Redox equilibrium of sperm-whale myoglobin, Aplysia myoglobin, and Chironomus thummi hemoglobin

Fluoride binding to characteristic heme-pocket centers: Insights into ligand stability

The studies on the L. pectinata hemoglobins (HbI, HbII, and HbIII) are essential because of their biological roles in hydrogen sulfide transport and metabolism. Variation in the pH could also play a role in the transport of hydrogen sulfide by HbI and oxygen by HbII and HbIII, respectively. Here, fluoride binding was used to further understand the structural properties essential for the molecular mechanism of ligand stabilization as a function of pH. The data allowed us to gain insights into how the physiological roles of HbI, HbII, HbIII, adult hemoglobin (A-Hb), and horse heart myoglobin (Mb) have an impact on the heme-bound fluoride stabilization. In addition, analysis of the vibrational assignments of the met-cyano heme complexes shows varied strength interactions of the heme-bound ligand. The heme pocket composition properties differ between HbI (GlnE7 and PheB10) and HbII/HbIII (GlnE7 and TyrB10). Also, the structural GlnE7 stereo orientation changes between HbI and HbII/HbIII. In HbI, its carbonyl group orients towards the heme iron, while in HbII/HbIII, the amino group occupies this position. Therefore, in HbI, the interactions to the heme-bound fluoride ion, cyanide, and oxygen with GlnE7 via H-bonding are not probable. Still, the aromatic cage PheB10, PheCD1, and PheE11 may contribute to the observed stabilization. However, a robust H-bonding networking stabilizes HbII and HbIII, heme-bound fluoride, cyanide, and oxygen ligand with the OH and NH2 groups of TyrB10 and GlnE7, respectively. At the same time, A-Hb and Mb have moderate but similar ligand interactions controlled by their respective distal E7 histidine.

Iron and redox cycling. Do's and don'ts

A major form of toxicity arises from the ability of iron to redox cycle, that is, to accept an electron from a reducing compound and to pass it on to H₂O₂ (the Fenton reaction). In order to do so, iron must be suitably complexed to avoid formation of Fe₂O₃. The ligands determine the electrode potential; this information should be known before experiments are carried out. Only one-electron transfer reactions are likely to be significant; thus two-electron potentials should not be used to determine whether an iron(III) complex can be reduced or oxidized. Ascorbate is the relevant reducing agent in blood serum, which means that iron toxicity in this compartment arises from the ascorbate-driven Fenton reaction. In the cytosol, an iron(II)-glutathione complex is likely to be the low-molecular weight iron complex involved in toxicity. When physiologically relevant concentrations are used the window of redox opportunity ranges from +0.1 V to +0.9 V. The electrode potential for non-transferrin-bound iron in the form of iron citrate is close to 0 V and the reduction of iron(III) citrate by ascorbate is slow. The clinically utilised chelators desferrioxamine, deferiprone and deferasirox in each case render iron complexes with large negative electrode potentials, thus being effective in preventing iron redox cycling and the associated toxicity resulting from such activity. There is still uncertainty about the product of the Fenton reaction, HO^• or FeO²⁺.

Effect of Bioconjugation on the Reduction Potential of Heme Proteins

The modification of protein surfaces employing cationic and anionic species enables the assembly of these biomaterials into highly sophisticated hierarchical structures. Such modifications can allow bioconjugates to retain or amplify their functionalities under conditions in which their native structure would be severely compromised. In this work, we assess the effect of this type of bioconjugation on the redox properties of two model heme proteins, that is, cytochrome c (CytC) and myoglobin (Mb). In particular, the work focuses on the sequential modification by 3-dimethylamino propylamine (DMAPA) and 4-nonylphenyl 3-sulfopropyl ether (S1) anionic surfactant. Bioconjugation with DMAPA and S1 are the initial steps in the generation of pure liquid proteins, which remain active in the absence of water and up to temperatures above 150 °C. Thin-layer spectroelectrochemistry reveals that DMAPA cationization leads to a distribution of bioconjugate structures featuring reduction potentials shifted up to 380 mV more negative than the native proteins. Analysis based on circular dichroism, MALDI-TOF mass spectrometry, and zeta potential measurements suggest that the shift in the reduction potentials are not linked to protein denaturation, but to changes in the spin state of the heme. These alterations of the spin states originate from subtle structural changes induced by DMAPA attachment. Interestingly, electrostatic coupling of anionic surfactant S1 shifts the reduction potential closer to that of the native protein, demonstrating that the modifications of the heme electronic configuration are linked to surface charges.

Intermediate Tyrosyl Radical and Amyloid Structure in Peroxide-Activated Cytoglobin

We characterized the peroxidase mechanism of recombinant rat brain cytoglobin (Cygb) challenged by hydrogen peroxide, tert-butylhydroperoxide and by cumene hydroperoxide. The peroxidase mechanism of Cygb is similar to that of myoglobin. Cygb challenged by hydrogen peroxide is converted to a Fe⁴⁺ oxoferryl π cation, which is converted to Fe⁴⁺ oxoferryl and tyrosyl radical detected by direct continuous wave-electron paramagnetic resonance and by 3,5-dibromo-4-nitrosobenzene sulfonate spin trapping. When organic peroxides are used as substrates at initial reaction times, and given an excess of peroxide present, the EPR signals of the corresponding peroxyl radicals precede those of the direct tyrosyl radical. This result is consistent with the use of peroxide as a reducing agent for the recycling of Cygb high-valence species. Furthermore, we found that the Cygb oxidation by peroxides leads to the formation of amyloid fibrils. This result suggests that Cygb possibly participates in the development of degenerative diseases; our findings also support the possible biological role of Cygb related to peroxidase activity.

Ferryl derivatives of human indoleamine 2,3-dioxygenase

The critical role of the ferryl intermediate in catalyzing the oxygen chemistry of monooxygenases, oxidases, or peroxidases has been known for decades. In contrast, its involvement in heme-based dioxygenases, such as human indoleamine 2,3-dioxygenase (hIDO), was not recognized until recently. In this study, H(2)O(2) was used as a surrogate to generate the ferryl intermediate of hIDO. Spectroscopic data demonstrate that the ferryl species is capable of oxidizing azinobis(3-ethylbenzothiazoline-6-sulfonic acid) but not L-Trp. Kinetic studies reveal that the conversion of the ferric enzyme to the ferryl intermediate facilitates the L-Trp binding rate by >400-fold; conversely, L-Trp binding to the enzyme retards the peroxide reaction rate by ∼9-fold, because of the significant elevation of the entropic barrier. The unfavorable entropic factor for the peroxide reaction highlights the scenario that the structure of hIDO is not optimized for utilizing H(2)O(2) as a co-substrate for oxidizing L-Trp. Titration studies show that the ferryl intermediate possesses two substrate-binding sites with a K(d) of 0.3 and 440 μM and that the electronic properties of the ferryl moiety are sensitive to the occupancy of the two substrate-binding sites. The implications of the data are discussed in the context of the structural and functional relationships of the enzyme.

Mesohaem substitution reveals how haem electronic properties can influence the kinetic and catalytic parameters of neuronal NO synthase

NOSs (NO synthases, EC 1.14.13.39) are haem-thiolate enzymes that catalyse a two-step oxidation of L-arginine to generate NO. The structural and electronic features that regulate their NO synthesis activity are incompletely understood. To investigate how haem electronics govern the catalytic properties of NOS, we utilized a bacterial haem transporter protein to overexpress a mesohaem-containing nNOS (neuronal NOS) and characterized the enzyme using a variety of techniques. Mesohaem-nNOS catalysed NO synthesis and retained a coupled NADPH consumption much like the wild-type enzyme. However, mesohaem-nNOS had a decreased rate of Fe(III) haem reduction and had increased rates for haem-dioxy transformation, Fe(III) haem-NO dissociation and Fe(II) haem-NO reaction with O2. These changes are largely related to the 48 mV decrease in haem midpoint potential that we measured for the bound mesohaem cofactor. Mesohaem nNOS displayed a significantly lower Vmax and KmO2 value for its NO synthesis activity compared with wild-type nNOS. Computer simulation showed that these altered catalytic behaviours of mesohaem-nNOS are consistent with the changes in the kinetic parameters. Taken together, the results of the present study reveal that several key kinetic parameters are sensitive to changes in haem electronics in nNOS, and show how these changes combine to alter its catalytic behaviour.

A comparison of catalytic site intermediates of cytochrome c oxidase and peroxidases

Compounds I and II of peroxidases such as horseradish peroxidase and cytochrome c peroxidase are relatively well understood catalytic intermediates in terms of their structures and redox states of iron, heme, and associated radical species. The intermediates involved in the oxygen reduction chemistry of the cytochrome c oxidase superfamily are more complicated because of the need for four reducing equivalents and because of the linkage of the oxygen chemistry with vectorial proton translocations. Nevertheless, two of these intermediates, the peroxy and ferryl forms, have characteristics that can in many ways be considered to be counterparts of peroxidase compounds I and II. We explore the primary factors that minimize the generation of unwanted reactive oxygen species products and ensure that the principal enzymological function becomes either that of a peroxidase or an oxidase. These comparisons can provide insights into the nature of biological oxygen reduction chemistry and guidance for the engineering of biomimetic synthetic materials.

Evidence for heme release in layer-by-layer assemblies of myoglobin and polystyrenesulfonate on pyrolitic graphite

Layer-by-layer assemblies of myoglobin and polystyrenesulfonate (PSS) on pyrolitic graphite have been investigated with the goal of determining the origin of the voltammetric response of these films. From the similar midpoint potential, coverage and electron transfer behavior compared with those of adsorbed free heme, it was concluded that the observed voltammetric peak is due to heme adsorbed at the electrode surface. This suggests that the interactions between the pyrolitic graphite electrode, PSS and myoglobin can result in heme release from the protein followed by heme adsorption on the electrode.

Purification and spectroscopic characterization of Ctb, a group III truncated hemoglobin implicated in oxygen metabolism in the food-borne pathogen Campylobacter jejuni

Campylobacter jejuni is a food-borne bacterial pathogen that possesses two distinct hemoglobins, encoded by the ctb and cgb genes. The former codes for a truncated hemoglobin (Ctb) in group III, an assemblage of uncharacterized globins in diverse clinically and technologically significant bacteria. Here, we show that Ctb purifies as a monomeric, predominantly oxygenated species. Optical spectra of ferric, ferrous, O(2)- and CO-bound forms resemble those of other hemoglobins. However, resonance Raman analysis shows Ctb to have an atypical nu(Fe)(-)(CO) stretching mode at 514 cm(-)(1), compared to those of the other truncated hemoglobins that have been characterized so far. This implies unique roles in ligand stabilization for TyrB10, HisE7, and TrpG8, residues highly conserved within group III truncated hemoglobins. Because C. jejuni is a microaerophile, and a ctb mutant exhibits O(2)-dependent growth defects, one of the hypothesized roles of Ctb is in the detoxification, sequestration, or transfer of O(2). The midpoint potential (E(h)) of Ctb was found to be -33 mV, but no evidence was obtained in vitro to support the hypothesis that Ctb is reducible by NADH or NADPH. This truncated hemoglobin may function in the facilitation of O(2) transfer to one of the terminal oxidases of C. jejuni or, instead, facilitate O(2) transfer to Cgb for NO detoxification.

Tendency for oxidation of annelid hemoglobin at alkaline pH and dissociated states probed by redox titration

The redox titration of extracellular hemoglobin of Glossoscolex paulistus (Annelidea) was investigated in different pH conditions and after dissociation induced by pressure. Oxidation increased with increasing pH, as shown by the reduced amount of ferricyanide necessary for the oxidation of hemoglobin. This behavior was the opposite of that of vertebrate hemoglobins. The potential of half oxidation (E1/2) changed from -65.3 to +146.8 mV when the pH increased from 4.50 to 8.75. The functional properties indicated a reduction in the log P50 from 1.28 to 0.28 in this pH range. The dissociation at alkaline pH or induced by high pressure, confirmed by HPLC gel filtration, suggested that disassembly of the hemoglobin could be involved in the increased potential for oxidation. These results suggest that the high stability and prolonged lifetime common to invertebrate hemoglobins is related to their low tendency to oxidize at acidic pH, in contrast to vertebrate hemoglobins.

Assembly and activation of heme-deficient neuronal NO synthase with various porphyrins

The heme prosthetic group of NO synthase is critical for catalytic activity as well as assembly of the enzyme to the native homodimeric form. In the current study, we examined if structurally different metal porphyrins could substitute for the native heme prosthetic group in neuronal NO synthase (nNOS) with regard to assembly and catalysis. We established, with the use of a recently developed in vitro method that functionally reconstitutes heme-deficient apo-nNOS, that Fe-mesoporphyrin IX or Fe-deuteroporphyrin IX can substitute for heme and lead to assembly of a functional nNOS, albeit with lower activity. Fe-protoporphyrin IX dimethyl ester or the metal free protoporphyrin IX, however, lead to minimal assembly of nNOS. Protoporphyrin IX compounds where the native Fe was substituted with Zn, Mn, Co, or Sn lead to assembly of nNOS, but no detectable NO was synthesized in the presence of NADPH and L-arginine. Thus, the presence of the metal and propionic acid groups, but not the vinyl moieties, of heme are important structural features in assembly of nNOS. These studies establish that the mechanism of assembly and catalysis of nNOS can be probed with structurally diverse metal porphyrins.

The 'push' effect of the thiolate ligand in cytochrome P450: a theoretical gauging

The 'push' effect of the thiolate ligand in cytochrome P450 is investigated using density functional calculations. Theory supports Dawson's postulate that the 'push' effect is crucial for the heterolytic O-O bond cleavage of ferric-peroxide, as well as for controlling the Fe(III)/Fe(II) redox process and gating the catalytic cycle. Two energetic factors that contribute to the 'push' effect are revealed. The dominant one is the field factor (DeltaE(field)=54-103 kcal/mol) that accounts for the classical electrostatic repulsion with the negative charge of thiolate. The smaller factor is a quantum mechanical effect (DeltaE(QM)(sigma)=39 kcal/mol, DeltaE(QM)(pi)=4 kcal/mol), which is associated with the sigma- and pi-donor capabilities of thiolate. The effects of ligand replacement, changes in hydrogen bonding and dielectric screening are discussed in term of these quantities. In an environment with a dielectric constant of 5.7, the total 'push' effect is reduced to 29-33 kcal/mol. Manifestations of the 'push' effect on other properties of thiolate enzymes are discussed.

Concentration-dependent effects of anions on the anaerobic oxidation of hemoglobin and myoglobin

The redox potentials of hemoglobin and myoglobin and the shapes of their anaerobic oxidation curves are sensitive indicators of globin alterations surrounding the active site. This report documents concentration-dependent effects of anions on the ease of anaerobic oxidation of representative hemoglobins and myoglobins. Hemoglobin (Hb) oxidation curves reflect the cooperative transition from the T state of deoxyHb to the more readily oxidized R-like conformation of metHb. Shifts in the oxidation curves for Hb A(0) as Cl(-) concentrations are increased to 0.2 m at pH 7.1 indicate preferential anion binding to the T state and destabilization of the R-like state of metHb, leading to reduced cooperativity in the oxidation process. A dramatic reversal of trend occurs above 0.2 m Cl(-) as anions bind to lower affinity sites and shift the conformational equilibrium toward the R state. This pattern has been observed for various hemoglobins with a variety of small anions. Steric rather than electronic effects are invoked to explain the fact that no comparable reversal of oxygen affinity is observed under identical conditions. Evidence is presented to show that increases in hydrophilicity in the distal heme pocket can decrease oxygen affinity via steric hindrance effects while increasing the ease of anaerobic oxidation.

Spectroelectrochemistry of heme proteins: effects of active-site heterogeneity on Nernst plots

In order to detect and model the effect of functional chain heterogeneity on Nernst plots for heme proteins, we examined the redox properties of various myoglobins (Mbs) and their mixtures using an improved spectroelectrochemical method. Specific redox responses and formal half potentials (E1/2) were obtained for Aplysia, horse, and sperm whale Mbs, as well as 1:1 mixtures of Mbs consisting of Aplysia/sperm whale, sperm whale/horse, and horse/Aplysia. Linear Nernst plots with slopes near unity were observed for horse, sperm whale, and Aplysia Mbs, with E1/2 values of 14, 19, and 96 mV (vs. NHE) respectively, consistent with previous reports using indirect methods. The Nernst plot responses for mixtures of some of these Mbs allowed us to evaluate and model the non-Nernstian behavior that results from intrinsically different values of E1/2 and from incomplete spectral overlap. The data demonstrate that increasing the E1/2 differences between the components of a Mb mixture increases the changes in shape of the resulting Nernst plots, the dominant effect being a decrease in the observed Nernst coefficient (nNernst). Comparison of Nernst plots for redox data with Hill plots for O2 binding data shows that the redox process is more affected by the structural differences in the distal heme pockets of the Mbs studied than is O2 binding. Similar effects of chain heterogeneity may give rise to disproportionate reductions in the slopes of Nernst and Hill plots for hemoglobins (Hbs). This possibility is discussed in relation to Hbs investigated for redox and O2 binding activity in our laboratories where we find nNernst to be commonly less than nHill over a range of experimental conditions.

Design and Semisynthesis of Photoactive Myoglobin Bearing Ruthenium Tris(2,2'-bipyridine) Using Cofactor-Reconstitution

A new strategy for semisynthesis of a photoactivatable redox protein is described. Three protohemin molecules with ruthenium tris(2,2'-bipyridine) attached by different spacers were synthesized. The Ru(bpy)(3)-protohemins were incorporated into the heme crevice of apomyoglobin (apo-Mb) to yield semisynthetic Mbs carrying Ru(bpy)(3) as a photosensitizer (Ru(bpy)(3)-Mb). The photoactivation properties and the reaction mechanisms of Ru(bpy)(3)-Mbs were investigated by steady-state photoirradiation and laser flash photolysis. The photoactivation of Ru(bpy)(3)-Mbs was spectrophotometrically demonstrated by comparison with an intermolecular control, namely an equimolar mixture of Ru(bpy)(3) and native Mb. The spacer structure considerably influenced net activation efficiency over a wide pH range as measured by steady-state visible light irradiation and quantum yield. Laser flash photolysis yielded the rate of the photoinduced electron transfer (ET) from the lifetime of the excited Ru(bpy)(3) (k(et) = 4.4 x 10(7) s(-)(1) for Mb(1b) and k(et) = 3.7 x 10(7) s(-)(1) for Mb(1c)) and the back ET rate (k(back) = (2.0-3.7) x 10(7) s(-)(1) for Mb(1b) and k(back) = (1.4-2.4) x 10(7) s(-)(1) for Mb(1c)) from the decay of the transient absorption. These data consistently explained the results of the net photoreaction as follows. (i) The intermolecular control system was less photoactivated because little ET occurred from the excited state of Ru(bpy)(3) to Mb. (ii) The short lifetime of the charge-separated state after photoinduced ET greatly decreased the photoactivation efficiency of Ru(bpy)(3)-Mb with the shortest spacer. (iii) The photochemical and photophysical data of the other two Ru(bpy)(3)-Mb derivatives (the net photoreaction, quantum yield, and ET/back ET rates) were essentially identical, indicating that flexible spacers consisting of oxyethylene units do not rigidly fix the distance between Ru(bpy)(3) and the heme center of Mb. In addition, Ru(bpy)(3)-Mbs were highly photoactivated under aerobic conditions in a manner similar to that under anaerobic conditions, although O(2) usually quenches the photoexcited state of Ru(bpy)(3). This was probably due to the accelerated intramolecular ET from Ru(bpy)(3) to heme, not to O(2) in Ru(bpy)(3)-Mbs. We therefore showed that visible light affects the content of O(2)-bound Mb even in air.

Photoinduced electron transfer from carboxymethylated cytochrome c to plastocyanin

Photoinduced electron transfer from cytochrome c to plastocyanin was investigated using a novel method. Reduced carboxymethylated cytochrome c (CmCyt c), with carbon monoxide bound to the heme iron, and oxidized plastocyanin were mixed. At 1 mM CO the reduced state of CmCyt c is stabilized by about 350 meV. After flash photolysis of CO the apparent redox potential of CmCyt c drops resulting in electron transfer to plastocyanin. The electron transfer characteristics were investigated at approximately 30 different wavelengths in the range 390-460 nm. A global fit of the data showed that the electron transfer rate is 960+/-30 s-1 at pH 7.

Photochemical electron injection into redox-active proteins

A new method is presented that makes it possible to inject electrons rapidly into redox-active proteins by means of a short light flash. Reduced carboxymethylated cytochrome c (CmCyt c) with carbon monoxide bound to the heme iron is mixed with the oxidized acceptor protein. Upon rapid photodissociation of CO the apparent redox potential of CmCyt c drops, resulting in electron transfer to the electron acceptor. In this study we have used mitochondrial cytochrome c oxidase as the acceptor protein, but the method also can be used to investigate electron transfer to other proteins that can interact with cytochrome c. In principle, it can be used with any redox protein into which a CO binding site at the heme iron can be engineered.

Study of the coordination chemistry of prostaglandin G/H synthase by resonance Raman spectroscopy

Resonance Raman spectra of prostaglandin G/H synthase (PGHS) in its ferric and ferrous states have been obtained by Soret excitation. In native PGHS, which contained only 0.25 heme/monomeric apoprotein, the ferric heme was in a high-spin hexacoordinated state. The presence of a vibration at 289 cm-1 that was responsive to H(2)16O -> H(2)18O replacement was taken as evidence for the presence of a H-bonded H2O molecule as the sixth ligand of the Fe. A study, by CD and resonance Raman spectroscopy, of heme incorporation into the apoprotein showed that, for heme/protein ratios lower than 0.5, the heme was in the same ferric high-spin hexacoordinated state as in the native enzyme. For heme/protein ratios higher than 0.5, the concomitant formation of two minor species was observed: a low-spin hexacoordinated species which could be due to the axial coordination of a distal histidine to the Fe trans to its proximal histidine ligand; and a high-spin pentacoordinated species that corresponded to non-specific binding of the heme to the apoprotein. In the reduced state, the heme of PGHS contained a high-spin pentacoordinated Fe(II) with a histidine as the proximal ligand. However, this species shifted spontaneously towards a low-spin hexacoordinated Fe(II) species in which the iron was probably coordinated by a distal histidine as the sixth axial ligand. The PGHS Fe(II).CO derivative displayed an Fe-CO stretching mode at 529 cm-1, which is in the range observed for peroxidases. Such a high frequency could be due to H-bonding between the oxygen atom of the CO ligand and the distal histidine, His207. Since this histidine plays an important role, by coordination of Fe(II) or Fe(III) of PGHS and stabilization of the ligands of the Fe, H2O or CO by H-bonding, it is suggested that this histidine could also play a key role in the cleavage of the O-O bond of peroxides by peroxidases.

Recent advances in the characterization of the hexadecahemic cytochrome c from Desulfovibrio

The biochemical characterization of the high molecular mass cytochromes c (Hmc) isolated from Desulfovibrio vulgaris has led to some controversy as regards their molecular size and subunit structure as well as their heme content and redox properties. Recently developed genetic techniques have made it possible to reach some definite conclusions about the structural and functional properties of the cytochrome. The hexadecahemic Hmc comprises four domains which resemble the tetrahemic cytochrome c3: the structure-function relationship between these multihemic proteins is examined. An hypothesis is discussed according to which the Hmc might be a peripherally interacting protein associated with the outer face of the cytoplasmic membrane, where it might interact with periplasmic proteins - [Fe] hydrogenase - and membrane-bound components of the hmc operon.

The active site structure of E. coli HPII catalase. Evidence favoring coordination of a tyrosinate proximal ligand to the chlorin iron

E. coli produces 2 catalases known as HPI and HPII. While the heme prosthetic group of the HPII catalase has been established to be a dihydroporphyrin or chlorin, the identity of the proximal ligand to the iron has not been addressed. The magnetic circular dichroism (MCD) spectrum of native ferric HPII catalase is very similar to those of a 5-coordinate phenolate-ligated ferric chlorin complex, a model for tyrosinate proximal ligation, as well as of chlorin-reconstituted ferric horseradish peroxidase, a model for 5-coordinate histidine ligation. However, further MCD comparisons of chlorin-reconstituted myoglobin with parallel ligand-bound adducts of the catalase clearly rule out histidine ligation in the latter, leaving tyrosinate as the best candidate for the proximal ligand.

1H NMR study of the role of heme carboxylate side chains in modulating heme pocket structure and the mechanism of reconstitution of cytochrome b5

1H nuclear magnetic resonance spectroscopy was used to assign the hyperfine-shifted resonances and determine the position of a side chain in the heme cavity of wild-type rat apocytochrome b5 reconstituted with a series of synthetic hemins possessing systematically perturbed carboxylate side chains. The hemins included protohemin derivatives with individually removed or pairwise shortened and lengthened carboxylate side chains, as well as (propionate)n(methyl)8-nporphine-iron(III) isomers with n = 1-3 designed to force occupation of nonnative propionate sites. The resonance assignments were effected on the basis of available empirical heme contact shift correlations and steady-state nuclear Overhauser effect measurements in the low-spin oxidized proteins. The failure to detect holoproteins with certain hemins dictates that the stable holoproteins, unlike the case of myoglobin, demand the axial iron-His bonds and cannot accommodate carboxylate side chains at interior positions in the binding pocket. Hence, the heme pocket interior in cytochrome b5 is judged much less polar and less sterically accommodating than that of myoglobin. The propionate occupational preference was greatest as the native 7-propionate site, but also possible at the nonnative crystallographic 5-methyl or 8-methyl positions. Only for a propionate at the crystallographic 8-methyl position was a significant perturbation of the native molecular/electronic structure observed, and this was attributed to an alternative propionate-protein hydrogen bond at the crystallographic 8-methyl position. The structures of the transient protein complexes detected only shortly after reconstitution reveal that the initial encounter complexes during assembly of holoprotein from apoprotein and hemin involve one of the two alternate propionate-protein links at either the 7-propionate or native 8-methyl position. In a monopropionate hemin, this leads to the characterization of a new type of heme orientational disorder involving rotation about a N-Fe-N axis.

Preparation and spectral characterization of the heme d1.apomyoglobin complex: an unusual protein environment for the substrate-binding heme of Pseudomonas cytochrome oxidase

The heme d1 prosthetic group isolated from Pseudomonas cytochrome oxidase combines with apomyoglobin to form a stable, optically well-defined complex. Addition of ferric heme d1 quenches apomyoglobin tryptophan fluorescence suggesting association in a 1:1 molar ratio. Optical absorption maxima for heme d1.apomyoglobin are at 629 and 429 nm before, and 632 and 458 nm after dithionite reduction; they are distinct from those of heme d1 in aqueous solution but more similar to those unobscured by heme c in Pseudomonas cytochrome oxidase. Cyanide, carbon monoxide and imidazole alter the spectrum of heme d1.apomyoglobin demonstrating axial coordination to heme d1 by exogeneous ligands. The cyanide-induced optical difference spectra exhibit isosbestic points, and a Scatchard-like analysis yields a linear plot with an apparent dissociation constant of 4.2 X 10(-5) M. However, carbon monoxide induces two absorption spectra with Soret maxima at 454 or 467 nm, and this duplicity, along with a shoulder that correlates with the latter before binding, suggests multiple carbon monoxide and possibly heme d1 orientations within the globin. The 50-fold reduction in cyanide affinity over myoglobin is more consistent with altered heme pocket interactions than the intrinsic electronic differences between the two hemes. However, stability of the heme d1.apomyoglobin complex is verified further by the inability to separate heme d1 from globin during dialysis and column chromatography in excess cyanide or imidazole. This stability, together with a comparison between spectra of ligand-free and -bound derivatives of heme d1-apomyoglobin and heme d1 in solution, implies that the prosthetic group is coordinated in the heme pocket through a protein-donated, strong-field ligand. Furthermore, the visible spectrum of heme d1.apomyoglobin varies minimally with ligand exchange, in contrast to the Soret, which suggests that much spectral information concerning heme d1 coordination in the oxidase is lost by interference from heme c absorption bands. A comparison of the absorption spectra of heme d1.apomyoglobin and Pseudomonas cytochrome oxidase, together with a critical examination of the previous axial ligand assignments from magnetic resonance techniques in the latter, implies that it is premature to accept the assignment of bishistidine heme d1 coordination in oxidized, ligand-free oxidase and other iron-isobacteriochlorin-containing enzymes.

Glutathione peroxidase and oxidative hemolysis in trout red blood cells

Red blood cells from the trout Salmo irideus contain several hemoglobin components that are prone to oxidation with production of oxygen radicals. The rate of hemolysis has been correlated to the extent of methemoglobin formation. A difference in the rate of hemolysis between red blood cells saturated with either CO or O2 was evident only when diminished glutathione peroxidase activity was observed. These results confirm the important role of this enzyme in providing protection against or repair of oxidative damage to the red cell membrane.