The spectra of ferric haems and haemoproteins

Phosphorus(V) Tetraazaporphyrin with an Intense, Broad CT Band in the Near-IR Region

In phosphorus tetraazaporphyrins (PTAPs), the Q- and charge-transfer (CT) bands appear as a result of configuration interaction between their excited states. On the basis of this concept, a PTAP with an intense, broad CT band in the near-IR region has been rationally designed and realized by introducing eight diphenylaminophenyl (dPAP) groups. The order of the CT and Q-bands in ascending energy was supported by magnetic circular dichroism (MCD) spectroscopy and theoretical calculations. An intense two-photon absorption was also found in the deep near-IR region.

Synthesis of Cerium Oxide Nanoparticles in a Bacterial Nanocellulose Matrix and the Study of Their Oxidizing and Reducing Properties

A soft synthesis of nanoceria with non-stoichiometric composition (33% Ce3+/67% Ce4+) named CeO2 NPs in bacterial cellulose (BC) matrix in the form of aerogel and hydrogel with controlled CeO2 NPs content was proposed. The advantage of CeO2 NPs synthesis in BC is the use of systemic antacid API–trisamine as a precursor, which did not destruct cellulose at room temperature and enabled a reduction in the duration of synthesis and the number of washes. Moreover, this method resulted in the subsequent uniform distribution of CeO2 NPs in the BC matrix due to cerium (III) nitrate sorption in the BC matrix. CeO2 NPs (0.1–50.0%) in the BC matrix had a fluorite structure with a size of 3–5 nm; the specific surface area of the composites was 233.728 m2/g. CeO2 NPs in the BC-CeO2 NPs composite demonstrated SOD-like activity in the processes of oxidation and reduction of cytochrome c (cyt c3+/cyt c2+), as well as epinephrine to inhibit its auto-oxidation in aqueous solutions by 33–63% relative to the control. In vitro experiments on rat blood showed a decrease in the MDA level and an increase in the activity of antioxidant defense enzymes–SOD by 24% and G6PDH by 2.0–2.5 times. Therefore, BC-CeO2 NPs can be proposed for wound healing as antioxidant material.

Antioxidant Activity of New Copolymer Conjugates of Methoxyoligo(Ethylene Glycol)Methacrylate and Betulin Methacrylate with Cerium Oxide Nanoparticles In Vitro

The synthesis of two new copolymer conjugates of methoxyoligo(ethylene glycol)methacrylate MPEGMA and betulin methacrylate BM was developed via RAFT polymerization. The molar content of BM units was equal to 9–10 and 13–16 mol%, respectively (HPLC, ¹H and ¹³C NMR); molar weights were equal to 75000–115000. CeO₂ NPs as a component of the hybrid material were synthesized for the preparation of the composition with copolymer conjugates of MPEGMA and BM. We showed a significant increase in G6PDH and GR activities by 21–51% and 9–132%, respectively, which was due to the increase in NADPH concentration under the action of copolymers in vitro. The actions of copolymers and CeO₂ NPs combination were stronger than those of the individual components: the SOD activity increased by more than 30%, the catalase activity increased dose-dependently from 13 to 45%, and the GR activity increased to 49%. The maximum increase in enzyme activity was observed for the G6PDH from 54% to 151%. The MDA level dose-dependently increased by 3–15% under the action of copolymers compared with the control, and dose-dependently decreased by 3–12% in samples containing CeO₂ NPs only. CeO₂ NP–copolymer compositions can be used for the design of new biomimetic medical products with controlled antioxidant properties.

"Labile" heme critically regulates mitochondrial biogenesis through the transcriptional co-activator Hap4p in Saccharomyces cerevisiae

Heme (iron protoporphyrin IX) is a well-known prosthetic group for enzymes involved in metabolic pathways such as oxygen transport and electron transfer through the mitochondrial respiratory chain. However, heme has also been shown to be an important regulatory molecule (as "labile" heme) for diverse processes such as translation, kinase activity, and transcription in mammals, yeast, and bacteria. Taking advantage of a yeast strain deficient for heme production that enabled controlled modulation and monitoring of labile heme levels, here we investigated the role of labile heme in the regulation of mitochondrial biogenesis. This process is regulated by the HAP complex in yeast. Using several biochemical assays along with EM and epifluorescence microscopy, to the best of our knowledge, we show for the first time that cellular labile heme is critical for the post-translational regulation of HAP complex activity, most likely through the stability of the transcriptional co-activator Hap4p. Consequently, we found that labile heme regulates mitochondrial biogenesis and cell growth. The findings of our work highlight a new mechanism in the regulation of mitochondrial biogenesis by cellular metabolites.

Experimental Methods for Studying Cellular Heme Signaling

The study of heme is important to our understanding of cellular bioenergetics, especially in cancer cells. The function of heme as a prosthetic group in proteins such as cytochromes is now well-documented. Less is known, however, about its role as a regulator of metabolic and energetic pathways. This is due in part to some inherent difficulties in studying heme. Due to its slightly amphiphilic nature, heme is a "sticky" molecule which can easily bind non-specifically to proteins. In addition, heme tends to dimerize, oxidize, and aggregate in purely aqueous solutions; therefore, there are constraints on buffer composition and concentrations. Despite these difficulties, our knowledge of heme's regulatory role continues to grow. This review sums up the latest methods used to study reversible heme binding. Heme-regulated proteins will also be reviewed, as well as a system for imaging the cellular localization of heme.

Discovery of the magnetic behavior of hemoglobin: A beginning of bioinorganic chemistry

Two articles published by Pauling and Coryell in PNAS nearly 80 years ago described in detail the magnetic properties of oxy- and deoxyhemoglobin, as well as those of closely related compounds containing hemes. Their measurements revealed a large difference in magnetism between oxygenated and deoxygenated forms of the protein and, along with consideration of the observed diamagnetism of the carbonmonoxy derivative, led to an electronic structural formulation of oxyhemoglobin. The key role of hemoglobin as the main oxygen carrier in mammalian blood had been established earlier, and its allosteric behavior had been described in the 1920s. The Pauling-Coryell articles on hemoglobin represent truly seminal contributions to the field of bioinorganic chemistry because they are the first to make connections between active site electronic structure and the function of a metalloprotein.

Langmuir monolayers and thin films of amphifilic thiacalix[4]arenes. Properties and matrix for the immobilization of cytochrome c

Formation and properties of Langmuir films of thiacalix[4]arene (TCA) derivatives containing N-donor groups on the lower rim (Y═O(CH2)3CN; OCH2CN; NH2; OCH2ArCN-p) in 1,3-alternate conformation on aqueous subphase and solid substrates have been studied. Only tetra-cyanopropoxy-p-tert-butylthiacalix[4]arene 1 forms a typical monomolecular layer with perpendicular orientation of the macrocycle relative to the water-air interface that is able to immobilize cytochrome c in the entire range of the surface pressure. Obtained monolayers were transferred by Langmuir-Schaefer technique onto quartz, indium-tin oxide (ITO), and silicon. It was demonstrated that protein activity is retained after immobilization on the substrate.

Phosphorus(v) tetraazaporphyrins: porphyrinoids showing an exceptionally strong CT band between the Soret and Q bands

Phosphorus(v) tetraazaporphyrins which absorb over the complete UV-visible region have been synthesized for the first time.

Unexpected weak magnetic exchange coupling between haem and non-haem iron in the catalytic site of nitric oxide reductase (NorBC) from Paracoccus denitrificans1

Bacterial NOR (nitric oxide reductase) is a major source of the powerful greenhouse gas N2O. NorBC from Paracoccus denitrificans is a heterodimeric multi-haem transmembrane complex. The active site, in NorB, comprises high-spin haem b3 in close proximity with non-haem iron, FeB. In oxidized NorBC, the active site is EPR-silent owing to exchange coupling between FeIII haem b3 and FeBIII (both S=5/2). On the basis of resonance Raman studies [Moënne-Loccoz, Richter, Huang, Wasser, Ghiladi, Karlin and de Vries (2000) J. Am. Chem. Soc. 122, 9344–9345], it has been assumed that the coupling is mediated by an oxo-bridge and subsequent studies have been interpreted on the basis of this model. In the present study we report a VFVT (variable-field variable-temperature) MCD (magnetic circular dichroism) study that determines an isotropic value of J=−1.7 cm−1 for the coupling. This is two orders of magnitude smaller than that encountered for oxo-bridged diferric systems, thus ruling out this configuration. Instead, it is proposed that weak coupling is mediated by a conserved glutamate residue.

Characterization of alkaline transitions in ferricytochrome c using carbon-deuterium infrared probes

The alkaline-induced structural transitions of ferricytochrome c have been studied intensively as a model for how changes in metal ligation contribute to protein function and folding. Previous studies have demonstrated that multiple non-native species accumulate with increasing pH. Here, we used a combination of experiments and simulations to provide a high-resolution view of the changes associated with increasing alkaline conditions. Alkaline-induced transitions were characterized under equilibrium conditions by following changes in the IR absorptions of carbon-deuterium chromophores incorporated at Leu68, Lys72, Lys73, Lys79, and Met80. The data suggest that at least four intermediates are formed as the pH is increased prior to complete unfolding of the protein. The first alkaline transition observed appears to be driven by a single deprotonation and occurs with a midpoint of pH 8.8, but surprisingly, the intermediate formed does not appear to be one of the well-characterized lysine misligates. At higher pH, second and third deprotonations, with a combined apparent midpoint pH of 10.2, induce transitions to Lys73- or Lys79-misligated species. Interestingly, the lysine misligates appear to undergo iron reduction by the coordinated amine. A transition from the lysine misligates to another intermediate, likely a hydroxide-misligated species, is associated with a fourth deprotonation and a midpoint of pH 10.7. Finally, the protein loses tertiary structure with a fifth deprotonation that occurs with a midpoint of pH 12.7. Native topology-based models with enforced misligation are employed to help understand the structures of the observed intermediates.

Tyrosine-heme ligation in heme-peptide complex: design based on conserved motif of catalase

On the basis of evolutionary conservation of sequence in catalases, we have designed a heme-binding peptide (Ac-RLKSYTDTQISR12-(GGGG)-CRIVHC22-NH2) for the 'redox activity modulation' of heme. Heme-binding studies showed a blue-shifted Soret (369 nm) in the presence of TFE and a red-shifted Soret (418 nm) in the absence of TFE. These blue- and red-shifted Sorets suggest ligation through tyrosinate and histidine, respectively. This is the first designed peptide ligating to heme through tyrosine. NMR studies have confirmed that tyrosine ligation to heme in this heme-peptide complex occurs only in the presence of TFE. We suggest that TFE induces helicity in the peptide and brings the arginine and tyrosine in proximity, resulting in ionization of the phenolic side chain of tyrosine. In the absence of TFE, the unstructured peptide lacks the intra-molecular Arg(+)Tyr(-) ion pair, allowing heme binding to histidine. This peptide has significant peroxidase activity though it does not have catalase activity.

De novo Design of ?F -containing Heme-binding Peptides

The structural characterization of de novo designed metalloproteins together with determination of chemical reactivity can provide a detailed understanding of the relationship between protein structure and functional properties. Toward this goal, using the basic scaffold of 1pbz (Rosenblatt et al. (2003) Proc Natl Acad Sci U S A;100:13140) we have designed cyclic DeltaF-containing heme-binding peptides. The alpha- and beta-bands in UV-Vis spectroscopy are indicative of bis-His-ligated heme complex. Most of our DeltaF-containing peptides have more affinity to cobalt(III)Coproporphyrinate-I than heme because cobalt(III)Coproporphyrinate-I contains two additional propionate groups which can have salt bridge interactions with the lysine residues in the peptide. Helicity induction in peptide by DeltaF and aromatic interaction of DeltaF with heme have increased the heme affinity of CP-6-12pbz (cyclic peptide with substitutions of Ala at positions 6 and 12 by DeltaF; 905/mm) compared with 1pbz (279/mm). The nuclear magnetic resonance spectra are indicative of overall helical structure for CP-6-12pbz and CP-6-12pbz in complex with cobalt (III)Coproporphyrinate-I. The descending order of heme affinity in peptides (CP-6-12pbz > CP-12pbz > CP-5-12pbz) indicates that DeltaF at i + 3 or i - 3 from the central H9 favors heme binding but disrupts the same when placed at i - 4.

The nature of the exchange coupling between high-spin Fe(III) heme o3 and CuBII in Escherichia coli quinol oxidase, cytochrome bo3: MCD and EPR studies

Fully oxidized cytochrome bo3 from Escherichia coli has been studied in its oxidized and several ligand-bound forms using electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopies. In each form, the spin-coupled high-spin Fe(III) heme o3 and CuB(II) ion at the active site give rise to similar fast-relaxing broad features in the dual-mode X-band EPR spectra. Simulations of dual-mode spectra are presented which show that this EPR can arise only from a dinuclear site in which the metal ions are weakly coupled by an anisotropic exchange interaction of J 1 cm-1. A variable-temperature and magnetic field (VTVF) MCD study is also presented for the cytochrome bo3 fluoride and azide derivatives. New methods are used to extract the contribution to the MCD of the spin-coupled active site in the presence of strong transitions from low-spin Fe(III) heme b. Analysis of the MCD data, independent of the EPR study, also shows that the spin-coupling within the active site is weak with J approximately 1 cm-1. These conclusions overturn a long-held view that such EPR signals in bovine cytochrome c oxidase arise from an S' = 2 ground state resulting from strong exchange coupling (J > 10(2) cm-1) within the active site.

Expression and characterization of cyanobacterium heme oxygenase, a key enzyme in the phycobilin synthesis. Properties of the heme complex of recombinant active enzyme

An efficient bacterial expression system of cyanobacterium Synechocystis sp. PCC 6803 heme oxygenase gene, ho-1, has been constructed, using a synthetic gene. A soluble protein was expressed at high levels and was highly purified, for the first time. The protein binds equimolar free hemin to catabolize the bound hemin to ferric-biliverdin IX alpha in the presence of oxygen and reducing equivalents, showing the heme oxygenase activity. During the reaction, verdoheme intermediate is formed with the evolution of carbon monoxide. Though both ascorbate and NADPH-cytochrome P450 reductase serve as an electron donor, the heme catabolism assisted by ascorbate is considerably slow and the reaction with NADPH-cytochrome P450 reductase is greatly retarded after the oxy-heme complex formation. The optical absorption spectra of the heme-enzyme complexes are similar to those of the known heme oxygenase complexes but have some distinct features, exhibiting the Soret band slightly blue-shifted and relatively strong CT bands of the high-spin component in the ferric form spectrum. The heme-enzyme complex shows the acid-base transition, where two alkaline species are generated. EPR of the nitrosyl heme complex has established the nitrogenous proximal ligand, presumably histidine 17 and the obtained EPR parameters are discriminated from those of the rat heme oxygenase-1 complex. The spectroscopic characters as well as the catabolic activities strongly suggest that, in spite of very high conservation of the primary structure, the heme pocket structure of Synechocystis heme oxygenase isoform-1 is different from that of rat heme oxygenase isoform-1, rather resembling that of bacterial heme oxygenase, H mu O.

Site-specific independent double labeling of proteins with reporter atoms

Many types of physical, spectroscopic, and biological studies of proteins and other macromolecules are facilitated by the incorporation of reporter groups. In many cases these are single atom substitutes, for example isotopes (13C for C), or light (F for H) and heavy (Se for S) atom homologs. In some circumstances the incorporation of two different labels in the same molecule would be greatly desirable. Commonly used protein engineering methods for incorporating them can rarely cope with differential double labeling, and have other limitations such as universal, non-specific, or random incorporation. Although de novo peptide synthesis has the power to achieve highly specific labeling, the difficulties inherent in creating long sequences lead us to propose protein semisynthesis as the most practical approach. By ligating combinations of natural and labeled synthetic fragments to reform holoproteins, we can overcome any of the limitations discussed. Using cytochrome c as a model protein we show that two reporter atoms, selenium and bromine, can be simultaneously and site-specifically incorporated without significant consequences to structure and (or) function. This capability opens up the prospect of advances in a number of areas in structural biology.

Key words: semisynthesis, peptide synthesis, reporter groups, cytochrome c, structural biology.

Heme-Peptide Models for Hemoproteins. 2. N-Acetylmicroperoxidase-8: Study of the pi-pi Dimers Formed at High Ionic Strength Using a Modified Version of Molecular Exciton Theory

AcMP8 is the Cys-14-acetylated water-soluble heme-octapeptide fragment obtained proteolytically from cytochrome c. Two successive dimerization equilibria are observed with increasing ionic strength in aqueous solution at neutral pH (part 1, preceding article). The electronic spectra of the two pi-pi dimers were extracted from the absorption envelopes at 2.01 and 4.02 M ionic strength and resolved by Gaussian analysis. The principal transitions were assigned using a tailored version of molecular exciton theory based on coupling of the main x- and y-polarized transition dipole moments of the interacting heme groups. The spectra of both pi-pi dimers indicate that the y-polarized exciton states are blue-shifted relative to the excited states of the monomer, while the x-polarized exciton states exhibit a red shift. These shifts were correctly predicted by a simple dipole-dipole coupling model. From an analysis of the resultant transition dipole moments to the exciton states with B(x)()(0,0) and B(y)()(0,0) character and the magnitudes of their red and blue exciton shifts, respectively, we have determined the dipole-dipole interaction geometries for both dimers. The principal difference between the interaction geometry in the first dimer and that in the second is a stronger interaction for the y-polarized transition dipoles and somewhat weakened interaction for the x-polarized transition dipoles. From an analysis of available crystallographic data for porphyrin and metalloporphyrin pi-pi dimers (Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1) and the results of our exciton model, we conclude that the origin of the coordinate system for the Soret transition dipole moments of AcMP8 is not metal-centered. Furthermore, since the true directions of the x- and y-axes of the low-symmetry heme chromophore in AcMP8 are unknown, we have not been able to determine the structures of the pi-pi dimers from a knowledge of their transition dipole-dipole interaction geometries. This study therefore highlights one of the shortfalls of molecular exciton theory.

pH-dependent properties of a mutant horseradish peroxidase isoenzyme C in which Arg38 has been replaced with lysine

Arg38 in the active site of horseradish peroxidase isoenzyme C (HRP-C) has been replaced with lysine by site-directed mutagenesis. As a preclude to a detailed kinetic analysis of this variant, the present study characterizes a pH-dependent cycle of reactions for recombinant horseradish peroxidase isoenzyme C with Arg38 replaced by lysine ([R38K]HRP-C*), which involves time-dependent changes in both specific activity and the electronic absorption spectrum of the enzyme. This pH-dependent cycle resembles that previously suggested for a cytochrome-c peroxidase variant in which Asp235 was replaced with asparagine. When the pH of a solution of resting [R38K]HRP-C* at pH 6.6 (form AH) is raised to pH 8.6, a rapid alkaline transition occurs. This results in spectral changes characteristic of a shift from a predominantly pentacoordinate to a completely hexacoordinate high-spin haem iron (form A-) with a pKa of 7.5. When the pH of a solution of form A- is raised from 8.7 to 12.0, no further spectral changes are observed. The reaction is reversible, but when the high-pH form of the enzyme (A-) is allowed to stand at pH 8.6, it slowly becomes converted into a third enzyme form (form I-) at a rate which is independent of pH (k = 0.56 h-1). When the pH of a sample of form I- is lowered from 8.6 to 6.6, the original low-pH form (AH) of the enzyme is recovered. Recovery of form AH from form I- does not occur via form A-, but via at least one further intermediate, form X. Following a downward pH jump, the rate constant for the formation of form X from form I- shows a small dependence on pH, changing from 48 s-1 at pH 6.8 to 39 s-1 at pH 7.4. The rate of formation of form AH from form X is also pH dependent and biphasic in nature, with measured rate constants ranging from 11.9-2.1 h-1. The possible structures of the different forms of [R38K]HRP-C* are discussed in the light of similar data in the literature for variants of cytochrome-c peroxidase. The properties may be indicative of a greater degree of conformational flexibility within the active site of this mutant caused by the smaller bulk of the lysine side-chain and the probable disruption of a part of the haem-linked hydrogen-bonding network in the distal haem pocket. The wild-type enzyme undergoes no such pH induced changes.

UV resonance Raman and excited-state relaxation rate studies of hemoglobin

We have measured the UV resonance Raman (UVRR) spectra of human methemoglobin fluoride (metHbF) and examined the Raman saturation behavior of the metHbF trytophyl (Trp) and tyrosyl (Tyr) residues. Our high-quality UVRR spectra devoid of Raman saturation with 229- and 238.3-nm CW laser excitation allow us to determine small changes in Trp and Tyr residue Raman band frequencies and intensities caused by the hemoglobin R-T quaternary structural change induced by the allosteric effector inositol hexaphosphate. At 238.3-nm excitation, we observe a ca. 15 and 8% intensity increase for the Trp and Tyr bands, respectively, upon the R-T transition. In contrast, a small intensity decrease is observed with 225-nm excitation. These intensity alterations result from Trp and Tyr absorption and Raman excitation profile red-shifts which correlate with a strong 231.5-nm R-T absorption spectral change. These absorption and Raman excitation profile red-shifts and our model compound absorption studies together suggest a T-state increase in the hydrogen bond donation of the Trp-beta(2)37 and Tyr-alpha(1)42 residues at the alpha 1 beta 2 subunit interface. The Tyr-alpha 42 residue appears to be a hydrogen bond donor, rather than an acceptor. We determined the electronic excited-state relaxation rates of the Trp and Tyr residues in hemoglobin by using Raman saturation spectroscopy with 225-nm pulsed laser excitation. The observed average excited-state relaxation rate of the Trp residues is ca. 1/120 ps and is independent of the quaternary structure. This rate is slower that that observed for Trp residues of horse myoglobin. The average excited-state relaxation rate of the Tyr residues is ca. 1/60 ps for both the R and T quaternary forms. These are the first Tyr relaxation rates measured for any heme protein.

Characterisation and amino acid sequence of cytochrome c-550 from Thiosphaera pantotropha

A cytochrome c-550, with mid-point potential +265 mV, has been purified from Thiosphaera pantotropha. The cytochrome was recognised by antibodies to Paracoccus denitrificans cytochrome c-550, but the two proteins were not immunologically identical. Amino acid sequencing of the cytochrome c-550 showed 85.9% and 95.5% identities, respectively, with the cytochromes c-550 of P. denitrificans and Thiobacillus versutus; these are amongst the highest values reported for similarities between class I c-type cytochromes of the c2 group. These similarities are consistent with the published values of 85% for the overall DNA similarity of P. denitrificans and T. pantotropha, but contrast with published 16S rRNA analyses which indicate identity between T. pantotropha and P. denitrificans and 97.5% similarity of T. versutus with these two organisms. Analysis by plasma-desorption mass spectrometry of the peptide containing the haem-binding motif isolated from the apocytochrome has shown that an Hg atom binds to one or both of the two thiol groups.

The Soret magnetic circular dichroism of ferric high-spin myoglobins. A probe for the distal histidine residue

To find a simple criterion for the presence of the distal (E7) histidine residue in myoglobins and hemoglobins, the Soret magnetic-circular-dichroic spectra were examined for ferric metmyoglobins from various species. A distinct and symmetric dispersion-type curve was obtained for myoglobins containing the distal histidine, whereas a relatively weak and unsymmetric pattern was observed for myoglobins lacking this residue, such as those from three kinds of gastropodic sea molluscs, a shark and the African elephant. The magnetic-circular-dichroic spectra obtained would thus be a direct reflection of the presence or absence of a water molecule at the sixth coordinate position of the heme iron(III), this axial water ligand being stabilized by hydrogen-bond formation to the distal histidine residue. On the basis of these Soret magnetic-circular-dichroic signals, we also examined the structure of a protozoan myoglobin (or a monomeric hemoglobin) from Paramecium caudatum of particular interest for the evolution of these proteins from protozoa to higher animals.

Spectroscopic identification of the haem ligands of cellobiose oxidase

A spectroscopic study of the flavocytochrome b enzyme, cellobiose oxidase, employing optical, NMR, EPR and near infra-red MCD techniques, has identified the axial ligands of the b-type haem. These are a histidine and a methionine, and this ligation set is discussed in relation to the functional role of the haem group.

Spectroscopic studies of Rhodobacter capsulatus cytochrome c' in the isolated state and in intact cells

Ferricytochrome c' from Rhodobacter capsulatus was investigated by 1H-NMR, EPR and optical spectroscopies. A haem-linked ionisation, occurring with a pKa of 8.4 at 25 degrees C, was observed and assigned to the ionisation of the axial histidine ligand by comparison with data for related proteins. At pH values below this pKa the spin-state of the haem Fe3+ is shown to be a quantum mechanically admixed S = 3/2, 5/2 state. Above the pKa the Fe3+ is high-spin. EPR studies of intact cells grown photoheterotrophically reveal that in situ cytochrome c' exists largely in the ferrous state. Upon the addition of [Fe(CN)6]3- the protein becomes oxidised and EPR spectra reveal that the Fe3+ spin-state is a quantum mechanically admixed S = 3/2, 5/2 state. These data indicate that the unusual spin-state of ferricytochrome c' is not a consequence of changes to the protein on its isolation, as had been suggested previously. They also indicate that in situ cytochrome c' is located in an environment with a pH less than 7.

Reversible unfolding of cytochrome c upon interaction with cardiolipin bilayers. 2. Evidence from phosphorus-31 NMR measurements

31P NMR measurements were conducted to determine the structural and chemical environment of beef heart cardiolipin when bound to cytochrome c. 31P NMR line shapes infer that the majority of lipid remains in the bilayer state and that the average conformation of the lipid phosphate is not greatly affected by binding to the protein. An analysis of the spin-lattice (T1) relaxation times of hydrated cardiolipin as a function of temperature describes a T1 minimum at around 25 degrees C which leads to a correlation time for the phosphates in the lipid headgroup of 0.71 ns. The relaxation behavior of the protein-lipid complex was markedly different, showing a pronounced enhancement in the phosphorus spin-lattice relaxation rate. This effect of the protein increased progressively with increasing temperature, giving no indication of a minimum in T1 up to 75 degrees C. The enhancement in lipid phosphorus T1 relaxation was observed with protein in both oxidation states, being somewhat less marked for the reduced form. The characteristics of the T1 effects and the influence of the protein on other relaxation processes determined for the lipid phosphorus (spin-spin relaxation and longitudinal relaxation in the rotating frame) point to a strong paramagnetic interaction from the protein. A comparison with the relaxation behavior of samples spinning at the "magic angle" was also consistent with this mechanism. The results suggest that cytochrome c reversibly denatures on complexation with cardiolipin bilayers, such that the electronic ground state prevailing in the native structure of both oxidized and reduced protein can convert to high-spin states with greater magnetic susceptibility.

Internal intensity standards for heme protein UV resonance Raman studies: excitation profiles of cacodylic acid and sodium selenate

We examine the utility of SO4(2-), ClO4-, cacodylic acid, and SeO4(2-) as internal intensity standards for Raman spectral measurements of protein structure. We find that 0.1 M SO4(2-) and ClO4- perturb the protein tertiary structure of aquomethemoglobin (met-Hb) and its fluoride (met-HbF) and azide (met-HbN3) complexes. Changes occur for the tryptophan near-UV absorption bands, the iron spin state is altered, and the fluoride ligand affinity decreases. Concentrations of ClO4- and SO4(2-) as low as 0.1 M suppress the met-HbF quaternary R----T transition induced by the allosteric effector inositol hexaphosphate (IHP). In contrast, similar concentrations of cacodylic acid and SeO4(2-) show little effect on the hemoglobin tertiary or quaternary protein structures or upon the R----T transition induced by IHP. We measure the Raman cross sections of cacodylic acid and SeO4(2-) between 218 and 514.5 nm and find that for UV excitation they are ca. 5-fold larger than ClO4- or SO4(2-). Thus, cacodylic acid and selenate can be used at lower concentrations. Cacodylic acid and SeO4(2-) are superior Raman internal intensity standards for protein structural studies.

Haem binding to horse spleen ferritin

Horse spleen ferritin, a spherical protein shell of 24 subunits, contains no haem when extracted. This contrasts with ferritins isolated from bacterial sources which have the capacity to bind up to 24 haem groups [(1990) FEBS Lett. 271, 141-143] via two methionine residues [(1990) Nature 341, 771]. Here it is shown that horse spleen ferritin can bind between 15 and 17 haems per 24 subunits with an apparent association constant of 2.2-3.2 x 10(4) M-1. The strength of haem binding appears to be unaffected either by the presence of the core or by the oxidation state of the haem. The demonstration of the ability of animal ferritin to bind haem strengthens the similarity between it and bacterioferritin and could have major consequences for its mechanism of action in physiological iron uptake and release processes.

Bis-methionine axial ligation of haem in bacterioferritin from Pseudomonas aeruginosa

The iron-containing bacterioferritins contain the protoporphyrin IX haem group. It has been established that Escherichia coli cytochrome b1, cytochrome b557 and bacterioferritin are identical. The optical spectra at room temperature of the haem group show it to be predominantly low-spin in both the ferrous and ferric states. The nature of the axial ligands binding the haem group to the polypeptide has, however, remained unknown. Low-spin, bis-coordinate haem centres in proteins typically have a role in rapid electron transfer as redox changes at the metal ion lead to little structural rearrangement. There are only four amino acids with side-chains that have ligand field strengths sufficient to generate the low-spin state of haem, namely, histidine, lysine, methionine and cysteine. Hence there are, potentially, ten different pairs of these four ligands which could be discovered in electron transfer haemoproteins. To date only three have been established with certainty. They are bis-histidine, as in mammalian cytochrome b5, methionine-histidine, typified by cytochrome c and lysine-histidine, recently recognized by spectroscopic methods in cytochrome f. Here we report the electron paramagnetic resonance and near infrared magnetic circular dichroism spectra of the oxidized state of Ps. aeruginosa bacterioferritin which enable the axial ligands to be identified as the thioether side chains of two methionine residues, a ligation scheme not previously reported for haem in any protein.

Horse heart ferricytochrome c: conformation and heme configuration of low ionic strength acidic forms

Resonance Raman, absorption and circular dichroism spectroscopic studies of the stable forms of horse heart ferricytochrome c in the pH range 6-0.8 and at the lowest possible ionic strengths, in water, and at 30 degrees C are reported. The neutral pH form, state III, changes to the acidic pH form, state I, through a three-step process: state III in equilibrium with IIIa in equilibrium with state II in equilibrium with state I, with pKa's of 3.6 +/- 0.3, 2.7 +/- 0.2, and 1.2 +/- 0.2, depending on the monitoring probe, respectively. State IIIa ferricytochrome c is like state III (i.e., with the Met-80-sulfur-iron linkage and a closed heme crevice) but with a higher degree of folding and a slightly larger porphyrin core. State II ferricytochrome c is an unfolded form with an open heme crevice and no Met-80-sulfur-iron linkage. The heme iron is high-spin, and hexacoordinated with weak ligand-field groups, water, and nitrogen of the protonated/hydrogen-bonded imidazole of the His-18 residue at the axial positions. The state I form also lacks the Met-80-sulfur-iron linkage and has an open heme crevice like the state II form; however, it is less unfolded and has a high-spin pentacoordinated heme iron, with the nitrogen of the imidazole of His-18 as the axial ligate, which is out of the porphyrin plane by about 0.45 A.

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.

Characterization of Cu2+ and Fe3+ -mesoporphyrin complexes with histidine-rich glycoprotein: evidence for Cu2+ -Fe3+ -mesoporphyrin interaction

One equivalent of Fe3+ -mesoporphyrin (heme) is coordinated by two axial histidine ligands to a preferred site on histidine-rich glycoprotein (HRG). This study shows that titration of this stochiometric heme.HRG complex with 0-20 equivalents of Cu2+ produces a series of pronounced spectral changes indicative of multiple, sequential alterations of the heme environment. A monotonic low- to high-spin heme transition characterized by a decrease in resonance amplitude at g = 2.99, an increase at g = 6.0, and an increase in absorptivity at 620 nm is induced with the addition of the first 10 Cu2+ equivalents. Furthermore, optical absorption and circular dichroism spectra exhibit isosbestic and isodichroic points throughout the addition of the first 8 and 12 equivalents, respectively. The isosbestic points imply a transition between two optically well defined axial heme coordinations, and the isodichroic points suggest that these axial coordinations also represent two distinct protein conformations. A second isosbestic is formed during the addition of 14-20 equivalents of Cu2+, again suggesting well-defined coordinations; however, changes in the EPR spectra over this range are more complex. Whereas the amount of low-spin (g = 2.99) heme.HRG complex continues to decrease with the addition of 10-20 Cu2+ equivalents, the amount of the high-spin (g = 6.0) complex reaches a maximum near 14 equivalents and decreases markedly thereafter. Of potentially greater significance is the appearance of signals at g = 9.3 (maximum), 7.7 (maximum), 4.8 (crossover), and 1.61 (minimum) after addition of 10 or more Cu2+ equivalents. Some of these signals are similar to those exhibited by cardiac cytochrome c oxidase upon reduction and reoxidation. Thus, even without the addition of exogenous reductants and oxygen, the interaction of Cu2+ with the stoichiometric heme.HRG complex may produce structural features similar to those found in a mechanistically important but poorly understood form of cardiac cytochrome c oxidase.

A change in the heme stereochemistry of cytochrome c upon addition of sodium dodecyl sulfate: electron paramagnetic resonance and electronic absorption spectral study

Electron paramagnetic resonance and electronic absorption spectral changes upon addition of sodium dodecyl sulfate (SDS) to ferric and ferrous cytochrome c have been measured at 77 degrees K and at room temperature. The spectral changes upon addition of SDS to ferric cytochrome c were performed, in two steps, from native low-spin to another low-spin spectrum and subsequently to high-spin-like spectrum. On the other hand, the spectral changes upon addition of SDS to ferrous cytochrome c proceeded, in one step, from native low-spin to high-spin spectrum. The high-spin-like spectrum of ferric cytochrome c and the high-spin spectrum of ferrous cytochrome c in the presence of high concentrations of SDS are, respectively, apparently similar to those of ferric and ferrous cytochrome c' at physiological pH in spectral features. These spectral similarities suggest the similarities in the heme stereochemistry and the ground state of heme iron. Further, the spectra of cytochrome c in the presence of SDS varied with the change of pH values. The ferric high-spin-like and ferrous high-spin spectra were stable at neutral pH and below it. Conformational changes of cytochrome c upon addition of SDS are also discussed.

Spin, oxidation, and ligand states of p-nitrothiophenolatoiron(III) complex of protoporphyrin-IX-dimethylester in the presence of 1-MeIm: magnetic circular dichroism and 1H nuclear magnetic resonance spectroscopic studies

Complex formation of 5-coordinated iron(III) heme containing thiolate anion (p-nitrothiophenol) with imidazole (1-methylimidazole) showed very interesting features depending on the nature of the solvent and the ratio of the ligand to heme. The complexes formed under different conditions were not only low spin iron(III) complexes with a thiolate anion and an imidazole or with two imidazoles, but also reduced (iron(II] complexes with a thiolate and an imidazole or with two imidazoles. Absorption, magnetic circular dichroism, and 1H NMR spectroscopies could identify the complex formed when they were used concurrently. The dependence of polarity of the solvents used on the resultant chemical species was ascribed to the stability of Fe(III) or Fe(II) complex in the different solvents. The iron(III) complex with a thiolate anion and an imidazole was found to be reduced automatically to the iron(II) complex with a thiolate and an imidazole which exchanged ligand to the iron(II) bisimidazoles in the presence of excess imidazole. This study showed that the ligands of heme are easily exchanged and that the heme iron(III) is automatically reduced in several conditions. Possible significance with respect to biological systems containing a sulfur ligand is discussed.

Rate constants for one-electron oxidation by the CF3O2., CCl3O2., and CBr3O2. radicals in aqueous solutions

The peroxyl radicals CF3O2., CCl3O2. and CBr3O2. were produced by radiolysis of aerated aqueous-alcohol solutions of CF3Br, CF3Cl, CCl4 or CBr4. Kinetic spectrophotometric pulse radiolysis experiments were carried out in the presence of various substrates: urate, ascorbate, xanthine, hydroquinone, p-methoxyphenol, phenol and chlorpromazine. Absolute rate constants for one-electron oxidation of these substrates by the alkylperoxyl radicals were found to vary from less than 10(5) to greater than 10(9) M-1 s-1, depending to some extent on the redox potential of the substrate. For all substrates the order of reactivity was CF3O2. greater than CBr3O2. greater than CCl3O2. . Because of its high reactivity, CF3O2., may have deleterious effects on biological systems. Its likely environmental precursor, CF3Br, which is used as a fire extinguisher and a refrigerant, was found to be reduced by a ferrous porphyrin model for cytochrome P-450 only very slowly and thus is not expected to have a major toxic effect if inhaled.