"Tension" on heme by the proximal base and ligand reactivity: conclusions drawn from model compounds for the reaction of hemoglobin.

Abstract

The kinetic data on model compounds of hemoglobin indicate that in oxy derivatives ligand dissociation rates are sensitive to the "tension" exerted by the proximal base on the metal-to-ligand bond;-the corresponding rates for carboxy derivatives are not sensitive to the tension. It is suggested that the metal-to-ligand bond becomes weaker with increased "pull" (or tension) on Fe from the proximal base due to the steric and/or electronic interaction between the ligand, the porphyrin ring, and the proximal base. In model compounds the linear heme Fe-to-CO bond vis-a-vis the bent heme Fe-to-02 bond probably makes such interactions less significant in carboxy derivatives. It is proposed that the kinetic a,#-chain nonequivalence in Hb4(02)4 is due to the difference in the tension in the two chains on Fe by the proximal base. The absence of a,#-chain differences large enough to show up in CO dissociation rates from Hb4(CO)4 is explained on the basis of lack of sensitivity of the Fe-CO bond to tension from the proximal base. The implications of the results for the observed cooperative effects in ligand combination (for CO) and dissociation (for 02 and NO) rates of hemoglobin have also been discussed.The stereochemical mechanism for cooperativity proposed by Perutz (1) rationalizes a large amount of data on the functional behavior of hemoglobins. From its inception the model was in disagreement with the kinetic results regarding the relative order of reactivity of a and # chains. Since then, the presence of a water molecule in the heme pocket of the a chains has been demonstrated, and on this basis the reactivity of a chains might well be lower than that of fl chains (2); this is consistent with the kinetic data (3). The kinetic differences in a and ft chains are either small or absent in buffers of low ionic strength but become significant in more concentrated phosphate buffers. For human hemoglobin (Hb A), the 02 off rates for the two chains differ by a factor of two; for pig and rabbit hemoglobins they differ by a factor of seven (4, 5). Recent studies on nitrosylhemoglobin show rather large chain differences in NO off rates in the presence of phosphate, particularly inositol hexaphosphate (inositol-P6) (unpublished data). When the ligand is CO, the a,#-chain differences are observed in the ligand combination rates but not in CO off rates. Details of the mechanism by which organic phosphates generate or accentuate the chain differences are not very well understood. Is it the narrowing or steric crowding of the heme pocket on the binding of phosphates and phosphate analogs that results in the accentuated kinetic nonequivalence of a and A chains? Or is it the difference in the "tension" on iron from the proximal base that results in different ligand dissociation and combination rates in the two chains. Or are both factors responsible?The cooperativity in the reactions of hemoglobin, if considered in terms of ligand combination and dissociation rates, is highly dependent on the nature of the ligand. Thus, whi...

Nitric oxide and carbon monoxide equilibriums of horse myoglobin and (N-methylimidazole)protoheme. Evidence for steric interaction with the distal residues

“…6 Mesoheme mono-4-(l-imidazolyl)butylamide monomethyl ester (Sharma et al, 1978;Geibel et al, 1978). ® FeIIPP(NMeIm)NO.…”

Control and pH dependence of ligand binding to heme proteins

“…The pH dependence implies that the charge on His E7 can change the activation barrier by 2-4 kJ/mol. The globin may affect the barrier by restricting the motion of the heme group or by "pulling" on the imidazole-iron bond (Sharma et al, 1978;Traylor, 1981). Comparison of j3A with /3HgBz0 and with Mb indicates that interaction of the globin with the iron and the porphyrin through the covalent iron-histidine bond and through hydrogen bonds can contribute as much as 6 kJ/mol to the barrier.…”

Synthesis, Kinetic Study, and Spectroscopic Analysis of Peroxidase-like Pinch-Porphyrin Fe(III) Complexes

“…The cc values for species A were between 4.3 and 4.05. 11,20 Values of g ⊥ = 6 and g ∥ = 2 are reported to belong to the pure high spin, s = 5/2, while the signals with g ⊥ = 4 and g ∥ < 2 value would correspond to the pure intermediate spin, s = 3/2. Signals from the species with s = 1/2 are not analyzed in the present work because high spin s = 5/2 and low spin s = 1/2 compounds have been largely studied.…”

“…In a low spin octahedral environment, the unpaired electron is initially assigned to d x 2 – y 2 and partially delocalized in the d xy orbital. 11 It is well known that if the native protein horseradish peroxidase (HRP) lacks a substituent in the sixth coordination position, the energy gap of the nondegenerated d x 2 – y 2 and d z 2 orbitals is narrower. However, the coordination of a sixth ligand can result in the displacement of the Fe ion located nearest to the porphyrin plane.…”

“…The electronic arrangement of Fe­(III) corresponds to the five unpaired 3 d electrons, giving various possible spin states such as low spin s = 1/2, the not observed intermediate spin s = 3/2, high spin s = 5/2, and the quantum mixed spin (qms) s = 3/2, s = 5/2. In a low spin octahedral environment, the unpaired electron is initially assigned to d x 2 – y 2 and partially delocalized in the d xy orbital . It is well known that if the native protein horseradish peroxidase (HRP) lacks a substituent in the sixth coordination position, the energy gap of the nondegenerated d x 2 – y 2 and d z 2 orbitals is narrower.…”