The Bohr effect of hemoglobin intermediates and the role of salt bridges in the tertiary/quaternary transitions.

Understanding mechanisms in cooperative proteins requires the analysis of the intermediate ligation states. The release of hydrogen ions at the intermediate states of native and chemically modified hemoglobin, known as the Bohr effect, is an indicator of the protein tertiary/quaternary transitions, useful for testing models of cooperativity. The Bohr effects due to ligation of one subunit of a dimer and two subunits across the dimer interface are not additive. The reductions of the Bohr effect due to the chemical modification of a Bohr group of one and two alpha or beta subunits are additive. The Bohr effects of monoliganded chemically modified hemoglobins indicate the additivity of the effects of ligation and chemical modification with the possible exception of ligation and chemical modification of the alpha subunits. These observations suggest that ligation of a subunit brings about a tertiary structure change of hemoglobin in the T quaternary structure, which breaks some salt bridges, releases hydrogen ions, and is signaled across the dimer interface in such a way that ligation of a second subunit in the adjacent dimer promotes the switch from the T to the R quaternary structure. The rupture of the salt bridges per se does not drive the transition.

Assay of soluble guanylate cyclase activity by isocratic high-performance liquid chromatography.

A HPLC method alternative to labelled or unlabelled procedures was developed for the assay of guanylate cyclase (GC) activity. The substrate (GTP) and the product (cGMP) of the enzymatic reaction were separated in the isocratic mode on a muBondapak C18 column. The activity of GC was linearly dependent on the amount of cGMP produced in the presence of sodium nitroprusside. This approach was applied to follow the purification of GC from bovine lung and to evaluate its stability in different storage conditions.

Functional heterogeneity of the alpha and beta subunits in the association reaction between hemoglobin and carbon monoxide.

A technique is described for the rapid inactivation and removal of excess ferricyanide used for the non-cryogenic oxidation of the unliganded subunits of the intermediates in the association reaction between hemoglobin and carbon monoxide. Under these conditions the asymmetric oxidized intermediates, which dissociate into non-identical dimers, disproportionate into their parent tetramers and four species, Hb+, HbCO, alpha 2+ beta 2CO, alpha 2CO beta 2+, are isolated by non-cryogenic isoelectric focusing. The relative concentrations of species alpha 2CO beta 2+ and alpha 2+ beta 2CO measure the overall distribution of the ligand between the alpha and beta subunits in the association reaction. At 20 degrees C in 0.1 M KCl, pH 7, preferential CO binding to the beta subunits was observed, in agreement with observations made by the cryogenic technique for the isolation of the intermediates [M. Perrella, N. Davids and L. Rossi-Bernardi, J. Biol. Chem. 267 (1992) 8744].

Bohr effect in hemoglobin deoxy/cyanomet intermediates.

The Bohr protons released by oxygen exposure of the unliganded subunits of intermediates (alpha +CN-beta) (alpha +CN-beta) and (alpha beta +CN-) (alpha beta +CN-) were obtained by titrations of concentrated solutions of these species. The Bohr protons released by oxygen exposure of the other intermediates were obtained from titrations of equilibrium mixtures of two parental species, (alpha beta) (alpha beta), (alpha +CN-beta) (alpha +CN-beta), (alpha beta +CN-) (alpha beta +CN-), and (alpha +CN-beta +CN-) (alpha +CN-beta +CN-), in which the concentration of the hybrid intermediate was determined by cryogenic electrophoretic techniques. The Bohr effect of the intermediates was calculated by subtracting the Bohr protons released by oxygen exposure of the intermediates from the total Bohr protons of deoxyhemoglobin at the same pH. The Bohr effects of intermediates (alpha +CN-beta) (alpha beta) and (alpha beta +CN-) (alpha beta) were similar and vanished at pH 8 where the total Bohr effect of deoxyhemoglobin is still significant. This suggests that the Bohr effect in these intermediates is tertiary in the quaternary T structure. The curve of the Bohr effect of intermediate (alpha +CN-beta +CN-) (alpha beta), which was close to the curve obtained by adding the Bohr effects of the two monoliganded intermediates at acidic and physiological pH values, was significantly different from the curve obtained by adding the Bohr effects of one liganded subunit of intermediate (alpha +CN-beta) (alpha +CN-beta) and one liganded subunit of intermediate (alpha beta +CN-) (alpha beta +CN-).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

What the intermediate compounds in ligand binding to hemoglobin tell about the mechanism of cooperativity.

The populations of the intermediates in concentrated solutions of hemoglobin A0 equilibrated at various PCO values, pH 7.0, 0.1 M KCl, and 20 degrees C, have been determined using cryogenic methods. Data on CO saturations and distributions of intermediates were analysed in terms of the free energies of dimer-tetramer assembly of the intermediates (G.K. Ackers and F.R. Smith, Annu. Rev. Biophys. Chem. 16 (1987) 583). The cooperative free energy value of the singly ligated species was approximately one-half the total cooperative energy. The cooperative free energy value of the doubly ligated species was not significantly different from that of carboxyhemoglobin. Because of experimental error, the observed difference in concentrations among the populations of the doubly ligated species cannot be taken as indicative of their functional heterogeneity. Additional studies on some NO intermediates have emphasized that (alpha 1 beta 1)(alpha 2 beta 2)X, a key intermediate in the formulation of the 'third-state' hypothesis in the deoxy/cyanomethemoglobin system, has a free energy value for dimer-tetramer assembly which is critically dependent on the nature of the ligand X as suggested by Ackers and Smith (reference as cited above).

Subunit hybridization studies of partially ligated cyanomethemoglobins using a cryogenic method. Evidence for three allosteric states.

Reaction of tetrameric hemoglobin with ligands at the four heme sites yields nine species that have structurally unique combinations of ligated and unligated subunits. Using hemoglobins where the ligated subunits contain cyanomethemoglobin, Smith and Ackers studied the dimer-tetramer assembly reactions in all nine of the partially ligated species (F. R. Smith and G. K. Ackers, Proc. Natl. Acad. Sci. U.S.A. 82 (1985) 5347). They found a third assembly free energy in addition to those of unligated hemoglobin and fully ligated cyanomethemoglobin. The observed distribution of the three assembly free energies among the ten species was found to be incompatible with the two-state mechanism of allosteric control (J. Monod, J. Wyman and J. P. Changeaux, J. Mol. Biol. 12 (1965) 81). The results indicated a mechanism of 'combinatorial switching' in which the binding free energies per site change with configuration of occupied sites and not just their number. In this study, we have confirmed the existence of three assembly free energies among the ten ligation species using a cryogenic method (M. Perrella and L. Rossi-Bernardi, Methods Enzymol. 76 (1981) 133). For one of the species we find a different free energy assignment from that reported by Smith and Ackers; for all other species we observe the same assignments as in earlier work. The revised distribution also requires a 'combinatorial' mechanism of allosteric switching among the three states.

Gas exchange properties of goat hemoglobins A and C.

Hypoxic or anemic goats with the A hemoglobin genotype switch to the production of hemoglobin C, resulting in a reduced blood oxygen affinity. However, the physiologic consequences of this switch are not clear. We therefore studied the gas exchange properties of the two hemoglobin types. We found that purified hemoglobins A and C have very similar oxygen affinities and H+ Bohr effects, but in the presence of CO2, the affinity of hemoglobin C is substantially less than that of hemoglobin A. That this is not a nonspecific ionic effect is suggested by identical effects of NaCl on O2 binding to the two proteins and by a 2-fold higher capacity of hemoglobin C to bind CO2. The data can be explained by a class of CO2 binding sites in the beta C chain whose affinity is much higher than that of either of the primary sites or of those in Hb A. Our results suggest that in hemoglobin C-containing red cells CO2 acts as a potent allosteric effector, analogous to the role played by 2,3-diphosphoglycerate in human red blood cells. Goat hemoglobin C may have advantages over hemoglobins A or B in O2 transport under hypoxic conditions or in anemia.

BliI, a restriction endonuclease from Bacillus licheniformis.

From Bacillus licheniformis a site-specific restriction endonuclease, named BliI, has been purified and characterized. BliI was able to digest lambda DNA at pH 9.1 over a wide temperature range (25-65 degrees C). Digestion of lambda and psi X174 DNAs with BliI produced banding patterns identical to those seen with HaeIII. Therefore, BliI and HaeIII endonculeases are isoschizomers.

Subzero temperature quenching and electrophoretic methods for the isolation of protein reaction intermediates.

A quenching technique for the study of rapid protein reactions is described which consists of injecting a small volume of aqueous solution of reactants into a large volume (X10) of hydro-organic solvent cooled at subzero temperature and mechanically shaken. The protein reaction intermediates, stabilized at subzero temperature and brought into a hydro-organic solution, can then be separated by subzero temperature electrophoretic methods, such as isoelectric focusing, in the same solvent. The alkaline hydrolysis of 2,4-dinitrophenylacetate was studied by the use of this quenching technique in order to compare the quenching time and the rate constants of the reaction with those obtained by normal rapid quenching methods. It was found that first-order reactions having rate constants up to about 5 s-1 can be satisfactorily studied by this technique. The technique is not suitable for the study of faster reactions because of the high value of the quenching time (40-100 ms). The hybridization reaction of carboxyhemoglobins A and C in aqueous solution at 22 degrees C was studied as an example of the application of this quenching technique and of the isoelectric focusing method at subzero temperature to the isolation of unstable intermediates in a protein reaction.

Isolation of intermediate compounds between hemoglobin and carbon monoxide.

A human hemoglobin solution partially saturated with carbon monoxide was rapidly quenched at -25 degrees C into a hydro-organic buffer containing ferricyanide. Under the experimental conditions of pH, ionic strength, and buffer composition used in this work, it was found that the deoxy hemes were rapidly transformed into their met form, whereas practically no carbon monoxide-bound hemes were oxidized before the separation of the mixture from the oxidizing agent. As a preliminary step to the analysis of the resulting solution, carbonylhemoglobin solutions partially oxidized with ferricyanide were studied by isoelectric focusing at -25 degrees C under identical conditions. The relative position in the gel of all nine possible valence hybrids was established as follows (going from the anodic to the cathodic side of the gel) alpha CO2 beta CO2, (alpha CO beta +)(alpha CO beta CO) (alpha CO beta CO), (alpha CO2 beta +2), (alpha + beta CO), (alpha + beta +)-(alpha CO beta CO), (alpha + beta +)(alpha CO beta +), (alpha +2 beta CO2), (alpha + beta +)(alpha + beta CO), alpha +2 beta +2. When carbonylhemoglobin and methemoglobin were mixed in equal proportion at -25 degrees C and then analyzed by isoelectric focusing at the same temperature, it was found that the contribution of valence hybrids other than alpha CO2 beta CO2 and alpha +2 beta +2 to the total amount of hemoglobin in the gel was no more than 6%. When carbonylhemoglobin and deoxyhemoglobin were mixed in the same proportion and incubated at 20 degrees C so to allow the redistribution of the carbon monoxide molecules between all possible binding sites to occur, a substantially higher amount of valence hybrids, derived from the oxidation of intermediate compounds of hemoglobin with carbon monoxide, was found. The isoelectric focusing separation indicated the presence in the original solution of intermediate species other than carbonylhemoglobin and deoxyhemoglobin at a concentration of about 10% of the total.

Isolation of intermediate valence hybrids between ferrous and methemoglobin at subzero temperatures.

Quenching a hemoglobin solution partially saturated with carbon monoxide into a hydro-organic solvent containing ferricyanide will produce under suitable conditions a population of partially oxidized and CO-bound hemoglobin molecules. Since each Fe3+ heme carries one extra charge, it should be possible, in theory, to resolve the spectrum of intermediate compounds between hemoglobin and carbon monoxide, which was originally present in solution. In this study we report: 1) the development of a simple and rapid method to quench aqueous hemoglobin solutions into a hydro-organic solvent at subzero temperatures; 2) the determination of suitable experimental conditions to isolate valence hybrids between carbonmonoxy- and methemoglobin by isoelectric focusing at temperatures as low as -25 degrees C; and 3) the identification and isolation of all valence hybrids of different charge between carbonmonoxy- and methemoglobin.