One electron reduction of metmyoglobin and methemoglobin and the reaction of the reduced molecule with oxygen

Reaction Intermediates of Nitric Oxide Synthase from Deinococcus radiodurans as Revealed by Pulse Radiolysis: Evidence for Intramolecular Electron Transfer from Biopterin to FeII-O2 Complex

Nitric oxide synthase (NOS) is a cytochrome P450-type mono-oxygenase that catalyzes the oxidation of l-arginine (Arg) to nitric oxide (NO) through a reaction intermediate N-hydroxy-l-arginine (NHA). The mechanism underlying the reaction catalyzed by NOS from Deinococcus radiodurans was investigated using pulse radiolysis. Radiolytically generated hydrated electrons reduced the heme iron of NOS within 2 μs. Subsequently, ferrous heme reacted with O2 to form a ferrous-dioxygen intermediate with a second-order rate constant of 2.8 × 108 M-1 s-1. In the tetrahydrofolate (H4F)-bound enzyme, the ferrous-dioxygen intermediate was found to decay an another intermediate with a first-order rate constant of 2.2 × 103 s-1. The spectrum of the intermediate featured an absorption maximum at 440 nm and an absorption minimum at 390 nm. In the absence of H4F, this step did not proceed, suggesting that H4F was reduced with the ferrous-dioxygen intermediate to form a second intermediate. The intermediate further converted to the original ferric form with a first-order rate constant of 4 s-1. A similar intermediate could be detected after pulse radiolysis in the presence of NHA, although the intermediate decayed more slowly (0.5 s-1). These data suggested that a common catalytically active intermediate involved in the substrate oxidation of both Arg and NHA may be formed during catalysis. In addition, we investigated the solvent isotope effects on the kinetics of the intermediate after pulse radiolysis. Our experiments revealed dramatic kinetic solvent isotope effects on the conversion of the intermediate to the ferric form, of 10.5 and 2.5 for Arg and NHA, respectively, whereas the faster phases were not affected. These data suggest that the proton transfer in DrNOS is the rate-limiting reaction of the intermediate with the substrates.

Spectroscopic and equilibrium studies of ligand and organic substrate binding to indolamine 2,3-dioxygenase

The binding of a number of ligands to the heme protein indolamine 2,3-dioxygenase has been examined with UV-visible absorption and with natural and magnetic circular dichroism spectroscopy. Relatively large ligands (e.g., norharman) which do not readily form complexes with myoglobin and horseradish peroxidase (HRP) can bind to the dioxygenase. Except for only a few cases (e.g., 4-phenylimidazole) for the ferric dioxygenase, a direct competition for the enzyme rarely occurs between the substrate L-tryptophan (Trp) and the ligands examined. L-Trp and small heme ligands (CN-,N3-,F-) markedly enhance the affinity of each other for the ferric enzyme in a reciprocal manner, exhibiting positive cooperativity. For the ferrous enzyme, L-Trp exerts negative cooperativity with some ligands such as imidazoles, alkyl isocyanides, and CO binding to the enzyme. This likely reflects the proximity of the Trp binding site to the heme iron. Other indolamine substrates also exert similar but smaller cooperative effects on the binding of azide or ethyl isocyanide. The pH dependence of the ligand affinity of the dioxygenase is similar to that of myoglobin rather than that of HRP. These results suggest that indolamine 2,3-dioxygenase has the active-site heme pocket whose environmental structure is similar to, but whose size is considerably larger than, that of myoglobin, a typical O2-binding heme protein. Although the L-Trp affinity of the ferric cyanide and ferrous CO enzyme varies only slightly between pH 5.5 and 9.5, the unligated ferric and ferrous enzymes have considerably higher affinity for L-Trp at alkaline pH than at acidic pH. L-Trp binding to the ferrous dioxygenase is affected by an ionizable residue with a pKa value of 7.3.

Kinetics of the reaction of superoxide anion with ferric horseradish peroxidase

Reaction of horseradish peroxidase A2 and C with superoxide anion (O2-) has been studied using pulse radiolysis technique. Peroxidase C formed Compound I and an oxy form of the enzyme due to reaction of ferric enzyme with hydrogen peroxide (H2O2) and O2-, respectively. At low concentrations of O2- (less than 1 mM), O2- reacted with ferric peroxidase C nearly quantitatively and formation of H2O2 was negligible. The rate constant for the reaction was found to be increased below pH 6 and this phenomenon can be explained by assuming that HO2 reacts with peroxidase C more rapidly than O2-. In contrast the formation of oxyperoxidase could not be detected in the case of peroxidase A2 after the pulse, and only Compound I of the enzyme was formed. Peroxidase A2, however, produced the oxy form upon aerobic addition of NADH, suggesting that O2- can also react with peroxidase A2 to form the oxy form. The results at present indicate that the rate constant for the reaction of O2- with peroxidase A2 is smaller than 103 M-1.s-1.

Parameters controlling the kinetics of ferric and ferrous hemeproteins reduction by hydrated electrons

To clarify the processes of hemeproteins reduction, three classes of these proteins (ferric, ferrous and desFe) were reduced by hydrated electrons generated by pulse radiolysis. Spectral and kinetic investigations were made on alpha hemoglobin chain and myoglobin. Human alpha chain has been chosen to avoid all ferric contaminations and horse ferric myoglobin to eliminate all ferrous protein fractions. We have successively studied the influences of: the iron presence, its oxidation state (II and III), the protein charge and the iron-ligand nature (H2O, OH-, N3- and CN-). For alpha human hemoglobin chain without metallic ion or with ferrous iron, the reduction rates are the same: 1.1 +/- 0.2.10(10) M-1.s-1. In the case of horse ferric myoglobin, the reduction rates depend principally on the protein charge (from pH 6.3 to pH 9.5, the reduction rate of Mb(FeIII)N3- decreases from 2.5 +/- 0.5.10(10) M-1.s-1 to 1.2 +/- 0.2.10(10) M-1.s-1) and are also modulated by the equilibrium constant of the hemeprotein-ligand association (1.2 +/- 0.2.10(10) M-1.s-1 for Mb(FeIII)N3- and 0.8 +/- 0.2.10(10) M-1.s-1 for Mb(FeIII)CN-, at pH 9.8).

Pulse radiolysis kinetics of the reaction of hydrated electrons with ferric-, ferrous-, protoporphyrin IX- and apo-myoglobin

The kinetics of the reaction of hydrated electron (e-aq) with ferric-, ferrous-, metal-free protoporphyrin IX-and apo-myoglobin have been studied by pulse radiolysis so that a direct kinetic measure of the relative reactivities of the heme and the protein part of myoglobin can be made. The second-order association rate constant with ferric Mb is about 3-times that for ApoMb, while ferric Mb, ferrous Mb and protoporphyrin IX-Mb all react at about the same rate, indicating that it is mainly the porphyrin that is the electron-attracting site. The magnitude of the rate constants (8-25 nM-1 X S-1) indicates that the encounter of e-aq with the protein is almost certainly diffusion-controlled. The initial encounter is probably followed by electron migration along parallel paths to the heme and most likely several of the 12 histidine residues. The heme competes very effectively (approx. 70%) with these other sites. The kinetically measured reduction yield of heme is consistent with that found spectrally, indicating that a histidine radical on the protein does not effectively transfer an electron intramolecularly to the heme. The spectral changes found upon the completion of the fast reaction (approx. 40 microseconds) for protoporphyrin IX-Mb and ferrous Mb are consistent with the formation of a porphyrin anion radical. For ApoMb the spectral changes are consistent with the formation of a histidine free radical.

A non-equilibrium state of deoxyhaemoglobin. Temperature-dependence and oxygen binding

After reduction of human methaemoglobin by solvated electrons a non-equilibrium low-spin state of deoxyhaemoglobin is formed which has the characteristic haemochrome spectrum. This haemochrome state is ascribed to a weakly 6-coordinated structure of the haem, which is stabilised by the protonated distal histidine. Oxygen binding is not inhibited by the presence of the weak interaction in the haemochrome state. From the pH dependence of the biphasic behaviour of the oxygen binding a pK of about 8.8 is obtained which is ascribed to the deprotonation of the distal histidine which is in the proximity of a negative ion. A model is proposed to explain the complex spin-equilibria observed in methaemoglobin. The enthalpy of activation of the decay of the haemochrome state is about 53 kJ x mol(-1) and increases to 90 kJ x mol(-1) in the presence of 1 M methanol, indicating a strong interaction between methanol and haemoglobin. Around pH 8.4 the rate constant of the binding of oxygen to the haemochrome state is so high that it may well be diffusion controlled.

Chain inequivalence in bovine methemoglobin

Using pulse radiolysis, a single heme in the tetramer of bovine methemoglobin was reduced within a few microseconds to the ferro state, producing a valence intermediate. The kinetics of oxygen binding to the valence intermediate as well as the re-oxidation of the ferro-heme to the ferric state were studied as a function of pH. The kinetics of the oxygenation revealed the existence of two species, characterized by high and low affinities for oxygen that are associated with two quaternary structures (R and T, respectively). A sigmoidal curve representing a transition between the two states as a function of pH was derived. Above pH 7.7 only the R state could be observed, while below pH 6.5 the T state was dominant. The reaction between the valence intermediate and ferricyanide at pH 7.75 (R state) consisted of two (about) equal contributions (k1 = 23 x 10(4) M-1 S-1; k2 = 2.1 x 10(4) M-1 S-1) attributed to the beta and alpha subunits within the tetramer, respectively. At pH 6.3 (T state) a similar phenomenon was observed (k1 = 69 x 10(4) M-1 S-1; k2 = 3.7 x 10(4) M-1 S-1), indicating chain inequivalences both in the T and the R states of methemoglobin. In the presence of inositol hexakisphosphate the T leads to R transition, as monitored by oxygenation of the valence intermediate, was shifted up to a higher pH by about 0.35. Yet similar rate constants exhibiting similar chain inequivalences have been measured.

Applications of pulse radiolysis to protein chemistry

Since its introduction, pulse radiolysis has been an important technique for examining the properties of organic and inorganic radicals, and for enumerating those reactions responsible for cellular damage by ionizing radiation. Biochemists, and biophysicists outside the area of radiation biology appear, perhaps for historical reasons, to have an incomplete appreciation of the technique's potential. Protein chemists in particular, have been only dimly aware of the numerous reports of, and the significant results obtained from pulse radiolysis studies of proteins. Our purpose here is to bring some of these results together in order to emphasize the power and usefulness of pulse radiolysis experiments both for elucidating enzyme reaction mechanisms, and for gaining information on the structure of proteins in aqueous solutions. Reviews containing related, or in part the same material to be covered here have appeared previously; for example, Land (1970), Adamset al.(1972a), Shafferman & Stein (1975), Adams & Wardman (1977). This review updates these earlier works, but more importantly approaches the topic of protein pulse radiolysis with a different emphasis.

Heterogeneity in the kinetics of oxygen binding to partially reduced human methemoglobin. A pulse-radiolysis study of oxygenated solutions of methemoglobin

The pulse-radiolysis technique has been introduced because it permits a rapid reduction (in a few microseconds) of one heme group of the methemoglobin tetramer by hydrated electrons. The kinetics of the binding of oxygen to this particular valence intermediate (Hb3+) with one reduced alpha or beta subunit has been studied. It appears that the hydrated electrons preferentially reduce one type of subunit of methemoglobin at acid and neutral pH-values as is shown by the biphasic behaviour of Hb3+ on oxygenation. The second-order on-rate constants measured for the binding of oxygen to Hb3+ are 14 +/- 3 mM-1 ms-1 and 56 +/- 9 mM-1 ms-1, respectively. The relative contribution of the faster fraction is about 0.63 +/- 0.08 of the total oxygenation process. A comparison of the kinetic absorbance difference spectrum for the reduction of methemoglobin with the static difference spectrum of deoxyhemoglobin and methemoglobin in the Soret-region revealed a decreased absorbance of the unliganded subunit of Hb3+ at 430 nm. This fact suggests that Hb3+ is in the relaxed quaternary conformation, which is in agreement with the observed on-rate constants.