The Spin‐State Transition of the Hemochrome Non‐equilibrium Conformation in Partially Reduced Human Methemoglobin

Abstract

The effect of external parameters on the relaxation process of the hemochrome-type non-equilibrium conformation in partially reduced methemoglobin has been investigated. The relaxation of the intermediate ferrous low-spin state to the high-spin equilibrium conformation of hemoglobin appears to be facilitated particularly by protons and phosphate ions. In addition to studying the spin-state transition in aquomethemoglobin we have also studied it in complexes of the heme group in methemoglobin with fluoride, azide and cyanide anions.The observation of a spin-state transition of the iron atom after reduction of the ferric heme group in methemoglobin by solvated electrons prompted us to examine this process in more detail. In a preliminary communication we reported that the visible absorbance spectrum observed immediately after the actual reduction process of the heme group revealed a low-spin ferrous heme spectrum which is characteristic for a hemochrorne state [l] T(t) state, the tense quaternary (tertiary) struc-~LII-C: I<" \(ate. a n activated state of the quaternary R structure; d; I and d: 2 , thc low-spin and high-spin state of the ferric ion; d: and d$, the low-spin and high-spin state of the Ferrous ion. Bistris, 2,2~bis(hydroxymethy1)-2,2',2"-nitrilotrietIianol. globin [3]. Previously we suggested [l] that the distal hi\tidyl residue is probably the primary electron acceptor of the solvated electron; subsequently it acts as an electron donor during the electron transfer process ; in the course of the process this residue becomes bound to the iron atom of the heme group and remains bound until the spin-state transition removes it again. This suggests that the spin-state transition will certainly be accompanied by structural changes within the protein conformation.From our studies on the binding kinetics of carbon monoxide [4,5] and oxygen [6] for partially reduced methemoglobin and for fully reduced valency hybrids of hemoglobin (consisting of two identical oxidized subunits and two identical CO-liganded subunits), we obtained information about the quaternary structure of the hemoglobin intermediates. In particular the study of the binding of carbon monoxide to fully reduced valency hybrids suggested the existence of an activated transient R" state which rapidly decays into the quaternary R state or T state. The non-equilibrium R* state has tentatively been ascribed by us to the intermediate hernochrome state [ 5 ] . In order to elucidate the physico-chemical properties of the hernochrome state we started an extensive investigation of the influence that the solute conditions and the ligand bound to the ferric heme group have on the decay rate of this non-equilibrium conformation.

Production of Superoxide from Hemoglobin-Bound Oxygen Under Hypoxic Conditions

“…Since distal histidine is the only nucleophilic side chain in the heme pocket, the possible interaction of the distal histidine with either the iron or with the oxygen bound could facilitate the displacement of the oxygen as a superoxide radical. The possibility that autoxidation proceeds through a peroxy-like intermediate (see above) raises the possibility that distal histidine interactions may facilitate autoxidation by the transfer of electron density directly to the iron-oxygen bond (Raap et al, 1978). Caughey and co-workers (Wallace & Caughey, 1975) have previously postulated such a hemoglobin oxidation process involving the transfer of electrons to oxygen by an exogenous reducing agent, which destabilized the iron oxygen bond.…”

Mechanisms and Prevention of Off-Odor Production and Color Changes in Irradiated Meat

“…Rapp et al (48) reported that solvated electrons attack the distal histidine of methemoglobin, which drives out the ligand at the sixth site to allow hemochrome formation via a covalent bond of the distal histidine to the iron atom. This process is accelerated when a substantial amount of hydroxide anion is present.…”