Proton NMR studies of hemoglobin in H2O

A proton nuclear magnetic resonance investigation of proximal histidyl residues in human normal and abnormal hemoglobins. A probe for the heme pocket

Proton nuclear magnetic resonance spectroscopy at 250 MHz has been used to investigate the conformations of proximal histidyl residues of human normal adult hemoglobin, hemoglobin Kempsey [beta 99(G1) Asp leads to Asn], hemoglobin Osler [beta 145(HC2) Tyr leads to Asp], and hemoglobin McKees Rocks [beta 145(HC2) Tyr leads to Term] around neutral pH in H2O at 27 degrees C, all in the deoxy form. Two resonances that occur between 58 and 76 ppm downfield from the water proton signal have been assigned to the hyperfine shifted proximal histidyl NH-exchangeable protons of the alpha- and beta-chains of deoxyhemoglobin. These two resonances are sensitive to the quaternary state of hemoglobin, amino acid substitutions in the alpha 1 beta 2-subunit interface and in the carboxy-terminal region of the beta-chain, and the addition of organic phosphates. The experimental results show that there are differences in the heme pockets among these four hemoglobins studied. The structural and dynamic information derived from the hyperfine shifted proximal histidyl NH-exchangeable proton resonances complement that obtained from the ferrous hyperfine shifted and exchangeable proton resonances of deoxyhemoglobin over the spectral region from 5 to 20 ppm downfield from H2O. The relationship between these findings and Perutz's stereochemical mechanism for the cooperative oxygenation of hemoglobin is discussed.

Proton nuclear magnetic resonance studies of hemoglobins Osler (beta145HC2 Tyr replaced by Asp) and McKee Rocks (beta145HC2 Tyr replaced by term): an assignment for an important tertiary structural probe in hemoglobin

High-resolution proton nuclear magnetic resonance studies of deoxyhemoglobins Osler (beta145HC2 Tyr replaced by Asp) and McKees Rocks (beta 145HC2 Tyr replaced by term) indicate that these hemoglobins are predominately in the oxy quaternary structure in 0.1 M [bis(2-hydroxyethyl)imino]-tris(hydroxymethyl) methane buffer at pH 7. Upon the addition of inositol hexaphosphate, the proton nuclear magnetic resonance spectra of these hemoglobins become similar to those characteristic of a hemoglobin molecule in the deoxy quaternary structure. The exchangeable proton resonance which is found at -6.4 ppm from H2O in the spectrum of normal human adult deoxyhemoglobin is absent in the spectra of these two mutant hemoglobins. Consequently we believe the hydrogen bond between the hydroxyl group of tyrosine-beta145HC2 and the carboxyl oxygen of valine-beta98FG5 gives rise to this resonance. This assignment allows us to use the -6.4ppm resonance as an important tertiary structural probe in the investigation of the cooperative oxygenation of hemoglobin.

Proton nuclear magnetic resonance studies of hemoglobin Providence (beta82EF6 Lys replaced by Asn or Asp): a residue involved in anion binding

High-resolution proton nuclear magnetic resonance studies of hemoglobins Providence-Asn (beta82EF6 Lys replaced by Asn) and Providence-Asp (beta82EF6 Lys replaced by Asp) show that different amino acid substitutions at the same position in the hemoglobin molecule have different effects on the structure of the protein molecule. Hemoglobin Providence-Asp appears to be in a low-affinity tertiary structure in both the deoxy and carbonmonoxy forms. Deoxyhemoglobin Providence-Asn has its beta heme resonance shifted downfield slightly from its position in normal adult hemoglobin; however, the tertiary structures of the heme pocket of hemoglobins A and Providence-Asn are very similar when both proteins are in the carbonmonoxy form. These results are consistent with the oxygen equilibrium measurements of Bonaventura, J., et al. [(1976) J. Biol. Chem. 251, 7563] which show that both Hb Providence-Asn and Hb Providence-Asp have oxygen affinities lower than normal adult hemoglobin, with Hb Providence-Asp having the lowest. Our studies of the effects of sodium chloride on the hyperfine shifted proton resonances of deoxyhemoglobins A, Providence-Asn, and Providence-Asp indicate that the beta82EF6 lysine is probably one, but not the only binding site for chloride ions.

Proton nuclear magnetic resonance studies of hemoglobin Malmö: implications of mutations at homologous positions of the alpha and beta chains

The abnormal human hemoglobin Malmö (beta97FG4 His leads to Gln) has been studied and its properties are compared with those of normal adult hemoglobin A. The data presented here show that the ring-current shifted proton resonances of both HbCO and HbO2 Malmö are very different from the corresponding forms of Hb A. The hyperfine shifted proton resonances of deoxy-Hb Malmö do not differ drastically from those of deoxy-Hb A. This result, together with the finding that the exchangeable proton resonances of the deoxy form of the two hemoglobins are similar, suggests that unliganded Hb Malmö can assume a deoxy-like quaternary structure both in the absence and presence of organic phosphates We have also compared the properties of Hb Malmö with those of Hb Chesapeake (alpha92FG4 Arg leads to Leu). This allows us to study the properties of two abnormal human hemoglobins with mutations at homologous positions of the alpha and beta chains in the three-dimenstional structure of the hemoglobin molecule. Our present results suggest that the mutaion at betaFG4 has its greatest effect on the teritiary structure of the heme pocket of the liganded forms of the hemoglobin while the mutation at alphaFG4 alters the deoxy structure of the hemoglogin molecule but does not alter the teriary structure of the heme pockets of the liganded form of the hemoglobin molecule. Both hemoglobins undergo a transition from the deoxy (T) to the oxy (R) quaternary structure upon ligation. The abnormally high oxygen affinities and low cooperativities of these two hemoglobins must therefore be due to either the structural differences which we have observed and/or to an altered transition between the T and R structures.

Assignment of the imidazole ring nitrogen protons of histidine 48 in the proton NMR spectrum of ribonuclease A in water solution

Several exchangeable resonances, designated a, b, c and d are observed in the 11-14 ppm (from 2,2-dimethyl-2-silapentane-5-sulfonate) region of the proton spectrum of ribonuclease A in water solution. We describe a number of lines of evidence suggesting the assignment of peaks b and c to the N1 and N3 protons of His 48, which occupies an interior position in the protein remote from the active site. This evidence includes the observation that the binding of Cu(II) and 3'-CMP (cytidine 3'-monophosphate) has no effect on these resonances. Further evidence includes pH titration data showing a pKa of approx. 2 for these protons, solvent exchange rates in the native state and with disulfide bridges IV-V and III-VIII cleaved, the observation of the carboxymethylated enzymes CM-His12-RNAase A and CM-His119-RNAase A, and of the modified enzymes Des(1-21)-RNAase A (S-protein) and Des(119-124)-RNAase A.

Allosteric interpretation of haemoglobin properties

It is our purpose to review recent experiments on haemoglobin in order to discuss them in terms of the two state model of cooperativity. Excellent previous reviews are available of the chemistry of haemoglobin (Antonini & Brunori, 1971; Gibson, 1959b) which are referred to when possible. The plethora of data necessitates that a selection must be made in a review. An intentionally wide range of experiments is selected to exhibit

A proton nuclear magnetic resonance study of the quaternary structure of human homoglobins in water

Proton nuclear magnetic resonance spectra of human hemoglobins in water reveal several exchangeable protons which are indicators of the quaternary structures of both the liganded and unliganded molecules. A comparison of the spectra of normal human adult hemoglobin with those of mutant hemoglobins Chesapeake (FG4alpha92 Arg yields Leu), Titusville (G1alpha94 Asp yields Asn), M Milwaukee (E11beta67 Val yields Glu), Malmo (FG4beta97 His yields Gln), Kempsey (G1beta99 Asp yields Asn), Yakima (G1beta99 Asp yields His), and New York (G15beta113 Val yields Glu), as well as with those of chemically modified hemoglobins Des-Arg(alpha141), Des-His(beta146), NES (on Cys-beta93)-Des-Arg(alpha141), and spin-labeled hemoglobin [Cys-beta93 reacted with N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)iodoacetamide], suggests that the proton in the important hydrogen bond between the tyrosine at C7alpha42 and the aspartic acid at G1beta99, which anchors the alpha1beta2 subunits of deoxyhemoglobin (a characteristic feature of the deoxy quaternary structure), is responsible for the resonance at -9.4 ppm from water at 27 degrees. Another exchangeable proton resonance which occurs at -6.4 ppm from H2O is a spectroscopic indicator of the deoxy structure. A resonance at -5.8 ppm from H2O, which is an indicator of the oxy conformation, is believed to originate from the hydrogen bond between the aspartic acid at G1alpha94 and the asparagine at G4beta102 in the alpha1beta2 subunit interface (a characteristic feature of the oxy quaternary structure). In the spectrum of methemoglobin at pH 6.2 both the -6.4- and the -5.8ppm resonances are present but not the -9.4-ppm resonance. Upon the addition of inositol hexaphosphate to methemoglobin at pH 6.2, the usual resonance at -9.4 ppm is shifted to -10 ppm and the resonance at 6.4 ppm is not observed. In the spectrum of methemoglobin at pH greater than or equal to 7.6 with or without inositol hexaphosphate, the resonance at -5.8 ppm is present, but not those at -10 and -6.4 ppm, suggesting that methemoglobin at high pH has an oxy-like structure. Two resonances (at -8.2 and -7.3 ppm) which remain invariant in the two quaternary structures could come from exchangeable protons in the alpha1beta1 subunit interface and/or other exchangeable protons in the hemoglobin molecule which undergo no conformational changes during the oxygenation process. These exchangeable proton resonances serve as excellent spectroscopic probes of the quaternary structures of the subunit interfaces in studies of the molecular mechanism of cooperative ligand binding to hemoglobin.

Spectral-kinetic heterogeneity in reactions of nitrosyl hemoglobin

When NO replaces CO in hemoglobin A in the presence of inositol hexaphosphate, the time course is heterogeneous in contrast to stripped hemoglobin A, where it is homogeneous. If nitrosyl hemoglobin is mixed with inositol hexaphosphate in the stopped-flow apparatus, an extra spectral change is observed which is the cause of the spectral-kinetic heterogeneity in the CO replacement reaction. At wavelengths isosbestic for this extra spectral change, the time courses show an accelerating rate of CO dissociation. On the other hand, the same reaction for NES-des-Arg hemoglobin (hemoglobin reacted with N-ethylmaleimide and carboxypeptidase B) in the presence of inositol hexaphosphate is homogeneous and slow, and shows isosbesty. High-resolution nuclear magnetic resonance spectra indicate that adult nitrosyl hemoglobin in the presence of inositol hexaphosphate is in the low ligand affinity state, thus offering a structural basis for the acceleration observed in the rate of CO dissociation. Hemoglobin Kansas, in the presence of inositol hexaphosphate, which starts and finishes the reaction in the low affinity state, shows a rate of CO dissociation about nine times faster than stripped hemoglobin A. We conclude from these results that (i) the CO to NO replacement reaction can include a functionally important change in the overall conformation of fully liganded hemoglobin, depending on solution conditions and protein type; (ii) the extra spectral change observed for nitrosyl hemoglobin is not the functionally dominating conformational change, but is a secondary effect within the low ligand affinity protein structure; and (iii) the functional properties of heme ligands are largely controlled by two conformational states of the protein, as seen by nuclear magnetic resonance spectroscopy.

Proton nuclear magnetic resonance studies of ribonuclease A in H 2 O

A resonance (designated a) due to an exchangeable proton titrates (pK(a) = 6.1) between 11.5 and 13 ppm in the nuclear magnetic resonance spectrum of RNase A-0.2 M NaCl in H(2)O at 20 degrees . Comparison with models has permitted assignment to a ring-nitrogen proton of histidine in slow exchange with solvent H(2)O. The pH and temperature-dependent line-width changes of resonance a are analyzed in terms of an exchange between histidine and protonated histidine, without the necessity to invoke any exchange processes associated with protein conformational changes. Several other resonances due to exchangeable protons are observed between 10 and 15 ppm in the nuclear magnetic resonance spectrum of RNase A in H(2)O.