogen peroxide as perturbing agents, we demonstrate a protective effect of the chaperone AHSP for the alpha-chains of Hb. High pressure induces an irreversible aggregation of the ferrous deoxy alpha-chains, whereas the AHSP/alpha-Hb complex shows reversible hexacoordination of the alpha-Hb without protein aggregation. Upon pressure release, the relaxation kinetics of the transition from the hexacoordinated to pentacoordinated form of alpha-Hb in the presence of AHSP exhibit a biphasic shape. High pressure did not induce dissociation of alpha-Hb from its chaperone, as evidenced by the ligand binding kinetics that show a unique rate for the AHSP/alpha-Hb complex. For both free alpha-Hb and the AHSP/alpha-Hb complex, the bimolecular rate constant of CO binding (k(CO)(on)) versus pressure exhibits a bell shape, attributed to the transition of the rate-determining step from the chemical barrier to the migration of CO within the protein matrix. These results reveal a plasticity of the alpha-Hb active site in the presence of the chaperone and indicate that the AHSP was still active at 300 MPa. The ferric state of the AHSP/alpha-Hb complex shows hexacoordination even at atmospheric pressures, indicating a His-Fe-His binding scheme as previously observed in neuroglobin and cytoglobin. The reaction with hydrogen peroxide of ferric alpha-Hb within the complex also demonstrates a protection against aggregation.

The effects of osmotic and hydrostatic pressures on macromolecular systems

Osmotic pressure and hydrostatic pressure can be used effectively to probe the behavior of biologically important macromolecules and their complexes. Using the two techniques requires a theoretical framework as well as knowledge of the more common pitfalls. Both are discussed in this review in the context of several examples.

Interaction of fully liganded valency hybrid hemoglobin with inositol hexaphosphate. Implication of the IHP-induced T state of human adult methemoglobin in the low-spin state

To gain further insight into the quaternary structures of methemoglobin derivatives in the low-spin state, the interaction of fully liganded valency hybrid human hemoglobins with IHP was studied by proton NMR spectroscopy. Upon addition of IHP to (alpha CO beta + N3-)2, the same resonances as the previously reported IHP-induced NMR peaks for azidomethemoglobin (alpha + N3-beta +N3-)2 appeared, whereas the binding of IHP did not significantly affect the NMR spectra for (alpha + N3-beta CO)2. The binding of IHP also brought about more pronounced spectral changes for (alpha CO beta + Im)2 and (alpha CO beta + H2O)2 than for (alpha + Im beta CO)2 and (alpha + H2O beta CO)2. Therefore, the IHP-induced NMR peaks for azidomethemoglobin are attributed to the beta heme methyl group. Such IHP-induced beta heme methyl resonances were also observed for (alpha NO beta + N3-)2, which undergoes quaternary structural change, analogously to the R-T transition by the binding of IHP. From the above results, it was suggested that the IHP-induced heme methyl resonances for azidomethemoglobin and (alpha CO beta +N3-)2 may also be associated with the quaternary structure of these Hbs, implying the presence of the IHP-induced "T-like" state in low-spin metHb A.

NMR study of hybrid hemoglobins containing unnatural heme: effect of heme modification on their tertiary and quaternary structures

The effect of heme modification on the tertiary and quaternary structures of hemoglobins was examined by utilizing the NMR spectra of the reconstituted [mesohemoglobin (mesoHb), deuterohemoglobin (deuteroHb)] and hybrid heme (meso-proto, deutero-proto) hemoglobins (Hbs). The heme peripheral modification resulted in the preferential downfield shift of the proximal histidine N1H signal for the beta subunit, indicating nonequivalence of the structural change induced by the heme modification in the alpha and beta subunits of Hb. In the reconstituted and hybrid heme Hbs, the exchangeable proton resonances due to the intra- and intersubunit hydrogen bonds, which have been used as the oxy and deoxy quaternary structural probes, were shifted by 0.2-0.3 ppm from that of native Hb upon the beta-heme substitution. This suggests that, in the fully deoxygenated form, the quaternary structure of the reconstituted Hbs is in an "imperfect" T state in which the hydrogen bonds located at the subunit interface are slightly distorted by the conformational change of the beta subunit. Moreover, the two heme orientations are found in the alpha subunit of deuteroHb, but not in the beta subunit of deuteroHb, and in both the alpha and beta subunits of mesoHb. The tertiary and quaternary structural changes in the Hb molecule induced by the heme peripheral modification were also discussed in relation to their functional properties.

High pressure NMR studies of hemoproteins. The effect of pressure on the quaternary structure of hemoglobin

Proton NMR spectra for nitrosyl-, aquomet - and deoxy des-Arg (alpha 141)-hemoglobin in H2O were studied at high pressures up to 1400 atm with attention to the exchangeable proton resonances due to the intra- and intersubunit hydrogen bonds. For aquomethemoglobin , the T state marker signal at 6.4 ppm is insensitive to pressure while the R state marker signal at 6.0 ppm exhibits progressive upfield shift upon pressurization . For nitrosylhemoglobin, the T state signals at 9.6 and 6.5 ppm decrease their intensities upon pressurization while the R state marker signal at 6. 0 ppm remains unchanged. Pressure-induced spectral changes for some of exchangeable resonances are also encountered for deoxy des-Arg (alpha 141)-hemoglobin while the R and T quaternary structural indicators at 6.0 and 9.4 ppm are insensitive to pressure. These pressure-induced spectral changes for these hemoglobin derivatives are significantly distinguished from those associated with the R-T transition induced by addition of IHP or by variation of pH. It is therefore concluded that pressure induces subtle quaternary structural changes in these hemoglobin derivatives without causing the R-T transition.