One- and two-dimensional NMR investigations of the heme pocket in free alpha(CO) chains from human hemoglobin

Two-dimensional nuclear magnetic resonance techniques were used to assign resonances corresponding to heme pocket residues of the isolated alpha(CO) subunits of the human adult hemoglobin (HbA). The assignment procedure was based on the partial identification of the amino acid spin system from the J-correlated (COSY) spectrum and on the nuclear Overhauser effect connectivities (from NOSEY spectra) with the heme substituents. We present here partial assignments corresponding to five amino acid residues: Leu86, Leu-91, Val-93, Leu-101 and Leu-136. Starting from the known crystallographic structure of the alpha subunit in the hemoglobin tetramer, we applied a dipolar model to compute the ring-current shift of the protons from fifteen amino acid residues in the heme pocket. Comparison of the predicted and observed chemical shifts suggests that there is a very close similarity between the heme pocket tertiary structure of the alpha(CO) subunits in crystals of HbA(CO) and of the free alpha(CO) chains. The one-dimensional NMR spectra were used to monitor the pH-induced structural changes, the effects of chemical modification and of ligand substitution. Upon increasing the pH from 5.6 to 9.0 the structure of the heme environment appears to be invariant with the exception of some residues in the CD corner. The structure is also largely conserved when p-chloromercuribenzoate is bound to Cys-104. In contrast, the substitution of CO by O2 as ligand induces many large changes in the heme cavity which can be partially characterized by NMR spectroscopy.     

Reconstitution of human fetal hemoglobin from isolated alpha and gamma chains

The reconstitution of hemoglobin F from isolated alpha and gamma chains was studied. An equimolar amounts of the alpha and gamma chains were mixed and incubated in 10 mM potassium phosphate buffer, pH 7.0, at 25 degrees C. Formation of hemoglobin F in the mixture was measured by separating hemoglobins on a cation exchange HPLC. Time courses of the formation of Hb F were independent of the protein concentration and could be analyzed on an exponential process with a first-order rate constant of (2.0 +/- 0.4) x 10(-3) h-1. Under the experimental conditions the isolated gamma chain existed as tetramer dominantly. These results suggest that the overall reaction of the reconstitution of hemoglobin F is limited by the dissociation step of the self-associated gamma chain.     

NMR characterization of a diamagnetic model of unliganded alpha chains from human hemoglobin.

This paper reports the reconstitution and spectroscopic characterization of a complex between alpha globin from human adult hemoglobin and protoporphyrin IX-Zn(II). Optical and proton one-dimensional (1-D) NMR spectra indicate that the prosthetic group binds in a 1:1 stoichiometry to the apoglobin in a single conformation. Using 2-D proton NMR techniques we assigned resonances corresponding to the majority of porphyrin substituents and to several side chains of amino acids in contact with the porphyrin. Analysis of nuclear Overhauser enhancement interactions between identified protons indicated that the complex contains only one rotation isomer of the prosthetic group. The diamagnetic Zn(II) ion is coordinated to the proximal histidine (His87) and does not bind O2 or CO as a sixth ligand. The ring current effects on protons from the distal valine (Val62) are considerably higher than in the liganded form providing strong evidence for a more compact ligand binding pocket relative to the carbon monoxy state. Therefore, protoporphyrin-Zn(II)/alpha globin complex is a suitable diamagnetic model for unliganded alpha chains and will be used for structure determination by NMR and modeling methods.     

Carbon 13 resonances of 13CO2 carbamino adducts of alpha and beta chains in human adult hemoglobin.

The principal component of normal adult human hemoglobin Ao, was equilibrated under various conditions with 13CO2. In addition, derivatives containing specifically carbamylated NH2-terinal groups in alpha or beta chains, or both, were prepared by treatment with cyanate, and equilibrated likewise to allow the identification of specific resonances observed by 13C nuclear magnetic resonance. In deoxyhemoglobin, a resonanance at 29.2 ppm upfield of external CS2 was assigned to the alpha chain terminal adduct, and one at 29.8 ppm to the beta chain terminal adduct. In the liganded state as the CO derivative, the terminal adduct on both chains showed a common resonance position at 29.8 ppm. Small effects of pH on the resonance positions were observed. Under certain conditions, a resonance was observed at 33.4 ppm, probably not ascribable to a carbamino compound. A carbamino resonance that became prominent at higher pH was found at 28.4 ppm, and is tentatively ascribed to one or more adducts on epsilon amino groups. The beta chain resonances in particular are minimized by the presence of inositol hexaphosphate or 2,3-diphosphoglycerate. Quantitative analysis of the resonance intensities shows that the effects of conversion from the deoxy to the liganded state in reducing the degree of carbamino adduct is much more pronounced for the beta than for the alpha chains.     

Rapid preparation of native alpha and beta chains of human hemoglobin.

1. Current procedures for the isolation of native chains of hemoglobin employ two ion exchange columns for each chain and result in readily autoxidizable chains with measurable contamination by Hb and Hg. 2. In the new procedure, altered buffer conditions on the first column reduce Hb contamination from 2 to 5% to less than 1%, the limit of detectability. 3. The second column and lengthy washes with beta mercaptoethanol are replaced by incubation with DTT for 1 min for alpha chains and, for beta chains, three incubations with DTT and separations by gel-filtration. The residual Hg is less than 0.1%. 4. Oxidations in the previous procedure resulted in low yields and unreliable spectroscopic assessments of bound Hg. The new procedure resulted in a simple UV assay for Hg-free chains. 5. Hemoglobin reconstituted from these oxy-chains was identical to native Hb in oxygen binding equilibria and in the kinetics of CO binding following laser photolysis.     

Assignment of proton resonances in the NMR spectrum of carbonmonoxy hemoglobin beta subunit tetramers

Isolated beta chains from human adult hemoglobin at millimolar concentration are mainly associated to form beta 4 tetramers. We were able to obtain relevant two-dimensional proton nuclear magnetic resonance (NMR) spectra of such supermolecular complexes (Mr approximately 66,000) in the carboxylated state. Analysis of the spectra enabled us to assign the major part of the proton resonances corresponding to the heme substituents. We also report assignments of proton resonances originating from 12 amino acid side chains mainly situated in the heme pocket. These results provide a basis for a comparative analysis of the tertiary heme structure in isolated beta(CO) chains in solution and in beta(CO) subunits of hemoglobin crystals. The two structures are generally similar. A significantly different position, closer to the heme center, is predicted by the NMR for Leu-141 (H19) in isolated beta chains. Comparison of the assigned resonances of conserved amino acids in alpha chains, beta chains and sperm whale myoglobin indicates a close similarity of the tertiary heme pocket structure in the three homologous proteins. Significant differences were noted on the distal heme side, at the position of Val-E11, and on Leu-H19 and Phe-G5 position on the proximal side.     

Carbon dioxide binding to human hemoglobin cross-linked between the alpha chains

The binding of carbon dioxide to human hemoglobin cross-linked between Lys alpha 99 residues with bis(3,5-di-bromosalicyl) fumarate was measured using manometric techniques. The binding of CO2 to unmodified hemoglobin can be described by two classes of sites with high and low affinities corresponding to the amino-terminal valines of the beta and alpha chains, respectively (Perrella, M., Kilmartin, J. V., Fogg, J., and Rossi-Bernardi, L. (1975b) Nature 256, 759-761. The cross-linked hemoglobin bound less CO2 than native hemoglobin at all CO2 concentrations in deoxygenated and liganded conformations, and the ligand-linked effect was reduced. Fitting the data to models of CO2 binding suggests that only half of the expected saturation with CO2 is possible. The remaining binding is described by a single affinity constant that for cross-linked deoxyhemoglobin is about two-thirds of the high affinity constant for deoxyhemoglobin A and that for cross-linked cyanomethemoglobin is equal to the high affinity constant for unmodified cyanomethemoglobin A or carbonmonoxyhemoglobin A. The low affinity binding constant for cross-linked hemoglobin in both the deoxygenated and liganded conformations is close to zero, which is significantly less than the affinity constants for either subunit binding site in unmodified hemoglobin. Comparing the low affinity sites in this modified hemoglobin to native hemoglobin suggests that cross-linking hemoglobin between Lys alpha 99 residues prevents CO2 binding at the alpha-subunit NH2 termini.     

Effect of phosphate on the kinetics of assembly of oxygenated hemoglobin from isolated alpha and beta chains

The kinetics of assembly of oxygenated hemoglobin from isolated alpha and beta chains was investigated under various buffer conditions by use of a circular dichroism (CD) stopped-flow apparatus. The difference CD spectra of hemoglobin against its constituent chains were independent of the buffer conditions, while the time courses of the Soret CD after mixing equimolar amounts of the alpha and beta chains changed with the buffer conditions. The time courses were analyzed on the basis of a scheme which included a monomer-tetramer equilibrium of the beta chain (beta 4 in equilibrium 4 beta), dissociation of the beta 4 (beta 4 leads to 4 beta), and a second-order combination of alpha and beta monomers (alpha + beta leads to alpha beta). The analysis showed that buffer conditions affected the dissociation of the beta 4 rather than the monomer combination: The rate of the dissociation of the beta 4 accelerated with decreasing phosphate concentration, while the rate of the monomer combination was less sensitive to the phosphate concentration. This result indicates that the stability of the beta 4 depends on the phosphate concentration. It was furthermore suggested that the inorganic phosphate was bound to the beta 4 with an association constant of 133 M-1 and a Hill coefficient of 1.2.     

Linear structure of the oligosaccharide chains in alpha1-protease inhibitor isolated from human plasma.

Two glycopeptides present in equal amounts were isolated from a pronase digest of alpha1-protease inhibitor of human plasma by gel filtration on Sephadex G-50 and chromatography on DEAE-cellulose. The carbohydrate side chains in both glycopeptides are linked through asparaginyl residues. The glycopeptides were digested sequentially with specific glycosidases; and after each step, the released sugars as well as the composition of the residual peptides were determined. The linear structures of these glycopeptides deduced from these data are shown below. Based on the total carbohydrate content of the intact protein and with these structural data, it is postulated that 4 oligosaccharide units are attached to 1 molecule of the protein; 2 of these were represented as in Equation 1, the other 2 as in Equation 2.     

Photodissociation of CO and O2 from alpha and beta hemoglobin chains studied by using picosecond absorption spectroscopy.

The picosecond photodissociation of the CO and O2 forms of alpha and beta chains of hemoglobin were studied by following pi pi Soret absorption changes using a Nd3+ phosphate-glass laser, 531-nm pump pulse, 8 ps full width half maximum, and a pump-probe double-beam absorption apparatus. Three intermediates were observed within the first 50 ps after photon absorption. The most notable differences between the two monomers are the extent and rate of geminate recombination with the two ligands. We attribute this result to differences between the tertiary protein structure of the alpha and beta forms of Hb, both distal and proximal.     

Complete assignment of the 1H NMR spectrum of a synthetic zinc finger from Xfin. Sequential resonance assignments and secondary structure

A 25-residue synthetic peptide corresponding to zinc finger 31 of the Xenopus protein Xfin adopts a compact, folded conformation in the presence of zinc. Complete 1H resonance assignments have been made. The peptide contains a helix, beginning as an alpha-helix and ending as a 3(10)-helix, that extends from residue 12 to 23. Several positively charged and polar residues located on this helix are likely to be involved in interactions with DNA. Residues 1-10 appear to adopt a hairpin-like structure.     

Equilibrium oxygen binding to human hemoglobin cross-linked between the alpha chains by bis(3,5-dibromosalicyl) fumarate

Oxygen equilibrium curves of human hemoglobin Ao (HbAo) and human hemoglobin cross-linked between the alpha chains (alpha alpha Hb) by bis(3,5-dibromosalicyl) fumarate were measured as a function of pH and chloride or organic phosphate concentration. Compared to HbAo, the oxygen affinity of alpha alpha Hb was lower, cooperativity was maintained, although slightly reduced, and all heterotropic effects were diminished. The major effect of alpha alpha-cross-linking appears to be a reduction of the oxygen affinity of R-state hemoglobin under all conditions. However, while the oxygen affinity of T-state alpha alpha Hb was slightly reduced at physiologic chloride concentration and in the absence of organic phosphates, KT was the same for both hemoglobins in the presence of 2,3-diphosphoglycerate (or high salt) and higher for alpha alpha Hb in the presence of inositol hexaphosphate. The reduced O2 affinity arises from smaller binding constants for both T- and R-state alpha alpha Hb rather than through stabilization of the low affinity conformation. All four Adair constants could be determined for alpha alpha Hb under most conditions, but a3 could not be resolved for HbAo without constraining a4, suggesting that the cross-link stabilizes triply ligated intermediates of hemoglobin.