Linear electric field effect in electron paramagnetic resonance for two bisimidazole-heme complexes, model compounds for B and H hemichromes of hemoglobin and for cytochrome b₅

Abstract

Bisimidazole-ferric heme is considered to be the structure at the heme site of cytochrome b5 and two different low spin ferric hemochromes spontaneously formed from ferric hemoglobin. The addition of strong base to bisimidazole-ferric heme in organic solvents alters the optical and magnetic properties of this compound. With the use of the linear electric field effect in the electron paramagnetic resonance, we demonstrate that addition of base does not lead to the exchange of hydroxide anion for ligated imidazole and that the bisimidazole structure is retained. Analysis of optical titrations indicates that 2 equiv of base react reversibly with bisimidazole-ferric heme. It is suggested that the two hemichromes formed from hemoglobin differ in structure from one another by the state of protonation of N-1 in the bound imidazoles.

Redox control of proton transfers in membrane b-type cytochromes: an absorption and resonance Raman study on bis(imidazole) and bis(imidazolate) model complexes of iron-protoporphyrin

“…1 C) We therefore can safely attribute the change in visible spectra accompanying the alkaline titration of Fe(III)PP(ImH)2 in aqueous CTABr solution to a sequential deprotonation of the two bound imidazoles, with pKa values of 9.0 and 10.8 (reaction (4)). As expected, the maxima of the visible absorption bands of the (ImH)(Im-) and bis (Ira-) complexes of ferriporphyrins are redshifted relative to those of the corresponding imidazole complexes (Mohr et al 1967;Davies 1973;Peisach and Mims 1977;Nappa et al 1977;Quinn et al 1982). Moreover, the spectral data of Fig.…”

“…In all hemoproteins of this class for which crystallographic structures are available, the metal-bound histidylimidazole ring is found to be hydrogen bonded to an electronegative group of the polypeptide chain (Mathews et al 1972a, b;Salemme et al 1973;Takano and Dickerson 1981;Pierrot et al 1982). The possibility that this hydrogen bond might play a functional role has been proposed by Peisach and coworkers (Peisach et al 1973;Peisach and Mims 1977) and, later, by Valentine and coworkers Swartz et al 1979). In particular, the influence of the hydrogen bond on histidylimidazole has been invoked as inducing a differential stabilization of oxidation states of cytochromes ).…”

Heterogeneity in the Kinetics of Oxygen Binding to Partially Reduced Human Methemoglobin

“…Thereafter the solutions were flushed very gently for one hour with a mixture of argon and oxygen obtained from a gas-mixing pump (Wosthoff M 3001 a-F). Following this procedure no hemichrome formation was observed, even in the presence of l M methanol [21]. The oxygen concentration was calculated using a solubility coefficient of l .64 pM/mmHg at room temperature 20…”

Coupling between oxidation state and hydrogen bond conformation in heme proteins

“…In those proteins for which high resolution x-ray crystal structural data are available, the metal-bound imidazole rings are invariably found to be hydrogen bonded to electronegative groups on the apoprotein (2-4). The possibility that this hydrogen bond might link the tertiary structure of the protein with the reactivity of the metal has been recognized by several investigators, particularly in the case of heme proteins (5)(6)(7)(8)(9)(10)(11)(12) in which the electronegative group is a backbone carbonyl oxygen. It is well known from studies of model compounds and of imidazole complexes of methemoglobin and metmyoglobin that the pKa of imidazole can drop several pH units upon complexation with a metal ion (1) [imidazole pKa = 14 (1), metmyoglobin imidazole pKa t 10 (13)].…”