Oxygen binding to fallow-deer (Dama dama) hemoglobin: stepwise enthalpies at pH 7.4

Iron porphyrin-cyclodextrin supramolecular complex as a functional model of myoglobin in aqueous solution

The 1:1 inclusion complex of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato iron(II) (Fe(II)TPPS) and an O-methylated beta-cyclodextrin dimer having a pyridine linker (1) binds dioxygen reversibly in aqueous solution. The O2 adduct was very stable (t(1/2) = 30.1 h) at pH 7.0 and 25 degrees C. ESI-MS and NMR spectroscopic measurements and molecular mechanics (MM) calculations indicated the inclusion of the sulfonatophenyl groups at the 5- and 15-positions of Fe(III)TPPS or Fe(II)TPPS into two cyclodextrin moieties of 1 to form a supramolecular 1:1 complex (hemoCD1 for the Fe(II)TPPS complex), whose iron center is completely covered by two cyclodextrin moieties. Equilibrium measurements and laser flash photolysis provided the affinities ( and ) and rate constants for O2 and CO binding of hemoCD1 (k(O2)(on), k(O2)(off), k(CO)(on), and k(CO)(off)). The CO affinity relative to the O2 affinity of hemoCD1 was abnormally high. Although resonance Raman spectra suggested weak back-bonding of d(pi)(Fe) --> pi(CO) and hence a weak CO-Fe bond, the CO adduct of hemoCD1 was very stable. The hydrophobic CO molecule dissociated from CO-hemoCD1 hardly breaks free from a shallow cleft in hemoCD1 surrounded by an aqueous bulk phase leading to fast rebinding of CO to hemoCD1. Isothermal titration calorimetry furnished the association constant (K(O2)), DeltaH degrees , and DeltaS degrees for O2 association to be (2.71 +/- 0.51) x 10(4) M(-1), -65.2 +/- 4.4 kJ mol(-1), and -133.9 +/- 16.1 J mol(-1) K(-1), respectively. The autoxidation of oxy-hemoCD1 was accelerated by H+ and OH-. The inorganic anions also accelerated the autoxidation of oxy-hemoCD1. The O2-Fe(II) bond is equivalent to the O2.--Fe(III) bond, which is attacked by the inorganic anions or the water molecule to produce met-hemoCD1 and a superoxide anion.

Different functional modulation by heterotropic ligands (2,3-diphosphoglycerate and chlorides) of the two haemoglobins from fallow-deer (Dama dama)

Two haemoglobin components have been identified and purified from fallow-deer (Dama dama) erythrocytes. They are present in similar amounts and the two tetrameric molecules share the same alpha chain, while two different beta chains are detected in the two components. The beta chains differ by 14 residues, even though they both have 145 amino-acid residues, which account for a molecular mass of 16,023 and 16,064 Da, respectively, while alpha chain has 141 residues, yielding a molecular mass of 15,142 Da. Compared with human Hb, the N-terminal region of both beta chains shows deletion of Val beta 1 and the replacement of His beta 2 by a methionyl residue, a modification which is common to most ruminant haemoglobins. Although both isolated components show a low intrinsic affinity for oxygen, meaningful differences between the two haemoglobins have been found with respect to the effect of heterotropic effectors, such as 2,3-diphosphoglycerate and chloride ions. In view of the high sequence homology between the two components, the different effect of heterotropic ligands has been tentatively correlated to possible localized structural variations between beta chains of the two haemoglobin components.

Entropy-driven intermediate steps of oxygenation may regulate the allosteric behavior of hemoglobin

When the oxygen binding isotherms of human, bovine and fallow deer (Dama-Dama) hemoglobins are measured at different temperatures either by optical or calorimetric techniques, analyses according to the Adair's formalism show that at least one of the intermediate steps of ligation has a positive enthalpy change, i.e., absorbs rather than emitting heat, indicating that it is entropy rather than enthalpy driven (Bucci, E., et al. 1991. Biochemistry. 30:3195-3199; Bucci, E., et al. 1993. Biochemistry. 32:3519-3526; Johnson, C., et al. 1992. Biochemistry. 31:10074-10082; Johnson, C., et al. 1995. Biophys. Chem. 59:107-117). This phenomenon is confirmed in systems in which the beta82 lysines of human hemoglobin are covalently cross-linked by acylation with dicarboxylic acids of increasing length, namely the fumaryl (four-carbon-long), adipoyl (six-carbon-long), and sebacoyl (10-carbon-long) residues. Consistently in all of the systems here reported, the enthalpy excursions are masked by compensatory entropy changes, which keep the free energy of ligand binding constant for the first three steps of oxygenation. Furthermore, the cooperativity index and the overall oxygen affinity seem to be correlated to the positive enthalpy excursions of the intermediate steps of ligation. Fumaryl-Hb (hemoglobin cross-linked with a fumaryl residue, four carbons) with the lowest absorption of heat has the highest affinity and lowest cooperativity index. Adipoyl-Hb (hemoglobin cross-linked with an adipoyl residue, six carbons) has the highest absorption of heat and the highest cooperativity index. It appears that nonuniform heat release by the intermediates of oxygenation is part of the allosteric phenomena in hemoglobin systems. There is not enough information that would allow assigning these phenomena to the interplay of the various conformations described for hemoglobin besides the classic T (Fermi et al. 1984. J. Mol. Biol. 175:159-174) and R (Shanaan. 1983. J. Mol. Biol. 171:31-59), as listed at the end of the Discussion. The possibility cannot be excluded that entropy-driven steps characterize new conformational transitions still to be described.

Temperature titration: a new approach to the thermodynamics of oxygen binding to hemoglobin

A cell was constructed in order to study hemoglobin's reaction with gaseous ligands. The temperature of the hemoglobin sample is systematically altered within a given temperature range (275-310 degrees K), while the percentage of oxygen in the equilibrating gas is kept constant. The equilibration time of the sample at each temperature step depends on sample concentration, ligand affinity, and absolute temperature; in most cases, the equilibration time is on the order of minutes. The construction of the optical compartment allows the experimenter to vary the optical pathlength using specially designed spacers, thus making it possible to study hemoglobin-ligand interactions over a wide range of protein concentrations (0.1-200 mg/ml). Optical glass is used in the construction of the cuvette in order to optimize its optical stability over a long period of time. At equilibrium the absorption spectrum of the sample is collected and decomposed into the relative contributions of oxy-Hb, deoxy-Hb, and ferric-Hb, thus revealing the fraction of oxyhemoglobin as well as any baseline drifts and protein degradation. Temperature steps of 1 degree K are already sufficient to change the absorption spectra in a significant way. This type of setup is also advantageous in that the experimenter can change the sample at any point (temperature) without having to restart the entire experiment. This makes it possible to study the oxygen binding characteristics of unstable hemoglobins. Analyses of the binding curves obtained with this technique immediately yield the overall oxygen binding constants beta i together with the respective standard enthalpies delta H(i).