Structure–Function of the Cytochrome b6f Complex†

The structure and function of the cytochrome b₆ f complex is considered in the context of recent crystal structures of the complex as an eight subunit, 220 kDa symmetric dimeric complex obtained from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. A major problem confronted in crystallization of the cyanobacterial complex, proteolysis of three of the subunits, is discussed along with initial efforts to identify the protease. The evolution of these cytochrome complexes is illustrated by conservation of the hydrophobic heme‐binding transmembrane domain of the cyt b polypeptide between b₆ f and bc₁ complexes, and the rubredoxin‐like membrane proximal domain of the Rieske [2Fe‐2S] protein. Pathways of coupled electron and proton transfer are discussed in the framework of a modified Q cycle, in which the heme c_n, not found in the bc₁ complex, but electronically tightly coupled to the heme b_n of the b₆ f complex, is included. Crystal structures of the cyanobacterial complex with the quinone analogue inhibitors, NQNO or tridecyl‐stigmatellin, show the latter to be ligands of heme c_n, implicating heme c_n as an n‐side plastoquinone reductase. Existing questions include (a) the details of the shuttle of: (i) the [2Fe‐2S] protein between the membrane‐bound PQH₂ electron/H⁺ donor and the cytochrome f acceptor to complete the p‐side electron transfer circuit; (ii) PQ/PQH₂ between n‐ and p‐sides of the complex across the intermonomer quinone exchange cavity, through the narrow portal connecting the cavity with the p‐side [2Fe‐2S] niche; (b) the role of the n‐side of the b₆ f complex and heme c_n in regulation of the relative rates of noncyclic and cyclic electron transfer. The likely presence of cyclic electron transport in the b₆ f complex, and of heme c_n in the firmicute bc complex suggests the concept that hemes b_n‐c_n define a branch point in bc complexes that can support electron transport pathways that differ in detail from the Q cycle supported by the bc₁ complex.     

Proline-40 is Essential to Maintaining Cytochrome b_5''''s Stability and Its Electron Transfer with Cytochrome c

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A Novel Role of Vesicles as Templates for the Oxidation and Oligomerization of p‐Aminodiphenylamine by Cytochrome c

The oxidation and subsequent oligo‐ or polymerization of aniline or the N‐C‐para coupled aniline dimer p‐aminodiphenylamine (N‐phenyl‐1,4‐phenylenediamine) with H₂O₂‐dependent heme peroxidases in aqueous medium at pH = 4.3 is strongly influenced in a positive way by the presence of anionic polymers, micelles or vesicles as soft ‘templates’ (macromolecular or polymolecular additives), to yield products which resemble the emeraldine salt form of polyaniline (PANI‐ES). The positive effect the templates exert on the reaction mainly originates from interactions between the templates and the monomers, reaction intermediates and products, whereby the reaction occurs localized in the vicinity of the templates, suppressing undesired side reactions. As shown in the present work, the templates may even play an additional role, depending on the type of catalyst. Through interactions between the heme protein cytochrome c and anionic vesicles, cytochrome c gains increased peroxidase activity. In this way, the templates not only serve for hosting the oxidation and oligo‐ or polymerization reactions for obtaining PANI‐ES type products, but also simultaneously activate the catalyst in order to trigger the reaction.     

Cytochrome c‐Capped Fluorescent Gold Nanoclusters: Imaging of Live Cells and Delivery of Cytochrome c

Cytochrome c‐capped fluorescent gold nanoclusters (Au‐NCs) are used for imaging of live lung and breast cells. Delivery of cytochrome c inside the cells is confirmed by covalently attaching a fluorophore (Alexa Fluor 594) to cytochrome c‐capped Au‐NCs and observing fluorescence from Alexa 594 inside the cell. Mass spectrometry studies suggest that in bulk water, addition of glutathione (GSH) to cytochrome c‐capped Au‐NCs results in the formation of glutathione‐capped Au‐NCs and free apo‐cytochrome c. Thus glutathione displaces cytochrome c as a capping agent. Using confocal microscopy, the emission spectra and decay of Au‐NCs are measured in live cells. From the position of the emission maximum it is shown that the Au‐NCs exist as Au₈ in bulk water and as Au₁₃ inside the cells. Fluorescence resonance energy transfer from cytochrome c–Au‐NC (donor) to Mitotracker Orange (acceptor) indicates that the Au‐NCs localise in the mitochondria of live cells.     

A spectroelectrochemical study of carboxymethylated cytochrome-c

Direct electrochemical reduction of dicarboxymethylated cytochrome c [(Cm)-cyt c] has been carried out to investigate the effect of the displacement of methionine 80 (Met 80) from the sixth coordination position on the redox potential of heme iron. Differential pulse and cyclic voltammetry were employed in the present study. At gold microelectrodes or gold-coated RVC electrodes, the reduction process occurs in the absence of mediators. The rate of heterogeneous charge transfer for (Cm)-cyt c appears to be greater than that operative in native cytochrome c. A value of −0.218 versus s.h.e. at neutral pH was determined for the formal standard potential (U′_0,7) from spectroelectrochemical measurements.The large change in U′₀ of the modified cytochrome c with respect to the native protein (ΔU ⋍0.5 V) is in agreement with rearrangement of the tertiary structure induced by rupture of the Met 80-heme iron bond, which leads to a high degree of heme exposure to the solvent. In addition, the results support the important role of Met 80 in assuring stability, through its bond with the heme iron, to the close crevice structure.     

Control of the Electronic Ground State on an Electron‐Transfer Copper Site by Second‐Sphere Perturbations

The Cu_A center is a dinuclear copper site that serves as an optimized hub for long‐range electron transfer in heme–copper terminal oxidases. Its electronic structure can be described in terms of a σ_u* ground‐state wavefunction with an alternative, less populated ground state of π_u symmetry, which is thermally accessible. It is now shown that second‐sphere mutations in the Cu_A containing subunit of Thermus thermophilus ba₃ oxidase perturb the electronic structure, which leads to a substantial increase in the population of the π_u state, as shown by different spectroscopic methods. This perturbation does not affect the redox potential of the metal site, and despite an increase in the reorganization energy, it is not detrimental to the electron‐transfer kinetics. The mutations were achieved by replacing the loops that are involved in protein–protein interactions with cytochrome c, suggesting that transient protein binding could also elicit ground‐state switching in the oxidase, which enables alternative electron‐transfer pathways.     

Study on the Gas Phase Stability of Heme-binding Pocket in Cytochrome Tb_5 and Its Mutants by Electrospray Mass Spectrometry

Ｃｙｔｏｃｈｒｏｍｅｂ5(Ｃｙｔｂ5)ｉｓｆｏｕｎｄｂｏｔｈａｓａｃｏｍｐｏ ｎｅｎｔｏｆｔｈｅｍｉｃｒｏｓｏｍａｌｍｅｍｂｒａｎｅｓａｎｄａｓａｓｏｌｕｂｌｅｆｏｒｍｉｎｅｒｙｔｈｒｏｃｙｔｅｓ .Ｉｔｐｌａｙｓａｎｉｍｐｏｒｔａｎｔｒｏｌｅｉｎｂｉｏｌｏｇｉｃａｌｓｙｓｔｅｍｓ ,ｉｎｗｈｉｃｈＣｙｔｂ5ｆｕｎｃｔｉｏｎｓａｓａｎｅｌｅｃｔｒｏｎｃａｒｒｉｅｒ,ｐａｒｔｉｃｉｐａｔｉｎｇｉｎａｓｅｒｉｅｓｏｆｅｌｅｃｔｒｏｎ ｔｒａｎｓｆｅｒｐｒｏｃｅｓｓｅｓ ,ｉｎ ｃｌｕｄｉｎｇｒｅｄｕｃｔｉｏｎｏｆ…     

Analysis of a high redox potential heme in tetraheme cytochrome c3 by direct electrochemistry

Cytochrome c₃ from Desulfovibrio vulgaris (Miyazaki F), a redox protein, contains four bis-histidine-coordinated hemes and has lower redox potential than other heme proteins. Direct electrochemical measurements of cytochrome c₃ were carried out using a pyrolytic graphite edge (PGE) electrode. A low redox potential, already measured by redox titration, and a high redox potential (− 245 mV vs. Ag/AgCl) were observed at room temperature. The high redox potential of cytochrome c₃ was similar to that observed for the loss of an axial ligand at heme. To investigate the loss of the histidine ligand, we explored the electrochemistry of four cytochrome c₃ mutants, in which the sixth coordinated histidine was replaced by methionine. The electrochemistry of the cytochrome c₃ mutants indicated that only Heme III undergoes loss of its axial histidine ligand.     

Influences of the Hydrophobicity of the Heme－binding Pocket on the Properties and Functions of Cytochrome b5 Mutants

The mutation sites of the four mutants F35Y, P40V, V45E and V45Y of cytochrome b5 are located at the edge of the heme-binding pocket. The solvent accessible areas of the “pocket inte-rior“ of the four mutants and the wild-type cytochrome b5 have been calculated based on their crystal structures at high resolu-tion. The change in the hydrophobicity of the heme-binding pocket resulting from the mutation can be quantitatively de-scribed using the difference of the solvent accessible area of the “pocket interior“ of each mutant from that of the wild-type cy-tochrome b5. The influences of the hydrophobicity of the heme-binding pocket on the protein stability and redox potential are discussed.     

Inhibition of Heme Uptake in Pseudomonas aeruginosa by its Hemophore (HasAp) Bound to Synthetic Metal Complexes

The heme acquisition system A protein secreted by Pseudomonas aeruginosa (HasA_p) can capture several synthetic metal complexes other than heme. The crystal structures of HasA_p harboring synthetic metal complexes revealed only small perturbation of the overall HasA_p structure. An inhibitory effect upon heme acquisition by HasA_p bearing synthetic metal complexes was examined by monitoring the growth of Pseudomonas aeruginosa PAO1. HasA_p bound to iron–phthalocyanine inhibits heme acquisition in the presence of heme‐bound HasA_p as an iron source.     

Flavin Redox Bifurcation as a Mechanism for Controlling the Direction of Electron Flow during Extracellular Electron Transfer

The iron‐reducing bacterium Shewanella oneidensis MR‐1 has a dual directional electronic conduit involving 40 heme redox centers in flavin‐binding outer‐membrane c‐type cytochromes (OM c‐Cyts). While the mechanism for electron export from the OM c‐Cyts to an anode is well understood, how the redox centers in OM c‐Cyts take electrons from a cathode has not been elucidated at the molecular level. Electrochemical analysis of live cells during switching from anodic to cathodic conditions showed that altering the direction of electron flow does not require gene expression or protein synthesis, but simply redox potential shift about 300 mV for a flavin cofactor interacting with the OM c‐Cyts. That is, the redox bifurcation of the riboflavin cofactor in OM c‐Cyts switches the direction of electron conduction in the biological conduit at the cell–electrode interface to drive bacterial metabolism as either anode or cathode catalysts.     

Direct determination of the primary binding site of cisplatin on cytochrome c by mass spectrometry

Protein—cisplatin interactions lie at the heart of both the effectiveness of cisplatin as a therapeutic agent and side effects associated with cisplatin treatment. Because a greater understanding of the protein—cisplatin interactions at the molecular level can inform the design of cisplatin-like agents for future use, mass spectrometric determination of the binding site of cisplatin on a model protein, cytochrome c, was undertaken in this paper. The monoadduct cytochrome c—Pt(NH₃)₂(H₂O) is found to be the primary adduct produced by the cytochrome c—cisplatin interactions under native conditions. To locate the primary binding site of cisplatin, both free cytochrome c and the cytochrome c adducts underwent trypsin digestion, followed by Fourier transform mass spectrometry (FT-MS) to identify unique fragments in the adduct digest. Four such fragments were found in the adduct digest. Tandem mass spectrometry (MS/MS and MS³ indicates that two fragments are Pt(NH₃)₂(H₂O) bound peptides (Gly56-Glu104 and Asn54-Glu104) with one water associated at the peptide bond Lys79∼Met80, and the other two fragments are heme containing peptides (acety1-Gly1-Lys53 and acety1-Gly1-Lys55). The product-ion spectra of the four fragments reveal that Met65 is the primary binding site of cisplatin on cytochrome c.     

纳米ZnO修饰电极的制备及其对血红蛋白, 细胞色素c的直接电化学研究

A novel electrochemical method as a sensitive and convenient technique for the determination of heme proteins based on their interaction with ZnO nanorods was developed. A ZnO nanorod modified glassy carbon electrode (ZnO/GCE) was prepared and the electrochemical behaviors of heme proteins, such as hemoglobin (HB) and cytochrome c (Cyt-c), on this modified electrode have been studied. The results showed that both HB and Cyt-c could be oxidized on the modified electrode and the oxidation currents were linear to the concentrations of the analytes in aqueous solutions. In addition, the results of flow injection analysis (FIA) further suggested the high stability and reproducibility of the ZnO nanorod modified electrode. So this method can be applied to the determination of HB and Cyt-c in biological systems.     

Striking Improvement in Peroxidase Activity of Cytochrome c by Modulating Hydrophobicity of Surface‐Functionalized Gold Nanoparticles within Cationic Reverse Micelles

This work demonstrates a remarkable enhancement in the peroxidase activity of mitochondrial membrane protein cytochrome c (cyt c) by perturbing its tertiary structure in the presence of surface‐functionalised gold nanoparticles (GNPs) within cetyltrimethylammonium bromide (CTAB) reverse micelles. The loss in the tertiary structure of cyt c exposes its heme moiety (which is buried inside in the native globular form), which provides greater substrate (pyrogallol and H₂O₂) accessibility to the reactive heme residue. The surfactant shell of the CTAB reverse micelle in the presence of co‐surfactant (n‐hexanol) exerted higher crowding effects on the interfacially bound cyt c than similar anionic systems. The congested interface led to protein unfolding, which resulted in a 56‐fold higher peroxidase activity of cyt c than that in water. Further perturbation in the protein’s structure was achieved by doping amphiphile‐capped GNPs with varying hydrophobicities in the water pool of the reverse micelles. The hydrophobic moiety on the surface of the GNPs was directed towards the interfacial region, which induced major steric strain at the interface. Consequently, interaction of the protein with the hydrophobic domain of the amphiphile further disrupted its tertiary structure, which led to better opening up of the heme residue and, thereby, superior activity of the cyt c. The cyt c activity in the reverse micelles proportionately enhanced with an increase in the hydrophobicity of the GNP‐capping amphiphiles. A rigid cholesterol moiety as the hydrophobic end group of the GNP strikingly improved the cyt c activity by up to 200‐fold relative to that found in aqueous buffer. Fluorescence studies with both a tryptophan residue (Trp59) of the native protein and the sodium salt of fluorescein delineated the crucial role of the hydrophobicity of the GNP‐capping amphiphiles in improving the peroxidase activity of cyt c by unfolding its tertiary structure within the reverse micelles.     

Synthesis and characterization of chitosan‐modified polymethyl methacrylate latex particles

Stable chitosan‐modified polymethyl methacrylate (PMMA) latex particles were prepared by using 2,2′‐azobis(2‐amidinopropane) dihydrochloride (V‐50) as the cationic initiator. The polymerization rate (R_p) is controlled by the V‐50 concentration ([V‐50]) and R_p is less sensitive to the chitosan concentration ([C]) used in the synthesis work. The reaction system follows Smith–Ewart Case III kinetics due to the relatively large particles produced. The zeta potential data show that the isoelectric point (pI) of the latex particles is 10.7. The amounts of V‐50 (C_V‐50) and chitosan (C_c) ultimately incorporated into the particles correlate reasonably well with [V‐50] and [C], respectively. At pH 7, the quantity of the negatively charged bovine serum albumin (BSA, pI = 4.8) adsorbed on the positively charged chitosan‐free particles (Q) via the electrostatic interaction increases with increasing C_V‐50. However, Q is relatively insensitive to changes in C_c. This result implies that only the outermost region of the hairy chitosan‐modified particles is available for adsorption of the relatively large protein species. Colloidal stability shows a significant influence on the BSA adsorption process. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1489–1499, 1999     

Spatially Resolved Confocal Resonant Raman Microscopic Analysis of Anode‐Grown Geobacter sulfurreducens Biofilms

When grown on the surface of an anode electrode, Geobacter sulfurreducens forms a multi‐cell thick biofilm in which all cells appear to couple the oxidation of acetate with electron transport to the anode, which serves as the terminal metabolic electron acceptor. Just how electrons are transported through such a biofilm from cells to the underlying anode surface over distances that can exceed 20 microns remains unresolved. Current evidence suggests it may occur by electron hopping through a proposed network of redox cofactors composed of immobile outer membrane and/or extracellular multi‐heme c‐type cytochromes. In the present work, we perform a spatially resolved confocal resonant Raman (CRR) microscopic analysis to investigate anode‐grown Geobacter biofilms. The results confirm the presence of an intra‐biofilm redox gradient whereby the probability that a heme is in the reduced state increases with increasing distance from the anode surface. Such a gradient is required to drive electron transport toward the anode surface by electron hopping via cytochromes. The results also indicate that at open circuit, when electrons are expected to accumulate in redox cofactors involved in electron transport due to the inability of the anode to accept electrons, nearly all c‐type cytochrome hemes detected in the biofilm are oxidized. The same outcome occurs when a comparable potential to that measured at open circuit (?0.30 V vs. SHE) is applied to the anode, whereas nearly all hemes are reduced when an exceedingly negative potential (?0.50 V vs. SHE) is applied to the anode. These results suggest that nearly all c‐type cytochrome hemes detected in the biofilm can be electrochemically accessed by the electrode, but most have oxidation potentials too negative to transport electrons originating from acetate metabolism. The results also reveal a lateral heterogeneity (x–y dimensions) in the type of c‐type cytochromes within the biofilm that may affect electron transport to the electrode.     

Polymer segmental dynamics and solvent thermal transitions in poly(ethyl acrylate)/p‐xylene mixtures

A poly(ethyl acrylate) polymer network was swollen with different concentrations of the nonpolar solvent p‐xylene, c_px, from xerogel until saturation (0 ≤ c_px ≤ 0.85). Differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) techniques were employed to study the polymer segmental dynamics and the solvent thermal transitions in homogeneous (c_px < 0.20) and partially crystallized (c_px ≥ 0.20) PEA/p‐xylene mixtures. Our DSC measurements indicate that p‐xylene undergoes cold crystallization for intermediate solvent concentrations, 0.20 ≤ c_px ≤ 0.30 while for higher c_px values crystallization takes place during cooling. The results show that for c_px ≤ 0.30 the T_g decreases with increasing c_px (plasticization effect) obeying the respective Fox equation. For the same c_px range we found that both the dielectric strength and the heat capacity increment of the segmental (α) relaxation process increase gradually with c_px whereas the distribution of relaxation times for the underlying molecular relaxations does not change. For c_px > 0.30 the partially crystallized mixtures exhibit a constant T_g corresponding to the gel phase of PEA with an amount of p‐xylene which is not able to crystallize under any conditions. The concentration of this noncrystallized p‐xylene, c_UCpx, has been estimated to be between 0.12 and 0.15, independent of the total p‐xylene concentration in the mixtures. When a separate p‐xylene crystal phase is formed (for c_px > 0.30) the segmental dielectric strength and heat capacity increment decrease significantly exhibiting values significantly lower than those measured for the homogeneous gels. In addition, we found that the presence of p‐xylene crystals may induce marginal spatial heterogeneity of polymer (or p‐xylene) concentration within the gel phase affecting thus slightly the breath of the segmental relaxation of PEA. We attribute these results to restrictions of polymer segmental configurations due to constraints imposed by the p‐xylene crystals and/or to the immobilization of a part of the polymer chains. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Direct electrochemistry of bacterial cytochrome c551 at surface-modified gold electrodes

The direct electrochemistry of the single heme cytochrome c₅₅₁ from the bacterium Pseudomonas aeruginosa has been investigated at gold electrodes surface-modified through chemisorption of polyfunctional organic molecules. The results have been compared and contrasted with those obtained under the same conditions for the eukaryotic cytochrome c from horse heart. Both cytochromes give a quasi-reversible electrode reaction at pH 6.0 at a modified interface presenting only 4-pyridyl groups to the solution suggesting the occurrence, in both cases, of a hydrogen bonding interaction from lysine side-chains on the protein to pyridyl-nitrogens on the electrode surface. However, in contrast, gold electrodes modified by Pyridine-n-AldehydeThioSemicarbazones (n = 2, 3, 4) give electrochemistry which is strongly isomer-dependent in the case of horse heart cytochrome c but completely isomer-independent in the case of cytochrome c₅₅₁. It is suggested that interaction of the eukaryotic protein with surfaces is dominated by its lysine residues only, but that interaction of the bacterial cytochrome is through hydrogen bonding from the surface to both lysines and carboxylate groups of aspartate residues. This is supported by observation of the loss of cytochrome c₅₅₁ electrochemistry at 4-pyridyl-only modified gold at pH 9.0 compared with the good, quasi-reversible electrochemistry maintained under the same conditions at PATS-4 modified gold. It is concluded that, while the two cytochromes show many similarities with respect to their structures and functions, they have quite different interfacial electron transfer reactions, particularly at PATS-modified electrodes. This may correlate with the known large differences between the two proteins in net electrostatic charge and surface charge distribution.     

Differential Pulse Polarography for a First‐Order EC Process and Its Diagnostic Parameters

Theoretical expressions for differential pulse polarography (DPP) for a reversible electron transfer coupled with an irreversible follow‐up first‐order chemical reaction (E_rC_i) is derived approximately. The peaks as given by the current expressions are analyzed in terms of several parameters such as a ratio of anodic‐to‐cathodic peak‐currents (i_p^a/i_p^c), a separation of peak‐potentials (E_p^c?E_p^a), and a ratio of anodic‐to‐cathodic half‐peak‐widths (W_1/2^a/W_1/2^c) in order to characterize the E_rC_i process and distinguish it from other types of electrode processes. The anodic peak is found to be more susceptible to the post kinetics than the cathodic peak. The new parameter of W_1/2^a/W_1/2^c ratio is much more sensitive to the post kinetics than the peak separation (E_p^c?E_p^a). The peak current ratio (i_p^a/i_p^c) and the peak‐width ratio (W_1/2^a/W_1/2^c) have comparable sensitivities to the kinetics. Hence, W_1/2^a/W_1/2^c ratio is a better diagnostic parameters than (E_p^c?E_p^a) which has a poor sensitivity. This phenomenon is different from cyclic voltammetry (CV) in which E_p^c?E_p^a is as sensitive as i_p^a/i_p^c. The new criteria for EC with DPV is tested and successfully applied to several Co(III) complex systems, including coenzyme B₁₂. The homogeneous rate constant (k) for the follow‐up step is estimated from the measurements of the experimental values of the parameters. The present treatment is valid quantitatively at lower values of k, yielding relatively larger errors for higher k values (k>10 s^?1).     

Near‐IR Absorbing Solar Cell Sensitized With Bacterial Photosynthetic Membranes

Current interest in natural photosynthesis as a blueprint for solar energy conversion has led to the development of a biohybrid photovoltaic cell in which bacterial photosynthetic membrane vesicles (chromatophores) have been adsorbed to a gold electrode surface in conjunction with biological electrolytes (quinone [Q] and cytochrome c; Magis et al. [2010] Biochim. Biophys. Acta 1798 , 637–645). Since light‐driven current generation was dependent on an open circuit potential, we have tested whether this external potential could be replaced in an appropriately designed dye‐sensitized solar cell (DSSC). Herein, we show that a DSSC system in which the organic light‐harvesting dye is replaced by robust chromatophores from Rhodospirillum rubrum, together with Q and cytochrome c as electrolytes, provides band energies between consecutive interfaces that facilitate a unidirectional flow of electrons. Solar I–V testing revealed a relatively high I _sc (short‐circuit current) of 25 μA cm^?2 and the cell was capable of generating a current utilizing abundant near‐IR photons (maximum at ca 880 nm) with greater than eight‐fold higher energy conversion efficiency than white light. These studies represent a powerful demonstration of the photoexcitation properties of a biological system in a closed solid‐state device and its successful implementation in a functioning solar cell.