Linear free-energy relationships in binding of oxygen and carbon monoxide with heme model compounds and heme proteins

Abstract

For almost a decade heme model compounds have been designed to test the influence of proximal base restraint or of distal steric hindrance upon the ligand affinity of hemoglobins. Despite the variety of molecular structures which have been successively proposed, the evaluation of the reported data is rendered difficult because of the small number of examples available within each series. In this paper we report on the kinetics of binding of oxygen and carbon monoxide with a series of nine closely related heme models. The 'basket-handle porphyrins' allow one to modify the constraints exerted upon a chelated proximal base as well as the chemical environment of the distal side of the heme. One salient feature of these models is the possibility of introducing a hydrogenbond stabilization of the oxygen by incorporating an amide group in the vicinity of the iron centre. The structural changes among models are sufficiently 'soft' to cause an almost continuous variation of the binding constants and rate parameters. The latter are found to obey a definite linear free energy relationship which proves that the series is homogenous from a thermodynamic viewpoint. This suggests an alternative way for comparing the trends in ligand binding in different heme model families with those of heme proteins, which is developed in the discussion using literature dataThe original concepts put forward by Perutz [I, 21 have greatly stimulated the investigations of heme model compounds capable of simulating certain aspects of hemoglobin reactivity. The central problem deals with the existence of a high-affinity form (R) and a low-affinity form (T) of hemoglobin and with the mechanism by which they are converted into each other. The absence of a protein structure in model compounds obviously precludes the elucidation of the trigger mechanism as such, but the idea that steric constraint of the proximal base may lead to reduced ligdnd affinities has been substantiated 13 -81.A more controversial question is concerned with the possibility that distal side steric hindrance might also serve to regulate ligand binding to the heme. The extent of steric hindrance is believed to be larger with linear (CO) than with bent (02) ligands and this process has been proposed to account principally for the kinetic difference between ligands in hemoglobin [9 -1 I].Scveral groups have designed model porphyrins to test either ( o r both) hypotheses. In most cases an extraneous 'proximal' base had to be supplied to the porphyrin in solution, although the advantages of a covalently attached internal proximal base had been recognized earlier [12, 131. In a recent comprehensive review of the various heme niodels, Jameson and Ibers [I41 have discussed CO and O 2 affinities in the light of stereochemical data. One of their Ahh/c~i.itr/iori.s. Unless otherwise stated, the shorthand expression BHP is to be understood as iron(I1) complexes of 'basket-handle porphyrins' 1-9 [18-201. Details concerning the structure of the othcr model compounds discussed in the text ca...

Energy barriers in binding of carbon monoxide and oxygen to heme model compounds.

“…Table 1 suggests that this barrier is mainly entropic, with a ratio of preexponential factors AI(o2)/AI(co) of approximately 10. This is consistent with the higher orientational degeneracy of the oxygenated complex compared to the CO adduct (3,4,22,23), which might be already expressed, in part, in the transition state. Although the numerical accuracy is low, the order of magnitude is correct and suggests that orientation factors, and the corresponding losses of degrees offreedom of the smaller ligand (24), are the principal factors governing binding from within a solvent-free distal pocket.…”

“…12. Photolysis of CO complexes of compounds [1][2][3][4] in the absence or presence of oxygen led to rebinding kinetics that were exponential and pseudo-first order with respect to the concentration of ligands over the whole temperature range (Fig. 2).…”

“…In fact, k1 is a complex rate that can be pictured as resulting from a fictitious barrier determined by the difference offree energy between transition state I and reactants (Fig. 4) 4 (e) at the two extreme working temperatures are displayed. The fraction N(t) of heme molecules that remained unliganded at time t after photodissociation is given in a log-log plot.…”

“…The synthesis of porphyrins 1-4 (13,14), the laser flash apparatus, and the procedures used for studying ligand photodissociation and recombination have been described (4,15,16). Fe(II)-heme-CO complexes were photodissociated (wavelength, 532 nm; pulse duration, 20 nsec), and the rebinding kinetics were recorded on a Tektronix 7834 storage oscilloscope.…”

“…The innermost barrier is produced by the heme, but, in a highly viscous glycerol/water medium, the outermost barrier is due to the solvent cage effect. More elaborate heme models like compounds [1][2][3][4] (Fig. 1) present several important features not found in the bare protoheme.…”