Energetic correlation of oxygen affinities and soret absorption maxima of deoxyhemeproteins

Abstract

The oxygen affinities of hemeproteins have been shown to exhibit a wide range of values [1]. The structural perturbations which influence the oxygen affinity and other properties of these proteins have not been fully elucidated. It has previously been observed that a shift in the positions of the Soret absorption maxima of the α and β chains of hemoglobin is associated with a change in oxygen affinity [2]. A comparison of the physical chemical properties of various monohemeproteins has revealed that a correlation exists between the oxygen affinities and the Soret absorption maxima of the deoxyhemeproteins. Table I shows the P 1 2 values, corresponding t001 O 2 Affinities and Soret Absorption Maxima of Deoxyhemeproteins. Hemeprotein P 1 2 Torr a λ nm Ref. AplysiaMb 2.7 438 1, 3 Horse Mb 0.70 435 4, 5 Sperm Whale Mb 0.51 434 6, 7 Human Hb α chain 0.46 430 8, 2 Human Hb β chain 0.40 428.5 8, 2 Soybean Lb 0.05 427 9, 10 a Pressure of oxygen for one-half oxygenation at pH 7.0 and 20 °C. to the partial pressures of oxygen at one half oxygenation, together with the Soret absorption maxima of the deoxyhemeproteins. Oxygen affinity appears to decrease as the absorption maximum increases. Furthermore, differences in the free energies of binding of the hemeproteins appear to be related to the differences in energies between the Soret absorption maxima. The standard free energy of binding may be given by Δ G° = R T1n P 1 2 The difference between the free energies of oxygen binding to two hemeproteins is then Δ(Δ G°) = R T1n 2 P 1 2 / 1 P 1 2 The energy associated with the Soret absorption is E = hc/λ The molar equivalent of the difference in energies between transitions of two hemeproteins is then ▪ Table II compares these energy differences for the proteins in Table I. The results show that in general the energy differences are strikingly close to one another. The correlation suggests that for these proteins the differences in oxygen affinities and the difference in wavelengths may be dependent on a common structural perturbation. Banerjee et al. [2] have proposed that the O 2 affinity and Soret absorption band are associated with the Fe(II)–histidine axial bond length. Changes in the bond length alter the Fe(II) spin-state equilibrium. It was suggested that alterations in the Fe(II)N bond length change the spin-pairing energy associated with conversion of the high-spin deoxyhemeprotein to the low-spin oxyhemeprotein. Changes in the Fe(II)histidine bond length may result from conformational restraints of the protein structure [11] as evidenced by model studies. [12, 13]. Other structural factors have also been suggested to influence the oxygen affinity of hemeproteins [14]. It has been shown that the oxygen affinity of model complexes increases with the polarity of the heme environment [14, 16]. This observation is consistent with spectroscopic studies which have demonstrated that the heme in soybean Lb is more exposed to solvent than the heme in sperm whale Mb [10, 17]. Likewise, the relative positions of absorption maxima may be indicative of different heme environments as indicated by the effect of solvent absorption maxima of model heme complexes and chromophores [10, 18–20]. Differences between the O 2 affinities of the monoheme-proteins in Table I may, therefore, also be associated with differences in the polarities of the heme environments. These seemingly different explanations may be rationalized by concluding that the polarity of the heme environment effects the Fe(II)histidine bond length and may thus represent an underlying structural parameter which determines the oxygen affinity as well as other physical chemical properties of the heme group. t002 Free Energy of Oxygen Binding and Soret Absorption Energy Differences between Hemeproteins. a2,b Hemeprotein Aplysia Mb HMb SWMb α-chain β-chain Lb Aplysia Mb 799 986 1050 1130 2360 450 602 1210 1450 1680 Horse Mb 799 187 248 331 1560 450 151 764 997 1230 Sperm Whale Mb 986 187 61 143 1370 602 151 613 846 1080 Human Hb α-chain 1050 248 61 83 1310 1210 764 613 233 467 Human Hb β-chain 1130 331 143 83 1230 1450 997 846 233 234 Soybean Lb 2360 1560 1370 1310 1230 1680 1230 1080 467 234 a2 Energy differences are in calories. b The upper number is the energy difference from P 1 2 data and the lower number is the energy difference from absorption maxima.