Cytochrome c6 from Monoraphidium braunii. A cytochrome with an unusual heme axial coordination

Alanine to serine substitutions drive thermal adaptation in a psychrophilic diatom cytochrome c6

In this study, we investigate the thermodynamic mechanisms by which electron transfer proteins adapt to environmental temperature by directly comparing the redox properties and folding stability of a psychrophilic cytochrome c and a mesophilic homolog. Our model system consists of two cytochrome c₆ proteins from diatoms: one adapted specifically to polar environments, the other adapted generally to surface ocean environments. Direct electrochemistry shows that the midpoint potential for the mesophilic homolog is slightly higher at all temperatures measured. Cytochrome c₆ from the psychrophilic diatom unfolds with a melting temperature 10.4 °C lower than the homologous mesophilic cytochrome c₆. Changes in free energy upon unfolding are identical, within error, for the psychrophilic and mesophilic protein; however, the chemical unfolding transition of the psychrophilic cytochrome c₆ is more cooperative than for the mesophilic cytochrome c₆. Substituting alanine residues found in the mesophile with serine found in corresponding positions of the psychrophile demonstrates that burial of the polar serine both decreases the thermal stability and decreases the midpoint potential. The mutagenesis data, combined with differences in the m-value of chemical denaturation, suggest that differences in solvent accessibility of the hydrophobic core underlie the adaptation of cytochrome c₆ to differing environmental temperature.

The structure of PccH from Geobacter sulfurreducens - a novel low reduction potential monoheme cytochrome essential for accepting electrons from an electrode

The structure of cytochrome c (GSU3274) designated as PccH from Geobacter sulfurreducens was determined at a resolution of 2.0 Å. PccH is a small (15 kDa) cytochrome containing one c-type heme, found to be essential for the growth of G. sulfurreducens with respect to accepting electrons from graphite electrodes poised at -300 mV versus standard hydrogen electrode. with fumarate as the terminal electron acceptor. The structure of PccH is unique among the monoheme cytochromes described to date. The structural fold of PccH can be described as forming two lobes with the heme sandwiched in a cleft between the two lobes. In addition, PccH has a low reduction potential of -24 mV at pH 7, which is unusual for monoheme cytochromes. Based on difference in structure, together with sequence phylogenetic analysis, we propose that PccH can be regarded as a first characterized example of a new subclass of class I monoheme cytochromes. The low reduction potential of PccH may enable the protein to be redox active at the typically negative potential ranges encountered by G. sulfurreducens. Because PccH is predicted to be located in the periplasm of this bacterium, it could not be involved in the first step of accepting electrons from the electrode but is very likely involved in the downstream electron transport events in the periplasm.

Purification of plastocyanin and cytochrome c6 from plants, green algae, and cyanobacteria

Plastocyanin and cytochrome c6 are widely distributed over the oxygen-evolving photosynthetic organisms. The two proteins are functionally equivalent, but strongly differ in their global electrostatic charge. In fact, they are acidic in eukaryotes, but either neutral or basic in cyanobacteria. Such a difference in their electrostatic features is a critical factor in designing the purification procedure, which must thus be modified and adapted accordingly. This chapter reports the methods for producing (including cell cultures), isolating, and purifying plastocyanin and cytochrome c6--which greatly differ in their isoelectric point--from a number of eukaryotic and prokaryotic organisms.

Analysis of the electrochemistry of hemes with E(m)s spanning 800 mV

The free energy of heme reduction in different proteins is found to vary over more than 18 kcal/mol. It is a challenge to determine how proteins manage to achieve this enormous range of E(m)s with a single type of redox cofactor. Proteins containing 141 unique hemes of a-, b-, and c-type, with bis-His, His-Met, and aquo-His ligation were calculated using Multi-Conformation Continuum Electrostatics (MCCE). The experimental E(m)s range over 800 mV from -350 mV in cytochrome c(3) to 450 mV in cytochrome c peroxidase (vs. SHE). The quantitative analysis of the factors that modulate heme electrochemistry includes the interactions of the heme with its ligands, the solvent, the protein backbone, and sidechains. MCCE calculated E(m)s are in good agreement with measured values. Using no free parameters the slope of the line comparing calculated and experimental E(m)s is 0.73 (R(2) = 0.90), showing the method accounts for 73% of the observed E(m) range. Adding a +160 mV correction to the His-Met c-type hemes yields a slope of 0.97 (R(2) = 0.93). With the correction 65% of the hemes have an absolute error smaller than 60 mV and 92% are within 120 mV. The overview of heme proteins with known structures and E(m)s shows both the lowest and highest potential hemes are c-type, whereas the b-type hemes are found in the middle E(m) range. In solution, bis-His ligation lowers the E(m) by approximately 205 mV relative to hemes with His-Met ligands. The bis-His, aquo-His, and His-Met ligated b-type hemes all cluster about E(m)s which are approximately 200 mV more positive in protein than in water. In contrast, the low potential bis-His c-type hemes are shifted little from in solution, whereas the high potential His-Met c-type hemes are raised by approximately 300 mV from solution. The analysis shows that no single type of interaction can be identified as the most important in setting heme electrochemistry in proteins. For example, the loss of solvation (reaction field) energy, which raises the E(m), has been suggested to be a major factor in tuning in situ E(m)s. However, the calculated solvation energy vs. experimental E(m) shows a slope of 0.2 and R(2) of 0.5 thus correlates weakly with E(m)s. All other individual interactions show even less correlation with E(m). However the sum of these terms does reproduce the range of observed E(m)s. Therefore, different proteins use different aspects of their structures to modulate the in situ heme electrochemistry. This study also shows that the calculated E(m)s are relatively insensitive to different heme partial charges and to the protein dielectric constant used in the simulation.

Purification and characterization of cytochrome c(6) from Acaryochloris marina

Cytochrome c(6), (cyt c(6)) a soluble monoheme electron transport protein, was isolated and characterized from the chlorophyll d-containing cyanobacterium Acaryochoris marina, the type strain MBIC11017. The protein was purified using ammonium sulfate precipitation, ion exchange and gel filtration column chromatography, and fast performance liquid chromatography. Its molecular mass and pI have been determined to be 8.87 kDa and less than 4.2, respectively, by mass spectrometry and isoelectrofocusing (IEF). The protein has an alpha helical structure as indicated by CD (circular dichroism) spectroscopy and a reduction midpoint potential (E(m)) of +327 mV versus the normal hydrogen electrode (NHE) as determined by redox potentiometry. Its potential role in electron transfer processes is discussed.

Structure and heme binding properties of Escherichia coli O157:H7 ChuX

For many pathogenic microorganisms, iron acquisition from host heme sources stimulates growth, multiplication, ultimately enabling successful survival and colonization. In gram-negative Escherichia coli O157:H7, Shigella dysenteriae and Yersinia enterocolitica the genes encoded within the heme utilization operon enable the effective uptake and utilization of heme as an iron source. While the complement of proteins responsible for heme internalization has been determined in these organisms, the fate of heme once it has reached the cytoplasm has only recently begun to be resolved. Here we report the first crystal structure of ChuX, a member of the conserved heme utilization operon from pathogenic E. coli O157:H7 determined at 2.05 A resolution. ChuX forms a dimer which remarkably given low sequence homology, displays a very similar fold to the monomer structure of ChuS and HemS, two other heme utilization proteins. Absorption spectral analysis of heme reconstituted ChuX demonstrates that ChuX binds heme in a 1:1 manner implying that each ChuX homodimer has the potential to coordinate two heme molecules in contrast to ChuS and HemS where only one heme molecule is bound. Resonance Raman spectroscopy indicates that the heme of ferric ChuX is composed of a mixture of coordination states: 5-coordinate and high-spin, 6-coordinate and low-spin, and 6-coordinate and high-spin. In contrast, the reduced ferrous form displays mainly a 5-coordinate and high-spin state with a minor contribution from a 6-coordinate and low-spin state. The nu(Fe-CO) and nu(C-O) frequencies of ChuX-bound CO fall on the correlation line expected for histidine-coordinated hemoproteins indicating that the fifth axial ligand of the ferrous heme is the imidazole ring of a histidine residue. Based on sequence and structural comparisons, we designed a number of site-directed mutations in ChuX to probe the heme binding sites and dimer interface. Spectral analysis of ChuX and mutants suggests involvement of H65 and H98 in heme coordination as mutations of both residues were required to abolish the formation of the hexacoordination state of heme-bound ChuX.

Hyperfine correlation spectroscopy and electron spin echo envelope modulation spectroscopy study of the two coexisting forms of the hemeprotein cytochrome c6 from Anabaena Pcc7119

Oxidized cytochrome c(6) from Anabaena PCC 7119 was studied by electron spin echo envelope modulation spectroscopy. Hyperfine couplings of the unpaired electron with several nuclei were detected, in particular those of the nitrogens bound to the iron atom. Combining the experimental information here presented and previous continuous wave-electron paramagnetic resonance and electron nuclear double resonance results, some details on the electronic structure of the heme center in the protein are obtained. These results are discussed on the basis of a molecular model that considers one unpaired electron localized mainly in the iron d orbitals and propagation of the spin density within the heme center via spin polarization of the nitrogen sigma-orbitals. The coexistence of two heme forms at physiological pH values in this c-type cytochrome is also discussed taking into account the experimental evidence.

Cloning, expression and purification of cytochrome c(6) from the brown alga Hizikia fusiformis and complete X-ray diffraction analysis of the structure

The primary sequence of cytochrome c(6) from the brown alga Hizikia fusiformis has been determined by cDNA cloning and the crystal structure has been solved at 1.6 A resolution. The crystal belonged to the tetragonal space group P4(1)2(1)2, with unit-cell parameters a = b = 84.58, c = 232.91 A and six molecules per asymmetric unit. The genome code, amino-acid sequence and crystal structure of H. fusiformis cytochrome c(6) were most similar to those of red algal cytochrome c(6). These results support the hypothesis that brown algae acquired their chloroplasts via secondary endosymbiosis involving a red algal endosymbiont and a eukaryote host.

Critical view on the monochlorodimedone assay utilized to detect haloperoxidase activity

The current study aimed to identify the halogenating enzymes involved in the biosynthesis of the ambigols A, B, C and tjipanazole D, isolated from the cyanobacterium Fischerella ambigua. Haloperoxidase (HPO) activity within F. ambigua was therefore assayed spectrophotometrically by using monochlorodimedone (MCD) during protein purification. This strategy revealed the isolation of a protein positive in the MCD-assay, but an involvement in halogenating processes could not be verified. N-terminal sequencing rather demonstrated homology to cytochrome c(6) from other cyanobacteria and green algae. From our findings it thus has to be concluded that the spectrophotometrical MCD-assay routinely used to detect HPO activity may yield false positive results, mainly since the assay focuses on the decline of the educt and not on the formation of the product. Our data indicate that the reaction of MCD with proteins of the cytochrome c- family leads to unspecific products.

Modulation of heme redox potential in the cytochrome c6 family

Cytochrome c6A is a unique dithio-cytochrome of green algae and plants. It has a very similar core structure to that of bacterial and algal cytochromes c6 but is unable to fulfill the same function of transferring electrons from cytochrome f to photosystem I. A key feature is that its heme midpoint potential is more than 200 mV below that of cytochrome c6 despite having His and Met as axial heme-iron ligands. To identify the molecular origins of the difference in potential, the structure of cytochrome c6 from the cyanobacterium Phormidium laminosum has been determined by X-ray crystallography and compared with the known structure of cytochrome c6A. One salient difference of the heme pockets is that a highly conserved Gln (Q51) in cytochrome c6 is replaced by Val (V52) in c6A. Using protein film voltammetry, we found that swapping these residues raised the c6A potential by +109 mV and decreased that of c6 by almost the same extent, -100 mV. X-ray crystallography of the V52Q protein showed that the Gln residue adopts the same configuration relative to the heme as in cytochrome c6 and we propose that this stereochemistry destabilizes the oxidized form of the heme. Consequently, replacement of Gln by Val was probably a key step in the evolution of cytochrome c6A from cytochrome c6, inhibiting reduction by the cytochrome b6f complex and facilitating establishment of a new function.

CW-EPR and ENDOR study of cytochrome c6 from Anabaena PCC 7119

The detailed analysis of the continuous-wave electron paramagnetic resonance and electron nuclear double resonance measurements on cytochrome c(6) from Anabaena PCC7119 reveals several electronic and structural properties of this hemeprotein. The oxidized protein shows two forms that differ in the arrangement of the residues that act as heme axial ligands. Information about the orientation of these residues is obtained for one of the forms, which turns out to differ from that found in the reduced protein from x-ray experiments. The biological significance of these results is discussed.

Characterization of a heme c nitrite reductase from a non-ammonifying microorganism, Desulfovibrio vulgaris Hildenborough

A cytochrome c nitrite reductase (NiR) was purified for the first time from a microorganism not capable of growing on nitrate, the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough. It was isolated from the membranes as a large heterooligomeric complex of 760 kDa, containing two cytochrome c subunits of 56 and 18 kDa. This complex has nitrite and sulfite reductase activities of 685 micromol NH(4)(+)/min/mg and 1.0 micromol H(2)/min/mg. The enzyme was studied by UV-visible and electron paramagnetic resonance (EPR) spectroscopies. The overall redox behavior was determined through a visible redox titration. The data were analyzed with a set of four redox transitions, with an E(0)' of +160 mV (12% of total absorption), -5 mV (38% of total absorption), -110 mV (38% of total absorption) and -210 mV (12% of total absorption) at pH 7.6. The EPR spectra of oxidized and partially reduced NiR show a complex pattern, indicative of multiple heme-heme magnetic interactions. It was found that D. vulgaris Hildenborough is not capable of using nitrite as a terminal electron acceptor. These results indicate that in this organism the NiR is not involved in the dissimilative reduction of nitrite, as is the case with the other similar enzymes isolated so far. The possible role of this enzyme in the detoxification of nitrite and/or in the reduction of sulfite is discussed.

Site-directed mutagenesis of cytochrome c(6) from Anabaena species PCC 7119. Identification of surface residues of the hemeprotein involved in photosystem I reduction

A number of surface residues of cytochrome c(6) from the cyanobacterium Anabaena sp. PCC 7119 have been modified by site-directed mutagenesis. Changes were made in six amino acids, two near the heme group (Val-25 and Lys-29) and four in the positively charged patch (Lys-62, Arg-64, Lys-66, and Asp-72). The reactivity of mutants toward the membrane-anchored complex photosystem I was analyzed by laser flash absorption spectroscopy. The experimental results indicate that cytochrome c(6) possesses two areas involved in the redox interaction with photosystem I: 1) a positively charged patch that may drive its electrostatic attractive movement toward photosystem I to form a transient complex and 2) a hydrophobic region at the edge of the heme pocket that may provide the contact surface for the transfer of electrons to P(700). The isofunctionality of these two areas with those found in plastocyanin (which acts as an alternative electron carrier playing the same role as cytochrome c(6)) are evident.

Extinction coefficients and midpoint potentials of cytochrome c(6) from the cyanobacteria Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803

Cytochrome c(6) is a soluble heme protein that serves as a photosynthetic electron transport component in cyanobacteria and algae, carrying electrons from the cytochrome bf complex to photosystem I. The rapid accumulation of cytochrome c(6) sequence data from a wide range of species, combined with significant advances in determining high resolution three-dimensional structures, provides a powerful database for investigating the relationship between structure and function. The fact that the gene encoding cytochrome c(6) can be readily modified in a number of species adds to the usefulness of cytochrome c(6) as a tool for comparative analysis. Efforts to relate cytochrome c(6) sequence information to structure, and structural information to function depend on knowledge of the physical and thermodynamic properties of the cytochrome from different species. To this end we have determined the optical extinction coefficient, the oxidation/reduction midpoint potential, and the pH dependence of the midpoint potential of cytochrome c(6) isolated from three cyanobacteria, Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803.

Amino acid sequence, crystallization and structure determination of reduced and oxidized cytochrome c6 from the green alga Scenedesmus obliquus

Cytochrome c6from the unicellular green alga Scenedesmus obliquus was sequenced, crystallized in its reduced and oxidized state and the three-dimensional structure of the protein in both redox states was determined by X-ray crystallography. Reduced cytochrome c6crystallized as a monomer in the space group P 21212, whereas the oxidized protein crystallized as a dimer in the space group P 3121. The structures were solved by molecular replacement and refined to 1. 9 and 2.0 A, respectively. Comparison of the structures of both redox states revealed only slight differences on the protein surface, whereas a distortion along the axis between the heme iron and its coordinating Met61 residue was observed. No redox-dependent movement of internal water molecules could be detected. The high degree of similarity of the surfaces and charge distributions of both redox states, as well as the dimerization of cytochrome c6as observed in the oxidized crystal, is discussed with respect to its biological relevance and its implications for the reaction mechanisms between cytochrome c6and its redox partners. The dimer of oxidized cytochrome c6may represent a molecular structure occurring in a binary complex with cytochrome b6f. This assembly might be required for the correct orientation of cytochrome c6with respect to its redox partner cytochrome b6f, facilitating the electron transfer within the complex. If the dimerization is not redox-dependent in vivo, the almost identical surfaces of both redox states do not support a long range differentiation between reduced and oxidized cyt c6, i.e. a random collision model for the formation of an electron transfer complex must be assumed.

PHOTOSYNTHETIC CYTOCHROMES c IN CYANOBACTERIA, ALGAE, AND PLANTS

The cytochromes that function in photosynthesis in cyanobacteria, algae, and higher plants have, like the other photosynthetic catalysts, been largely conserved in their structure and function during evolution. Cyanobacteria and algae contain cytochrome c6, which is not found in higher plants and which may enhance survival in their planktonic mode of life. Cyanobacteria and algae contain another cytochrome, low-potential c549, which is not found in higher plants. This cytochrome has a structural role in PSII and may contribute to anaerobic survival. There is a third unique cytochrome, cytochrome M, in the planktonic photosynthesizers, and its function is unknown. New evidence is appearing to indicate evolution of cytochrome interaction mechanisms during the evolution of photosynthesis. The ease of cytochrome gene manipulation in cyanobacteria and in Chlamydomonas reinhardtii now provides great advantages in understanding of photosynthesis. The solution of tertiary and quaternary structures of cytochromes and cytochrome complexes will provide structural and functional detail at atomic resolution.

Multiheme cytochromes from the sulfur-reducing bacterium Desulfuromonas acetoxidans

Two new multiheme cytochromes were isolated from the anaerobic sulfur reducing bacterium Desulfuromonas acetoxidans. They have monomeric molecular masses of 50 and 65 kDa and contain six and eight hemes, respectively. Visible and EPR spectroscopies, in the as-isolated (oxidised) cytochromes, show the presence of only low-spin hemes in the 50-kDa cytochrome, and of high-spin and low-spin hemes in the 65-kDa cytochrome. The EPR spectra of the native 65-kDa cytochrome indicate multiple heme-heme interactions, including integer-spin systems as judged by parallel-mode EPR. The 50-kDa cytochrome has a complex redox pattern, as shown by EPR redox titrations, and contains one heme with unusual characteristics. Both cytochromes cover an extremely wide range of reduction potentials, which go from +100 mV to -375 mV for the 50-kDa cytochrome, and +185 mV to -235 mV for the 65-kDa cytochrome. The two cytochromes were tested for hydroxylamine oxidoreductase activity and polysulfide reductase activity, but neither displayed any activity. In contrast, it was found for the first time that the previously characterised cytochrome c551.5, from the same bacterium is very active in the reduction of polysulfide, which suggests that it acts as a terminal reductase in D. acetoxidans.

Ab initio determination of the crystal structure of cytochrome c6 and comparison with plastocyanin

BACKGROUND

Electron transfer between cytochrome f and photosystem I (PSI) can be accomplished by the heme-containing protein cytochrome c6 or by the copper-containing protein plastocyanin. Higher plants use plastocyanin as the only electron donor to PSI, whereas most green algae and cyanobacteria can use either, with similar kinetics, depending on the copper concentration in the culture medium.

RESULTS

We report here the determination of the structure of cytochrome c6 from the green alga Monoraphidium braunii. Synchrotron X-ray data with an effective resolution of 1.2 A and the presence of one iron and three sulfur atoms enabled, possibly for the first time, the determination of an unknown protein structure by ab initio methods. Anisotropic refinement was accompanied by a decrease in the 'free' R value of over 7% the anisotropic motion is concentrated at the termini and between residues 38 and 53. The heme geometry is in very good agreement with a new set of heme distances derived from the structures of small molecules. This is probably the most precise structure of a heme protein to date.

CONCLUSIONS

On the basis of this cytochrome c6 structure, we have calculated potential electron transfer pathways and made comparisons with similar analyses for plastocyanin. Electron transfer between the copper redox center of plastocyanin to PSI and from cytochrome f is believed to involve two sites on the protein. In contrast, cytochrome c6 may well use just one electron transfer site, close to the heme unit, in its corresponding reactions with the same two redox partners.

Laser flash-induced photoreduction of photosynthetic ferredoxins and flavodoxin by 5-deazariboflavin and by a viologen analogue

Laser flash photolysis has been used to compare the kinetics of reduction of ferredoxin isoforms from the green alga Monoraphidium braunii, and the ferredoxin and flavodoxin from the cyanobacterium Anabaena PCC 7119, by 5-deazariboflavin semiquinone (dRfH.) and the viologen analogue 1,1'-propylene-2,2'-bipyridyl (PDQ.+). Similar ionic strength-independent second-order rate constants (1.4 x 10(8) M-1 s-1) were obtained for the reduction of both algal ferredoxin isoforms by dRfH.. For the reduction of oxidized flavodoxin by dRfH., a more complex behavior was observed, with a second-order rate constant for dRfH. decay of 1.8 x 10(8) M-1 s-1, and a first-order (i.e. protein concentration independent) rate constant of 450 s-1, that probably corresponds to the protonation of the FMN semiquinone cofactor, which occurs subsequent to electron transfer. A value of 5 x 10(7) M-1 s-1 was obtained for the second-order rate constant of flavodoxin semiquinone reduction by dRfH.. The reduction of ferredoxins and flavodoxin semiquinone by PDQ.+ showed nonlinear protein concentration dependencies, consistent with a minimal two-step mechanism involving complex formation followed by intracomplex electron transfer. A negative ionic strength effect on the kinetic constants was obtained, indicating the existence of attractive electrostatic interactions during electron transfer. With all the ferredoxins the k infinity values (rate constants extrapolated to infinite ionic strength) for the second-order step of the reduction process (complex formation) are smaller than previously reported for spinach ferredoxin, although Anabaena ferredoxin is somewhat more reactive than are the algal ferredoxins with the viologen.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of mutant Met100Lys of cytochrome c-550 from Thiobacillus versutus with lysine-histidine heme ligation

The heme iron in cytochrome c-550 from Thiobacillus versutus has a methionine and a histidine as axial ligands. In order to study the characteristics of a possible lysine-histidine ligation in a heme protein, the methionine has been replaced by a lysine. This residue acts as a ligand between pH 3 and 12. The midpoint potential of the mutant has shifted -329 mV compared to wild type, but apart from this shift the pH dependence of the midpoint potential is unchanged, suggesting that the large drop is caused by specific ligand effects and not by protein refolding. While the EPR spectrum of wild-type cytochrome c-550 shows one species with gz = 3.35, in the spectrum of the mutant two species occur with gz values of 3.53 and 3.30. The intensity ratio of both species depends on the presence of organic cosolvents. In the low frequency region (-4 to -1 ppm) of the 1H NMR spectrum of mutant ferrocytochrome c-550, four one-proton peaks replace the resonances of the ligand methionine side chain protons. Using two-dimensional NMR spectroscopy (COSY and NOESY), these protons and five others have been assigned to the lysine ligand. The spectroscopic results obtained for this mutant show similarities with those observed for the alkaline form of cytochrome c, supporting the Lys-His ligation proposed for this protein. The data are consistent with the evidence for amine ligation in cytochrome f: the EPR spectrum of M100K cytc-550 is similar to that of cytochrome f. However, the NMR spectra show significant differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and correct expression in E. coli of the petJ gene encoding cytochrome c6 from Synechocystis 6803

Cytochrome c6 from the cyanobacterium Synechocystis 6803 has been isolated and purified to electrophoretic homogeneity. The gene coding for such a heme protein (petJ) has been cloned and properly expressed in E. coli. This procedure yields a protein preparation completely identical to that obtained from the cyanobacterial cells. The N-terminal amino acid sequences of cytochrome c6 synthesized in both organisms are the same, thus allowing us to conclude that the petJ gene product is correctly processed in E. coli. To the best of our knowledge, this is the first time that any cytochrome c6 is produced in the enterobacterium. The identical physicochemical and kinetic properties of the proteins isolated from both sources confirm that expression of the petJ gene in E. coli is an adequate tool to address the study of Synechocystis cytochrome c6 by site-directed mutagenesis in a parallel way to that carried out with plastocyanin from the same organism.

A thermodynamic study by laser-flash photolysis of plastocyanin and cytochrome c6 oxidation by photosystem I from the green alga Monoraphidium braunii

Plastocyanin and cytochrome c6 from the green alga Monoraphidium braunii reduce the photo-oxidized algal photosystem I (PSI) reaction center chlorophyll (P700) with similar kinetics, as expected from their functional equivalence. The observed P700+ reduction rate constants show a non-linear dependence on metalloprotein concentration, which indicates a (minimal) two-step kinetic mechanism involving complex formation prior to electron transfer. The dependence of the observed rate constants on NaCl concentration suggests that the electrostatic interaction forces between the negatively charged donor proteins and PSI are repulsive at neutral pH and relatively low ionic strength (I), although attractive dipole-dipole interactions may play a role at higher ionic strengths. Activation parameters for P700+ reduction by cytochrome c6 and plastocyanin have been determined by studying the temperature dependence of the respective rate constants at varying ionic strength and pH. Changes in NaCl concentration and pH induce significant changes in the activation free energy of the overall reaction, even though the corresponding values for activation enthalpy and entropy undergo changes in opposite directions. Such a compensation effect between enthalpy and entropy is observed with both cytochrome c6 and plastocyanin. Protein concentration dependencies of the observed rate constants at different temperatures has allowed an estimate of the free energy change during complex association, as well as the activation parameters for electron transfer, according to a two-step kinetic model.