The roles of c-type cytochromes in algal photosynthesis. Extraction from algae of a cytochrome similar to higher plant cytochrome f

The discovery and function of plastocyanin: A personal account

A brief autobiographical account is presented of the early research that led to the discovery of the copper protein plastocyanin and the identification of its function as an electron carrier in plant photosynthesis. A discussion follows of different approaches employed for the determination of the functional site of plastocyanin in relation to cytochrome f. A summary is provided of a heated controversy about the involvement of two or three light reactions in photosynthesis and an experiment is described that has contributed to resolution of the controversy through the identification of the functional site of plastocyanin. An early history of photosynthesis research in Japan is also discussed.

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

A soluble monoheme c-type cytochrome (cytochrome c6) has been isolated from the green alga Monoraphidium braunii. It has a molecular mass of 9.3 kDa, an isoelectric point of 3.6 and a reduction potential of 358 mV at pH 7. The determined amino acid sequence allows its classification as a class-I c-type cytochrome. The ferric and ferrous cytochrome forms and their pH equilibria have been studied using 1H-NMR, ultraviolet/visible, EPR and Mössbauer spectroscopies. The pH equilibria are complex, several pKa values and pH-dependent forms being observed. The amino acid sequence, the reduction-potential value and the visible and NMR spectroscopies data in the pH range 4-9 indicate that the heme iron has a methionine-histidine axial coordination. However, the EPR and Mössbauer data obtained for the ferricytochrome show that in this pH range two distinct forms are present: form I, gz = 3.27, gy = 2.05 and gx = 1.05; form II, gz = 2.95, gy = 2.29 and gx = 1.43. While form I has crystal-field parameters typical of a methionine-histidine coordination, those associated with form II would suggest a histidine-histidine axial ligation. This possibility was extensively analyzed by spectroscopic methods and by chemical modification of a histidine residue. It was concluded that form II actually corresponds to an unusual type of methionine-histidine axial coordination. Straightforward examples of this type of coordination have recently been found in other c-type hemeproteins [Teixeira, M., Campos, A. P., Aguiar, A. P., Costa, H. S., Santos, H., Turner, D. L. & Xavier, A. V. (1993) FEBS Lett. 317, 233-236], corroborating our proposal. Since both forms, with very distinct crystal-field parameters, are shown to have the same reduction potential, it may be concluded that the axial and rhombic distortions of the heme-iron ligand field cannot be directly correlated with the heme-reduction potential. The pH-dependence studies have also shown that the form I and form II are interconvertible, with pKa approximately 5. To establish a possible physiological significance for this process, in particular for the interaction of the cytochrome with the membrane-bound electron-transfer complexes b6f and photosystem I, the effect of surfactants on the spectroscopic characteristics of cytochrome c6 has been studied.

Cytochrome f: Structure, function and biosynthesis

Cytochrome f is an intrinsic membrane component of the cytochrome bf complex, transferring electrons from the Rieske FeS protein to plastocyanin in the thylakoid lumen. The protein is held in the thylakoid membrane by a single transmembrane span located near its C-terminus with a globular hydrophilic domain extending into the lumen. The globular domain of the turnip protein has recently been crystallised, offering the prospect of a detailed three-dimensional structure. Reaction with plastocyanin involves localised positive charges on cytochrome f interacting with the acidic patch on plastocyanin and electron transfer via the surface-exposed tyrosine residue (Tyr83) of plastocyanin. Apocytochrome f is encoded in the chloroplast genome and is synthesised with an N-terminal presequence which targets the protein to the thylakoid membrane. The synthesis of cytochrome f is coordinated with the synthesis of the other subunits of the cytochrome bf complex.

A comparative laser-flash absorption spectroscopy study of algal plastocyanin and cytochrome c552 photooxidation by photosystem I particles from spinach

Laser-flash kinetic absorption spectroscopy has been used to compare the rate constants for electron transfer from reduced plastocyanin and cytochrome c552, obtained from the green alga Monoraphidium braunii, to photooxidized P700 (P700+) in photosystem I (PSI) particles from spinach Sigmoidal protein concentration dependence for the observed electron-transfer rate constants are obtained for both proteins. In the absence of added salts, the P700+ reduction rate increases as the pH decreases from approximately 8 to 5.5, then decreases to pH 3.5, this effect being more pronounced with cytochrome c552 than with plastocyanin. At neutral pH, plastocyanin is a more efficient electron donor to P700+ than cytochrome c552, whereas at pH 5.5, which is closer to physiological conditions, the two redox proteins react with approximately equal rate constants. In the presence of increasing concentrations of added salts, the P700+ reduction rate constants for both proteins increase at pH greater than 5.5, but decrease at pH less than 4. At neutral pH, the observed rate constants for both algal proteins have a biphasic dependence on sodium chloride concentration, increasing in a parallel manner with increasing salt concentration, reaching a maximum value at 50 mM NaCl, then decreasing. A similar biphasic dependence is obtained with magnesium chloride, but in this case the maximum value is reached at salt concentrations ten times smaller, suggesting a specific role for the divalent cations in the electron-transfer reaction.

Flavin-photosensitized oxidation of reduced c-type cytochromes. Reaction mechanism and comparison with photoreduction of oxidized cytochromes by flavin semiquinones

In order to compare the oxidation and reduction reactions of c-type cytochromes (cytochrome c552 from the green alga Monoraphidium braunii and horse heart cytochrome c) by different flavins (lumiflavin, riboflavin and FMN), laser flash photolysis studies have been carried out using either reduced or oxidized protein in the presence of triplet or semiquinone flavin, respectively. The reaction kinetics clearly demonstrate that cytochrome oxidation is mediated by the flavin triplet state. The rate constants for reduction are 20-100 times smaller than those for oxidation, indicating that the triplet state is a more effective reactant than is the semiquinone. This is attributed to its excited state nature and correspondingly high free energy content. The rate constants for both the reduction and oxidation of cytochrome c552 by riboflavin are significantly smaller than those obtained with lumiflavin, suggesting a steric interference of the ribityl side chain in the flavin-cytochrome interaction. The comparison between oxidation and reduction indicates that the former process is less affected by steric hindrance than the latter. Both reduction and oxidation of cytochrome c552 by FMN show an ionic strength dependence with the same sign, consistent with a negatively charged reaction site on the cytochrome. The magnitude of the electrostatic effect is slightly smaller for reduction than it is for oxidation. A pattern quite similar to that observed with cytochrome c552 was obtained when parallel experiments were carried out with horse cytochrome c, although differences were observed in the steric and electrostatic properties of the electron transfer site(s) in these two cytochromes. These results suggest that the same or closely adjacent sites on the proteins are involved in the oxidation and reduction reactions. The biochemical implications of this are discussed.

Plastocyanin cytochrome f interaction

Spinach plastocyanin and turnip cytochrome f have been covalently linked by using a water-soluble carbodiimide to yield an adduct of the two proteins. The redox potential of cytochrome f in the adduct was shifted by -20 mV relative to that of free cytochrome f, while the redox potential of plastocyanin in the adduct was the same as that of free plastocyanin. Solvent perturbation studies showed the degree of heme exposure in the adduct to be less than in free cytochrome f, indicating that plastocyanin was linked in such a way as to bury the exposed heme edge. Small changes were also observed when the resonance Raman spectrum of the adduct was compared to that of free cytochrome f. The adduct was incapable of interacting with or donating electrons to photosystem I. Peptide mapping and sequencing studies revealed two sites of linkage between the two proteins. In one site of linkage, Asp-44 of plastocyanin is covalently linked to Lys-187 of cytochrome f. This represents the first identification of a group on cytochrome f that is involved in the interaction with plastocyanin. The other site of linkage involves Glu-59 and/or Glu-60 of plastocyanin to as yet unidentified amino groups on cytochrome f. Euglena cytochrome c-552 could also be covalently linked to turnip cytochrome f, although with a lower efficiency than spinach plastocyanin. In contrast, a variety of cyanobacterial cytochrome c-553's and a cyanobacterial plastocyanin could not be covalently linked to turnip cytochrome f.

Resonance Raman spectroscopy indicates a lysine as the sixth iron ligand in cytochrome f

The resonance Raman spectrum of turnip cytochrome f is similar to that of other c-type cytochromes with the exception of a single band at 1532 cm-1 which is shifted to lower frequency relative to its position (1542-1545 cm-1) in other c-type cytochromes. Comparison of the frequency of this band with that in alkylated cytochrome c at high pH suggests that the sixth heme iron ligand in cytochrome f is a deprotonated lysine amino group rather than a methionine sulfur. Comparison of the amino-acid sequences of cytochromes f and c1 suggests lysine-145 as a likely candidate for the sixth heme iron ligand in cytochrome f.

Kinetics of reduction by free flavin semiquinones of algal cytochromes and plastocyanin

It had been shown that plastocyanin and cytochrome c-553 are functionally interchangeable in algae and that the physiological electron transfer reactions are sensitive to ionic strength. The isoelectric points of these proteins range from very acidic to basic depending upon species, and naturally occurring amino acid substitutions of charged residues have been shown to affect the kinetics of electron transfer, presumably through alteration of protein net charge. We have now shown that these naturally occurring amino acid substitutions also affect the kinetics of nonphysiological electron transfer reactions, and that we can quantitate the extent of nonconservation of charge. The reduction of plant and algal proteins by FMN semiquinone is sensitive to ionic strength and the effects can be correlated with net protein charge with regard to sign, but not to magnitude, with the charge at the site of electron transfer varying from +3 through 0 to -3. We had previously observed in a large variety of electron transfer proteins from bacteria (G. Tollin, T. E. Meyer, and M. A. Cusanovich (1986) Biochim. Biophys. Acta 853, 29-41) that charge localized at the site of electron transfer, rather than net protein charge, was more likely to affect kinetics. This also appears to be the case with the algal proteins. By comparison of protein structures, we have been able to predict which substitutions are likely to be responsible for the kinetic effects in the algal proteins and to discuss the implications of such changes for function.

Regulation by copper of the expression of plastocyanin and cytochrome c552 in Chlamydomonas reinhardi

Plastocyanin and cytochrome c552 are interchangeable electron carriers in the photosynthetic electron transfer chains of some cyanobacteria and green algae (P. M. Wood, Eur. J. Biochem. 87:9-19, 1978; G. Sandmann et al., Arch. Microbiol. 134:23-27, 1983). Chlamydomonas reinhardi cells respond to the availability of copper in the medium and accordingly accumulate either plastocyanin (if copper is available) or cytochrome c552 (if copper is not available). The response occurs in both heterotrophically and phototrophically grown cells. We have studied the molecular level at which this response occurs. No immunoreactive polypeptide is detectable under conditions where the mature protein is not spectroscopically detectable. Both plastocyanin and cytochrome c552 appear to be translated (in vitro) from polyadenylated mRNA as precursors of higher molecular weight. RNA was isolated from cells grown either under conditions favorable for the accumulation of plastocyanin (medium with Cu2+) or for the accumulation of cytochrome c552 (without Cu2+ added to the medium). Translatable mRNA for preapoplastocyanin was detected in both RNA preparations, although mature plastocyanin was detected in C. reinhardi cells only when copper was added to the culture. Translatable mRNA for preapocytochrome, on the other hand, was detected only in cells grown under conditions where cytochrome c552 accumulates (i.e., in the absence of copper). We conclude that copper-mediated regulation of plastocyanin and cytochrome c552 accumulation is effected at different levels, the former at the level of stable protein and the latter at the level of stable mRNA.

Implications of cytochrome b6/f location for thylakoidal electron transport

The cytochrome b6/f complex of higher plant chloroplasts is uniformly distributed throughout both appressed and nonappressed thylakoids, in contrast to photosystem II and photosystem I, the other major membrane protein complexes involved in electron transport. We discuss how this distribution is likely to affect interactions of the cytochrome b6/f complex with other electron transport components because of the resulting local stoichiometries, and how these may affect the regulation of electron transport.

Regulation by copper of the expression of plastocyanin and cytochrome c552 in Chlamydomonas reinhardi

Plastocyanin and cytochrome c552 are interchangeable electron carriers in the photosynthetic electron transfer chains of some cyanobacteria and green algae (P. M. Wood, Eur. J. Biochem. 87:9-19, 1978; G. Sandmann et al., Arch. Microbiol. 134:23-27, 1983). Chlamydomonas reinhardi cells respond to the availability of copper in the medium and accordingly accumulate either plastocyanin (if copper is available) or cytochrome c552 (if copper is not available). The response occurs in both heterotrophically and phototrophically grown cells. We have studied the molecular level at which this response occurs. No immunoreactive polypeptide is detectable under conditions where the mature protein is not spectroscopically detectable. Both plastocyanin and cytochrome c552 appear to be translated (in vitro) from polyadenylated mRNA as precursors of higher molecular weight. RNA was isolated from cells grown either under conditions favorable for the accumulation of plastocyanin (medium with Cu2+) or for the accumulation of cytochrome c552 (without Cu2+ added to the medium). Translatable mRNA for preapoplastocyanin was detected in both RNA preparations, although mature plastocyanin was detected in C. reinhardi cells only when copper was added to the culture. Translatable mRNA for preapocytochrome, on the other hand, was detected only in cells grown under conditions where cytochrome c552 accumulates (i.e., in the absence of copper). We conclude that copper-mediated regulation of plastocyanin and cytochrome c552 accumulation is effected at different levels, the former at the level of stable protein and the latter at the level of stable mRNA.

Multidisciplinary research in photosynthesis: A case history based on the green alga Chlamydomonas

This article examines the contribution of a unicellular green alga Chlamydomonas to progress in photosynthetic research. The objective is to focus on the aspects of Chlamydomonas that have provided an advantage over other photosynthetic organisms in investigating photosynthesis. To do this we discuss several examples that demonstrate the progress from a genetic study to a multidisciplinary approach that probes higher levels of complexity within the organism. These examples include the function and molecular regulation of electron transport components between photosystem II and photosystem I, the molecular genetics of the herbicide binding protein of photosystem II, and several different studies that have derived from a search for rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) mutants in Chlamydomonas, including chloroplast ribosome function, the regulation of the large subunit of rubisco, and the interaction between photosynthetic electron transport and carbon metabolism.

The amino acid sequence of the dihaem cytochrome c4 from the bacterium Azotobacter vinelandii

An amino acid sequence is proposed for the cytochrome c4 from the bacterium Azotobacter vinelandii strain OP. It is a single polypeptide chain of 190 residues, with two sets of haem-attachment cysteine residues at positions 14/17 and 119/122. Proteins with similar sequences are also present in denitrifying pseudomonads. There is similarity in sequence between the two halves of the cytochrome c4 molecule, and each half also shows similarity to the sequences of certain monohaem cytochromes c isolated from organisms that are not obviously closely related to A. vinelandii. Detailed evidence for the amino acid sequence of the protein has been deposited as Supplementary Publication SUP 50125 (17 pages) at the British Library Lending Division, Boston Spa, West Yorkshire LS23 7BQ, U.K., from whom copies are available on prepayment.

The cytochrome f-b electron-transport complex. A common link between photosynthesis and respiration in the cyanobacterium Anacystis nidulans

Reduced-minus-oxidized difference spectra were recorded on particle preparations of the cyanobacterium Anacystis nidulans. Physiological oxidation of anaerobic membranes was effected either by O2 or by light. In both cases the spectral changes observed in the 550-570nm region were essentially the same. The results were confirmed by dual-wavelength spectrophotometry. It is concluded that a membrane-bound cytochrome f-b complex participates in both respiratory and photosynthetic elevtron transport.

The effect on photosynthetic electron transport of temperature-dependent changes in the fluidity of the thylakoid membrane in a thermophilic blue-green alga

Various electron transport reactions in cell or isolated thylakoid membranes of the thermophilic blue-green alga, Synechococcus sp. were measured at different temperatures between 72 and 3 degrees C. They are classified into two groups with respect to their temperature dependency. The first group involves cytochrome 553 photooxidation, methyl viologen photoreduction with reduced 2,6-dichlorophenolindophenol as electron donor and 3-(3',4'-dichlorophenyl)-1,1-dimethylurea-resistant ferricyanide photoreduction determined in the presence or absence of silicomolybdate. The Arrhenius plot of these reactions showed a single straight line with the activation energy of about 10 kcal/mol throughout wide temperature ranges studied. Methyl viologen photoreduction with water as electron donor, reduction of flash-oxidized cytochrome 553, ferricyanide photoreduction and photosynthetic O2 evolution form the second group. Their arrhenius plots are characterized by discontinuities or breaks at about 30 and 10 degrees C, which respectively correspond to the upper and lower boundaries of the lateral phase separation of the membrane lipids. The first group reactions represent short spans of electron transport which are mediated either by Photosystem I or Photosystem II alone and not related to plastoquinone, whereas all the reactions of the second group involve plastoquinone. It is concluded therefore that the membrane fluidity affect electron transport specifically at the region of plastoquinone. It is proposed that the reaction center chlorophyll-protein complexes of both Photosystems I and II are closely associated with related electron carrier proteins to form functional supramolecular assemblies so that electron transfer within such a cluster of proteins proceeds independently of the phase changes in the membrane lipids. On the other hand, the role of plastoquinone as a mobile electron carrier mediating electron transfer from the protein assembly of Photosystem II to that of Photosystem I through the fluid hydrophobic matrix of the membranes is highly sensitive to the physical state of the membrane lipids.

Occurrence of cytochrome aa3 in Anacystis nidulans

The cytochrome content of membrane fragments prepared from the blue-green alga (cyanobacterium) Anacystis nidulans was examined by difference spectrophotometry. Two beta-type cytochromes and hitherto unknown cytochrome alpha could be characterized. In the reduced-minus-oxidised difference spectra the alpha-type cytochrome showed an alpha-band at 605 nm and a gamma-band at 445 nm. These bands shifted to 590 and 430 nm, respectively, in CO difference spectra, NADPH, NADH and ascorbate reduced the cytochrome through added horse heart cytochrome c as electron mediator. In presence of KCN the reduced-minus-oxidised spectrum showed a peak at 600 nm and a trough at 604 nm. Photoaction spectra of O2 uptake and of horse heart cytochrome c oxidation by CO-inhibited membranes showed peaks at 590 and 430 nm. These findings are consistent with cytochrome aa3 being the predominant respiratory cytochrome c oxidase in Anacystis nidulans.

The interrelation of the two c-type cytochromes in Rhodopseudomonas sphaeroides photosynthesis

Photosynthetic electron flow in the bacterium Rhodopseudomonas sphaeroides involves two c-type cytochromes, one membrane-bound and the other a soluble protein, cytochrome c2. Membranes deficient in cytochrome c2 were used for photo-oxidation studies, with and without the addition of purified cytochrome c2. The results favour a series interrelation, membrane cytochrome c-cytochrome c2-reaction centre.

Purification and characterization of cytochrome f-556.5 from the blue-green alga Spirulina platensis

The membrane-bound cytochrome f-556.5 from the blue-green alga Spirulina platensis was purified to apparent homogeneity. Most of its properties are comparable to cytochrome f isolated from higher plants and green algae. It is clearly distinguishable from soluble cytochrome c-554, also present in Spirulina, which probably replaces the function of plastocyanin in photosynthetic electron transport. 1. The reduced form of cytochrome f exhibits an asymmetrical alpha-band with a maximum at 556.5 nm, and a pronounced shoulder at 550 nm. The beta-, gamma and delta-bands coincide with those described for Scenedesmus cytochrome f-553, with maxima at 524 (532), 422, 331 and a protein peak at 276 nm. The maximum of ferricytochrome f is at 410.5 nm; there is no indication of a weak 695 nm band, described for soluble c-type cytochromes. The purest preparations had a delta/protein-peak ratio of 0.8; the gamma/alpha ratio was 7.3. Formation of a pyridine hemochromogen with a maximum at 550 nm indicated a c-type cytochrome. The molar extinction coefficient at 556.5 nm is 30200, the differential extinction coefficient 21 500. 2. The molecular weight determined by gel filtration or SDS-polyacrylamide gel electrophoresis is 33 000 and 34 000, respectively. 3. The redox properties differ from those described for other cytochromes f isolated from green algae and higher plants: the midpoint redox potential is significantly more negative (+318 mV, pH 7.0) and from pH 6 to 10 no pH dependence is observed. 4. The isoelectric point was determined at pH 3.95, which is more acidic as compared to other cytochromes f. 5. Comparison of the amino acid composition indicated a distant relationship to higher plant cytochrome f and a closer relationship to cytochrome f from green algae.

Plastoquinone as a common link between photosynthesis and respiration in a blue-green alga

The role of plastoquinone in a thermophilic blue-green alga, Shynechococcus sp., was studied by measuring reduction kinetics of cytochrome 553 which was oxidized with red flash preferentially exciting photosystem I. Sensitivity of the cytochrome reduction to DBMIB Abbreviations: DCMU = 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DBMIB = 2,5-dib romo-3-methyl-6-isopropyl-p-benzoquinone; HOQNO = 2-n-heptyl-4-hydroxyquinoline-N-oxide indicates that cytochrome 553 accepts electrons from reduced plastoquinone. Plastoquinone is in turn reduced in cells without electrons from photosystem II, since DCMU Abbreviations: DCMU = 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DBMIB = 2,5-dib romo-3-methyl-6-isopropyl-p-benzoquinone; HOQNO = 2-n-heptyl-4-hydroxyquinoline-N-oxide , which inhibited methyl viologen photoreduction more strongly than DBMIB, failed to affect the cytochrome reduction. Participation of cyclic electron transport around photosystem I in cytochrome reduction in the presence of DCMU was excluded, because methyl viologen and antimycin A had no effect on the cytochrome kinetics. On the other hand, electron donation from endogenous substrates to plastoquinone was suggested from decreases in rate of the cytochrome reduction by dark starvation of cells and also from restoration of fast reduction kinetics by the addition of exogenous substrates to or by reillumination of starved cells.KCN, which completely suppressed respiratory O2-uptake, induced a marked acceleration of the cytochrome reduction in starved cells. The poison was less or not effective in stimulating the cytochrome reduction in more extensively starved or reilluminated cells.Results indicate that plastoquinone is functioning not only in the photosynthetic but also in the respiratory electron transport chain, thereby forming a common link between the two energy conservation systems of the blue-green alga.

Do photosynthetic bacteria contain cytochrome c1?

A method is described for characterizing, c-type cytochromes in bacterial membrane preparations according to molecular weight on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Applied to the photosynthetic bacterium Rhodopseudomonas sphaeroides this technique is used, together with spectroscopic measurements, to demonstrate that a membrane-bound cytochrome c of mol.wt. 30000 is active in photosynthetic electron transport in addition to the well-known soluble cytochrome, cytochrome c2. The membrane cytochrome has a midpoint potential (E'0) at pH 7 of +290 mV, as compared with +360 mV for purified cytochrome c2. Its alpha-band has a peak near 552 nm, as compared with 550 nm for cytochrome c2. Evidence is presented that chromatophores contain roughly equal amounts of the two cytochromes.

Photosynthetic electron transport in a cell-free preparation from the thermophilic blue-green alga Phormidium laminosum

1. A cell-free preparation of membrane fragments was prepared from the thermophilic blue-green alga Phormidium laminosum by lysozyme treatment of the cells followed by osmotic shock to lyse the spheroplasts. The membrane fragments showed high rates of photosynthetic electron transport and O2 evolution (180-250 mumol of O2/h per mg of chlorophyll a with 2,6-dimethyl-1,4-benzoquinone as electron acceptor). O2-evolution activity was stable provided that cations (e.g. 10mM-Mg2+ or 100mM-Na+) or glycerol (25%, v/v) were present in the suspending medium. 2. The components of the electron-transport chain in P. laminosum were similar to those of other blue-green algae: the cells contained Pigment P700, plastocyanin, soluble high-potential cytochrome c-553, soluble low-potential cytochrome c-54 and membrane-bound cytochromes f, b-563 and b-559 (both low- and high-potential forms). The amounts and midpoint potentials of the membrane-bound cytochromes were similar to those in higher-plant chloroplasts. 3. Although O2 evolution in P. laminosum spheroplasts was resistant to high temperatures, thermal stability was not retained in the cell-free preparation. However, in contrast with higher plants, O2 evolution in P. laminosum membrane fragments was remarkably resistant to the non-ionic detergent Triton X-100.

Isolation and characterization of soluble cytochrome c-553 and membrane-bound cytochrome f-553 from thylakoids of the green alga Scenedesmus acutus

Soluble cytochrome c-553 and membrane-bound cytochrome f-553 from the alga Scenedesmus acutus were purified to apparent homogeneity. The properties of cytochrome c-553 are comparable to preparations obtained from other eukaryotic algae, whereas the thylakoid-bound species resembles higher plant cytochrome f. Common characteristics are: 1. An asymmetrical alpha-band at 553 nm. 2. A midpoint redox potential of +38 MV (pH 7.0), with a pH dependency above pH 8.0 of -60mV/pH unit. 3. Formation of a pyridine hemochromogen with a maximum at 550 nm; no adducts with CN- or CO are observed. Distinguishing features are: 1. Cytochrome f-553 has a more complicated beta-band, with maxima at 531.5 and 524 nm, and hence a more complex low-temperature spectrum. Also the positions of the gamma- and delta-bank at 421.5 and 331 nm, respectively, distinguish cytochrome f-553 from cytochrome c-553, with gamma- and delta-bands at 416 and 318 nm. 2. The ferricytochrome c-553 spectrum exhibits a weak band at 692 nm, which is not observed with cytochrome f.

High variability of the electron carrier plastocyanin in microalgae

1. The plastocyanin content of Scenedesmus can be dramatically varied with the copper content of the culture medium. Figures as high as 7 mmol plastocyanin/mol chlorophyll are possible. Electron paramagnetic resonance (EPR) spectroscopy has been used to determine this physiological response quantitatively in intact cells having different amounts of plastocyanin. The results obtained by the EPR technique were compared with data on isolated plastocyanin determined either by spectrophotometry or immunoelectrophoresis. Agreement was found for the amount of plastocyanin detected by the first two methods, whereas the last assay yielded data at least 25% higher on the average. Under all culture conditions a copper-free plastocyanin precursor is present. 2. The EPR properties of purified plastocyanin and those of cellular plastocyanin located within the thylakoids are practically identical in terms of g-values, hyperfine splittings, signal linewidths and saturation behavior at temperatures of 12--15 K. Our data indicate that plastocyanin is not present in a membrane-bound form but exists as a single soluble pool. 3. The studies have been extended to the algae Dunaliella parva and Bumilleriopsis filiformis. The first species exhibits a limited variation of plastocyanin with the copper content of the medium. Furthermore, no cytochrome c-553 could be detected in Dunaliella even under conditions of copper deficiency. In contrast, Bumilleriopsis does not contain plastocyanin regardless of the amount of copper offered.

The cytochromes c: paradigms for chemical recognition

The cytochromes c include subgroups which present a variety of redox functions based on well-defined changes in the basic three-dimensional structure exemplified by the mitochondrial and certain bacterial forms, in particular cytochromes c2. These proteins exhibit overlapping functionality and a graded sequence of structures which provide paradigms well suited for clarification of recognition mechanisms. The character and distribution of cytochromes c will be discussed and approaches to relatedness of structure and function will be described, based on kinetic analyses of cross reactivities of cytochromes c2 with mitochondrial cytochrome c oxidase.

Purification and primary structure of cytochrome c-552 from the cyanobacterium, Synechococcus PCC 6312

Cytochrome c-552 (soluble 'cytochrome f') from the unicellular cyanobacterium Synechococcus PCC 6312 (ATCC 27167) was purified and the primary structure determined. The proposed sequence consists of one polypeptide chain of 87 residues. The sequence was determined by a combination of chemical and enzymatic cleavage, manual and automatic sequencing and mass spectroscopy. This is the first amino acid sequence of this cytochrome from a unicellular cyanobacterium to be determined in a study of the variation in primary structure between phylogenetically distant cyanobacteria. The sequence is compared to the primary structures of the cytochrome from filamentous cyanobacteria and from eukaryotic algae. The significance of these sequence comparisons to the current hypotheses concerning the origin of eukaryotic cells and their chloroplasts is discussed.