Plastocyanin cytochrome f interaction

Altered coordination in a blue copper protein upon association with redox partner revealed by carbon-deuterium vibrational probes

Proteins tune the reactivity of metal sites; less understood is the impact of association with a redox partner. We demonstrate the utility of carbon-deuterium labels for selective analysis of delicate...

David W. Krogmann, 1931-2016

We provide here reflections on the life and career of David W. Krogmann (1931-2016), a great scientist, a mentor and an outstanding teacher, who had a remarkable impact on anyone who came in contact with him. Dave was a pillar of photosynthesis at Purdue University, and an international authority on electron transfer intermediates in oxygenic photosynthesis, particularly the soluble cytochromes. The photosynthetic system of his choice was cyanobacteria, and one of his major discoveries was the Orange Carotenoid Protein in these microrganisms.

Born in 1949 in postwar Japan

In this article, I would like to look back at my life as a researcher of photosynthesis. I was born in 1949, and grew up and was educated in postwar Japan in the 1950s and 1960s. I have studied photosynthesis, in particular Photosystem II, after research experiences in the USA and UK. My study of Photosystem II has continued over 43 years until now. Through the present retrospection, I would like to suggest that all photosynthesis researchers, including the members of the "49ers", many other established scientists, and young students as well, should not simply stay in the lab working hard on their studies and writing papers; but should also do something for the public. People want to learn from us about many critical social issues such as the environment, food, energy and, most importantly, peace. I believe that our knowledge must form an important basis for people to take action to create a peaceful and harmonious human society.

A Brownian Dynamics computational study of the interaction of spinach plastocyanin with turnip cytochrome f: the importance of plastocyanin conformational changes

Brownian Dynamics (BD) computer simulations were used to study electrostatic interactions between turnip cytochrome f (cyt f) and spinach plastocyanin (PC). Three different spinach PC structures were studied: The X-ray crystal structure of Xue and coworkers [(1998) Protein Sci 7:2099-2105] and the NMR structure of Musiani et al. [(2005) J Biol Chem 280:18833-18841] and Ubbink and co-workers [(1998) Structure 6:323-335]. Significant differences exist in the backbone conformation between the PC taken from Ubbink and coworkers and the other two PC structures particularly the regions surrounding G10, E59-E60, and D51. Complexes formed in BD simulations using the PC of Ubbink and colleagues had a smaller Cu-Fe distance than the other two. These results suggest that different PC conformations may exist in solution with different capabilities of forming electron-transfer-active docks. All three types of complexes show electrostatic contacts between D42, E43, and D44 on PC and K187 on cyt f as well as between E59 on PC and K58 on cyt f. However, the PC of Ubbink and coworkers reveals additional contacts between D51 and cyt f as a result of the difference in backbone configuration. A second minor complex component was observed for the PC of Ubbink and co-workers and Xue and co-workers which had contacts between K187 on cyt f and E59 and E60 on PC rather than between K187 on cyt f and D42-D44 on PC as observed for the major components. This second type of complex may represent an earlier complex which rearranges to form a final complex capable of electron transfer.

Reduction of plastocyanin by tyrosine-containing oligopeptides

Oxidized plastocyanin (PC) was reduced with TyrTyrTyr and LysLysLysLysTyrTyrTyr (KKKKYYY) oligopeptides at neutral pH. The TyrTyrTyr site of the peptides provided an electron to the copper active site of PC, whereas the tetralysine site of KKKKYYY functioned as the recognition site for the negative patch of PC. The reciprocal initial rate constant (1/k(int)) increased linearly with the reciprocal TyrTyrTyr concentration and proton concentration, although the electron transfer rate decreased gradually with time. The results showed that PC was reduced by the deprotonated species of TyrTyrTyr. A linear increase of log k(int) with increase in the ionic strength was observed due to decrease in the electrostatic repulsion between negatively charged PC and deprotonated (TyrTyrTyr)(-). PC was reduced faster by an addition of KKKKYYY to the PC-TyrTyrTyr solution, although KKKKYYY could not reduce PC without TyrTyrTyr. The ESI-LCMS spectrum of the products from the reaction between PC and TyrTyrTyr showed molecular ion peaks at m/z 1015.7 and 1037.7, which suggested formation of a dimerized peptide that may be produced from the reaction of a tyrosyl radical. The results indicate that PC and the tyrosine-containing oligopeptides form an equilibrium, PC(ox)/(oligopeptide)(-)-->/<--PC(red)/(oligopeptide)(*). The equilibrium is usually shifted to the left, but could shift to the right when the produced oligopeptide radical reacts with unreacted peptides. For the reaction of PC with KKKKYYY in the absence of TyrTyrTyr, the produced KKKK(YYY)(*) radical peptide could not react with other KKKKYYY peptides, since they were positively charged. In the presence of both KKKKYYY and TyrTyrTyr, PC may interact effectively with KKKKYYY through its tetralysine site and receive an electron from its TyrTyrTyr site, where the produced KKKK(YYY)(*) may interact with TyrTyrTyr peptides.

A Brownian dynamics study of the effects of cytochrome f structure and deletion of its small domain in interactions with cytochrome c6 and plastocyanin in Chlamydomonas reinhardtii

The availability of seven different structures of cytochrome f (cyt f) from Chlamydomonas reinhardtii allowed us, using Brownian dynamics simulations, to model interactions between these molecules and their redox partners, plastocyanin (PC) and cytochrome c6 (cyt c6) in the same species to study the effect of cyt f structure on its function. Our results showed that different cyt f structures, which are very similar, produced different reaction rates in interactions with PC and cyt c6. We were able to attribute this to structural differences among these molecules, particularly to a small flexible loop between A-184 and G-191 (which has some of the highest crystallographic temperature factors in all of the cyt f structures) on the cyt f small domain. We also showed that deletion of the cyt f small domain affected cyt c6 more than PC, due to their different binding positions on cyt f. One function of the small domain in cyt f may be to guide PC or cyt c6 to a uniform dock with cyt f, especially due to electrostatic interactions with K-188 and K-189 on this domain. Our results could serve as a good guide for future experimental work on these proteins to understand better the electron transfer process between them. Also, these results demonstrated the sensitivity and the power of the Brownian dynamics simulations in the study of molecular interactions.

A Brownian dynamics study of the interaction of Phormidium cytochrome f with various cyanobacterial plastocyanins

Brownian dynamics simulations were used to study the role of electrostatic forces in the interactions of cytochrome f from the cyanobacterium Phormidium laminosum with various cyanobacterial plastocyanins. Both the net charge on the plastocyanin molecule and the charge configuration around H92 (H87 in higher plants) are important in determining the interactions. Those plastocyanins (PCs) with a net charge more negative than -2.0, including those from Synechococcus sp. PCC7942, Synechocystis sp. 6803, and P. laminosum showed very little complex formation. On the other hand, complex formation for those with a net charge more positive than -2.0 (including Nostoc sp. PCC7119 and Prochlorothrix hollandica) as well as Nostoc plastocyanin mutants showed a linear dependence of complex formation upon the net charge on the plastocyanin molecule. Mutation of charged residues on the surface of the PC molecules also affected complex formation. Simulations involving plastocyanin mutants K35A, R93A, and K11A (when present) showed inhibition of complex formation. In contrast, D10A and E17A mutants showed an increase in complex formation. All of these residues surround the H92 (H87 in higher plant plastocyanins) ligand to the copper. An examination of the closest electrostatic contacts shows that these residues interact with D63, E123, R157, D188, and the heme on Phormidium cytochrome f. In the complexes formed, the long axis of the PC molecule lies perpendicular to the long axis of cytochrome f. There is considerable heterogeneity in the orientation of plastocyanin in the complexes formed.

The transient complex of poplar plastocyanin with cytochrome f: effects of ionic strength and pH

The orientation of poplar plastocyanin in the complex with turnip cytochrome f has been determined by rigid-body calculations using restraints from paramagnetic NMR measurements. The results show that poplar plastocyanin interacts with cytochrome f with the hydrophobic patch of plastocyanin close to the heme region on cytochrome f and via electrostatic interactions between the charged patches on both proteins. Plastocyanin is tilted relative to the orientation reported for spinach plastocyanin, resulting in a longer distance between iron and copper (13.9 A). With increasing ionic strength, from 0.01 to 0.11 M, all observed chemical-shift changes decrease uniformly, supporting the idea that electrostatic forces contribute to complex formation. There is no indication for a rearrangement of the transient complex in this ionic strength range, contrary to what had been proposed earlier on the basis of kinetic data. By decreasing the pH from pH 7.7 to pH 5.5, the complex is destabilized. This may be attributed to the protonation of the conserved acidic patches or the copper ligand His87 in poplar plastocyanin, which are shown to have similar pK(a) values. The results are interpreted in a two-step model for complex formation.

Brownian dynamics study of cytochrome f interactions with cytochrome c6 and plastocyanin in Chlamydomonas reinhardtii plastocyanin, and cytochrome c6 mutants

Using Brownian dynamics simulations, all of the charged residues in Chlamydomonas reinhardtii cytochrome c(6) (cyt c(6)) and plastocyanin (PC) were mutated to alanine and their interactions with cytochrome f (cyt f) were modeled. Systematic mutation of charged residues on both PC and cyt c(6) confirmed that electrostatic interactions (at least in vitro) play an important role in bringing these proteins sufficiently close to cyt f to allow hydrophobic and van der Waals interactions to form the final electron transfer-active complex. The charged residue mutants on PC and cyt c(6) displayed similar inhibition classes. Our results indicate a difference between the two acidic clusters on PC. Mutations D44A and E43A of the lower cluster showed greater inhibition than do any of the mutations of the upper cluster residues. Replacement of acidic residues on cyt c(6) that correspond to the PC's lower cluster, particularly E70 and E69, was observed to be more inhibitory than those corresponding to the upper cluster. In PC residues D42, E43, D44, D53, D59, D61, and E85, and in cyt c(6) residues D2, E54, K57, D65, R66, E70, E71, and the heme had significant electrostatic contacts with cyt f charged residues. PC and cyt c(6) showed different binding sites and orientations on cyt f. As there are no experimental cyt c(6) mutation data available for algae, our results could serve as a good guide for future experimental work on this protein. The comparison between computational values and the available experimental data (for PC-cyt f interactions) showed overall good agreement, which supports the predictive power of Brownian dynamics simulations in mutagenesis studies.

A Brownian dynamics study of the interaction of Phormidium laminosum plastocyanin with Phormidium laminosum cytochrome f

The interaction of Phormidium laminosum plastocyanin (PC) with P. laminosum cytochrome f (cyt f) was studied using Brownian dynamics (BD) simulations. Few complexes and a low rate of electron transfer were observed for wild-type PC. Increasing the positive electrostatic field on PC by the addition of a Zn(2+) ion in the neighborhood of D44 and D45 on PC (as found in crystal structure of plastocyanin) increased the number of complexes formed and the calculated rates of electron transfer as did PC mutations D44A, D45A, E54A, and E57A. Mutations of charged residues on Phormidium PC and Phormidium cyt f were used to map binding sites on both proteins. In both the presence and absence of the Zn(2+) ion, the following residues on PC interact with cyt f: D44, D45, K6, D79, R93, and K100 that lie in a patch just below H92 and Y88 and D10, E17, and E70 located on the upper portion of the PC molecule. In the absence of the Zn(2+) ion, K6 and K35 on the top of the PC molecule also interact with cyt f. Cyt f residues involved in binding PC, in the absence of the Zn(2+) ion, include E165, D187, and D188 that are located on the small domain of cyt f. The orientation of PC in the complexes was quite random in accordance with NMR results. In the presence of the Zn(2+) ion, K53 and E54 in the lower patch of the PC molecule also interact with cyt f and PC interacts with E86, E95, and E123 on the large domain of cyt f. Also, the orientation of PC in the complexes was much more uniform than in the absence of the Zn(2+) ion. The difference may be due to both the larger electrostatic field and the greater asymmetry of the charge distribution on PC observed in the presence of the Zn(2+) ion. Hydrophobic interactions were also observed suggesting a model of cyt f-PC interactions in which electrostatic forces bring the two molecules together but hydrophobic interactions participate in stabilizing the final electron-transfer-active dock.

Interaction of plastocyanin with oligopeptides: effect of lysine distribution within the peptide

We synthesized and purified four oligopeptides containing four lysines (KKKK, GKKGGKK, KKGGGKK, and KGKGKGK) as models for the plastocyanin (PC) interacting site of cytochrome f. These peptides competitively inhibited electron transfer between cytochrome c and PC. The inhibitory effect increased as the peptide concentrations were increased. The association constants between PC and the peptides did not differ significantly (3500-5100 M(-1)), although the association constant of PC-KGKGKGK was a little larger than the constants between PC and other peptides. Changes in the absorption spectrum of PC were observed when the peptides were added to the PC solution: peaks and troughs were detected at about 460 and 630 nm and at about 560 and 700 nm, respectively, in the difference absorption spectra between the spectra with and without peptides. These changes were attributed to the structural change at the copper site of PC by interaction with the peptides. The structural change was most significant when tetralysine was used. These results show that binding of the oligopeptide to PC is slightly more efficient when lysines are distributed uniformly within the peptide, whereas the structural change of PC becomes larger when the lysines are close to each other within the peptide.

Brownian dynamics simulations of the interaction of Chlamydomonas cytochrome f with plastocyanin and cytochrome c6

The interaction of Chlamydomonas cytochrome f (cyt f) with either Chlamydomonas plastocyanin (PC) or Chlamydomonas cytochrome c(6) (cyt c(6)) was studied using Brownian dynamics simulations. The two electron acceptors (PC and cyt c(6)) were found to be essentially interchangeable despite a lack of sequence homology and different secondary structures (beta-sheet for PC and alpha-helix for cyt c(6)). Simulations using PC and cyt c(6) interacting with cyt f showed approximately equal numbers of successful complexes and calculated rates of electron transfer. Cyt f-PC and cyt f-cyt c(6) showed the same types of interactions. Hydrophobic residues surrounding the Y1 ligand to the heme on cyt f interacted with hydrophobic residues on PC (surrounding the H87 ligand to the Cu) or cyt c(6) (surrounding the heme). Both types of complexes were stabilized by electrostatic interactions between K65, K188, and K189 on cyt f and conserved anionic residues on PC (E43, D44, D53, and E85) or cyt c(6) (E2, E70, and E71). Mutations on cyt f had identical effects on its interaction with either PC or cyt c(6). K65A, K188A, and K189A showed the largest effects whereas residues such as K217A, R88A, and K110A, which are located far from the positive patch on cyt f, showed very little inhibition. The effect of mutations observed in Brownian dynamics simulations paralleled those observed in experiments.

Surface interactions in the complex between cytochrome f and the E43Q/D44N and E59K/E60Q plastocyanin double mutants as determined by (1)H-NMR chemical shift analysis

A combination of site-directed mutagenesis and NMR chemical shift perturbation analysis of backbone and side-chain protons has been used to characterize the transient complex of the photosynthetic redox proteins plastocyanin and cytochrome f. To elucidate the importance of charged residues on complex formation, the complex of cytochrome f and E43Q/D44N or E59K/E60Q spinach plastocyanin double mutants was studied by full analysis of the (1)H chemical shifts by use of two-dimensional homonuclear NMR spectra. Both mutants show a significant overall decrease in chemical shift perturbations compared with wild-type plastocyanin, in agreement with a large decrease in binding affinity. Qualitatively, the E43Q/D44N mutant showed a similar interaction surface as wild-type plastocyanin. The interaction surface in the E59K/E60Q mutant was distinctly different from wild type. It is concluded that all four charged residues contribute to the affinity and that residues E59 and E60 have an additional role in fine tuning the orientation of the proteins in the complex.

The role of individual lysine residues in the basic patch on turnip cytochrome f for electrostatic interactions with plastocyanin in vitro

The role of electrostatic interactions in determining the rate of electron transfer between cytochrome f and plastocyanin has been examined in vitro with mutants of turnip cytochrome f and mutants of pea and spinach plastocyanins. Mutation of lysine residues Lys58, Lys65 and Lys187 of cytochrome f to neutral or acidic residues resulted in decreased binding constants and decreased rates of electron transfer to wild-type pea plastocyanin. Interaction of the cytochrome f mutant K187E with the pea plastocyanin mutant D51K gave a further decrease in electron transfer rate, indicating that a complementary charge pair at these positions could not compensate for the decreased overall charge on the proteins. Similar results were obtained with the interaction of the cytochrome f mutant K187E with single, double and triple mutants of residues in the acidic patches of spinach plastocyanin. These results suggest that the lysine residues of the basic patch on cytochrome f are predominantly involved in long-range electrostatic interactions with plastocyanin. However, analysis of the data using thermodynamic cycles provided evidence for the interaction of Lys187 of cytochrome f with Asp51, Asp42 and Glu43 of plastocyanin in the complex, in agreement with a structural model of a cytochrome f-plastocyanin complex determined by NMR.

The role of amino-acid residues in the hydrophobic patch surrounding the haem group of cytochrome f in the interaction with plastocyanin

Soluble turnip cytochrome f has been purified from the periplasmic fraction of Escherichia coli expressing a truncated petA gene encoding the precursor protein lacking the C-terminal 33 amino-acid residues. The protein is identical [as judged by 1H-NMR spectroscopy, midpoint redox potential (+ 365 mV) and electron transfer reactions with plastocyanin] to cytochrome f purified from turnip leaves. Several residues in the hydrophobic patch surrounding the haem group have been changed by site-directed mutagenesis, and the proteins purified from E. coli. The Y1F and Q7N mutants showed only minor changes in the plastocyanin-binding constant Ka and the second-order rate constant for electron transfer to plastocyanin, whereas the Y160S mutant showed a 30% decrease in the overall rate of electron transfer caused in part by a 60% decrease in binding constant and partially compensated by an increased driving force due to a 27-mV decrease in redox potential. In contrast, the F4Y mutant showed increased rates of electron transfer which may be ascribed to an increased binding constant and a 14-mV decrease in midpoint redox potential. This indicates that subtle changes in the hydrophobic patch can influence rates of electron transfer to plastocyanin by changing the binding constants and altering the midpoint redox potential of the cytochrome haem group.

Brownian dynamics study of the interaction between plastocyanin and cytochrome f

The electrostatic interaction between plastocyanin (PC) and cytochrome f (cyt f), electron transfer partners in photosynthesis was studied using Brownian dynamics (BD) simulations. By using the software package MacroDox, which implements the BD algorithm of Northrup et al. (Northrup, S. H., J. O. Boles, and J. C. L. Reynolds. 1987. J. Phys. Chem. 91:5991-5998), we have modeled the interaction of the two proteins based on crystal structures of poplar PC and turnip cyt f at pH 7 and a variety of ionic strengths. We find that the electrostatic attraction between positively charged residues (K58, K65, K187, and R209, among others) on cyt f and negatively charged residues (E43, D44, E59, and E60, among others) on PC steers PC into a single dominant orientation with respect to cyt f, and furthermore, that the single dominant orientation that we observe is one that we had predicted in our previous work (Pearson, D. C., E. L. Gross, and E. S. David. 1996. Biophys. J. 71:64-76). This dominant orientation permits the formation of hydrophobic interactions, which are not implemented in the MacroDox algorithm. This proposed complex between PC and cyt f implicates H87, a copper ligand on PC, as the residue that accepts electrons from the heme on cyt f (and possibly through Y1 as we proposed previously). We argue for the existence of this single dominant complex on the basis of observations that the most favorable orientations of the interaction between PC and cyt f, as determined by grouping successful BD trajectories on the basis of closest contacts of charged residues, tend to overlap one another and have very close distances between the metal centers on the two proteins (copper on PC, iron on cyt f). We use this knowledge to develop a model for PC/cyt f interaction that places a reaction between the two proteins occurring when the copper-to-iron distance is between 16 and 17 A. This reaction distance gives a good estimate of the experimentally observed rate constant for PC-cyt f interaction. Analysis of BD results as a function of ionic strength predicts an interaction that happens less frequently and becomes less specific as ionic strength increases.

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.

The structure of the complex of plastocyanin and cytochrome f, determined by paramagnetic NMR and restrained rigid-body molecular dynamics

BACKGROUND

The reduction of plastocyanin by cytochrome f is part of the chain of photosynthetic electron transfer reactions that links photosystems II and I. The reaction is rapid and is influenced by charged residues on both proteins. Previously determined structures show that the plastocyanin copper and cytochrome f haem redox centres are some distance apart from the relevant charged sidechains, and until now it was unclear how a transient electrostatic complex can be formed that brings the redox centres sufficiently close for a rapid reaction.

RESULTS

A new approach was used to determine the structure of the transient complex between cytochrome f and plastocyanin. Diamagnetic chemical shift changes and intermolecular pseudocontact shifts in the NMR spectrum of plastocyanin were used as input in restrained rigid-body molecular dynamics calculations. An ensemble of ten structures was obtained, in which the root mean square deviation of the plastocyanin position relative to cytochrome f is 1.0 A. Electrostatic interaction is maintained at the same time as the hydrophobic side of plastocyanin makes close contact with the haem area, thus providing a short electron transfer pathway (Fe-Cu distance 10.9 A) via residues Tyr1 or Phe4 (cytochrome f) and the copper ligand His87 (plastocyanin).

CONCLUSIONS

The combined use of diamagnetic and paramagnetic chemical shift changes makes it possible to obtain detailed information about the structure of a transient complex of redox proteins. The structure suggests that the electrostatic interactions 'guide' the partners into a position that is optimal for electron transfer, and which may be stabilised by short-range interactions.

Electrostatic effects on electron-transfer kinetics in the cytochrome f-plastocyanin complex

In a complex of two electron-transfer proteins, their redox potentials can be shifted due to changes in the dielectric surroundings and the electrostatic potentials at each center caused by the charged residues of the partner. These effects are dependent on the geometry of the complex. Three different docking configurations (DCs) for intracomplex electron transfer between cytochrome f and plastocyanin were studied, defined by 1) close contact of the positively charged region of cytochrome f and the negatively charged regions of plastocyanin (DC1) and by (2, 3) close contact of the surface regions adjacent to the Fe and Cu redox centers (DC2 and DC3). The equilibrium energetics for electron transfer in DC1-DC3 are the same within approximately +/-0.1 kT. The lower reorganization energy for DC2 results in a slightly lower activation energy for this complex compared with DC1 and DC3. The long heme-copper distance (approximately 24 A) in the DC1 complex drastically decreases electronic coupling and makes this complex much less favorable for electron transfer than DC2 or DC3. DC1-like complexes can only serve as docking intermediates in the pathway toward formation of an electron-transfer-competent complex. Elimination of the four positive charges arising from the lysine residues in the positive patch of cytochrome f, as accomplished by mutagenesis, exerts a negligible effect (approximately 3 mV) on the redox potential difference between cyt f and PC.

Analysis of the 1H-NMR chemical shifts of Cu(I)-, Cu(II)- and Cd-substituted pea plastocyanin. Metal-dependent differences in the hydrogen-bond network around the copper site

To compare cadmium-substituted plastocyanin with copper plastocyanin, the 1H-NMR spectra of CuI-, CuII- and Cd-plastocyanin from pea have been analyzed. Full assignments of the spectra of CuI- and Cd-plastocyanin indicate chemical shift differences up to 1 ppm. The affected protons are located in the four loops that surround the Cu site. The largest differences were found for protons in the hydrogen bond network which stabilizes this part of the protein. This suggests that the chemical shift differences are caused by very small but extensive structural changes in the network upon replacement of CuI by Cd. For CuII-plastocyanin the resonances of 72% of the protons observed in the CuI form have been identified. Protons within approximately 0.9 nm of the CuII were not observed due to fast paramagnetic relaxation. The protons between 0.9-1.7 nm from the CuII showed chemical shift differences up to 0.4 ppm compared to both CuI- and Cd-plastocyanin. These differences can be predicted assuming that they represent pseudocontact shifts. When corrected for the pseudocontact shift contribution, the CuII-plastocyanin chemical shifts were nearly all identical within error to those of the Cd form, but not of the CuI-plastocyanin, indicating that the CuII-plastocyanin structure, in as far as it can be observed, resembles Cd-rather than CuI-plastocyanin. In a single stretch of residues (64-69) chemical shift differences remained between all three forms after correction. The fact that pseudocontact shifts were observed for protons which were not broadened may be attributable to the weaker distance dependence of the pseudocontact shift effect compared to paramagnetic relaxation. This results in two shells around the Cu atom, an inner paramagnetic shell (0-0.9 nm), in which protons are not observed due to broadening, and an outer paramagnetic shell (0.9-1.7 nm), in which protons can be observed and show pseudocontact shifts. It is concluded that Cd-plastocyanin is a suitable redox-inactive substitute for Cu-plastocyanin.

Electrostatic properties of cytochrome f: implications for docking with plastocyanin

The electrostatic properties of cytochrome f (cyt f), a member of the cytochrome b6f complex and reaction partner with plastocyanin (PC) in photosynthetic electron transport, are qualitatively studied with the goal of determining the mechanism of electron transfer between cyt f and PC. A crystal structure for cyt f was analyzed with the software package GRASP, revealing a large region of positive potential generated by a patch of positively charged residues (including K58, K65, K66, K122, K185, K187, and R209) and reinforced by the iron center of the heme. This positive field attracts the negative charges of the two acidic patches on the mobile electron carrier PC. Three docked complexes are obtained for the two proteins, based on electrostatic or hydrophobic interactions or both and on steric fits by manual docking methods. The first of these three complexes shows strong electrostatic interactions between K187 on cyt f and D44 on PC and between E59 on PC and K58 on cyt f. Two other manually docked complexes are proposed, implicating H87 on PC as the electron-accepting site from the iron center of cyt f through Y1. The second complex maintains the D44/K187 cross-link (but not the E59/K58 link) while increasing hydrophobic interactions between PC and cyt f. Hydrophobic interactions are increased still further in the third complex, whereas the link between K187 on cyt f and D44 on PC is broken. The proposed reaction mechanism, therefore, involves an initial electrostatic docking complex that gives rise to a nonpolar attraction between the regions surrounding H87 on PC and Y1 on cyt f, providing for an electron-transfer active complex.

SOME NEW STRUCTURAL ASPECTS AND OLD CONTROVERSIES CONCERNING THE CYTOCHROME b6f COMPLEX OF OXYGENIC PHOTOSYNTHESIS

The cytochrome b6f complex functions in oxygenic photosynthetic membranes as the redox link between the photosynthetic reaction center complexes II and I and also functions in proton translocation. It is an ideal integral membrane protein complex in which to study structure and function because of the existence of a large amount of primary sequence data, purified complex, the emergence of structures, and the ability of flash kinetic spectroscopy to assay function in a readily accessible ms-100 mus time domain. The redox active polypeptides are cytochromes f and b6 (organelle encoded) and the Rieske iron-sulfur protein (nuclear encoded) in a mol wt = 210,000 dimeric complex that is believed to contain 22-24 transmembrane helices. The high resolution structure of the lumen-side domain of cytochrome f shows it to be an elongate (75 A long) mostly beta-strand, two-domain protein, with the N-terminal alpha-amino group as orthogonal heme ligand and an internal linear 11-A bound water chain. An unusual electron transfer event, the oxidant-induced reduction of a significant fraction of the p (lumen)-side cytochrome b heme by plastosemiquinone indicates that the electron transfer pathway in the b6f complex can be described by a version of the Q-cycle mechanism, originally proposed to describe similar processes in the mitochondrial and bacterial bc1 complexes.

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.

Characterization of plastocyanin from the cyanobacterium Phormidium laminosum: copper-inducible expression and SecA-dependent targeting in Escherichia coli

Plastocyanin from the thermophilic cyanobacterium Phormidium laminosum has been purified, a partial amino acid sequence obtained and the gene cloned and sequenced. The derived amino acid sequence indicates that the plastocyanin protein is initially synthesized with an N-terminal leader sequence of 34 amino acids to direct it across the thylakoid membrane. The leader sequence consists of a positively charged N-terminal region, a hydrophobic region and a cleavage site, which are characteristic both of higher-plant chloroplast thylakoid transfer domains and of bacterial leader peptides. The petE gene and flanking regions have been cloned in Escherichia coli, and the plastocyanin protein is expressed and directed to the periplasmic space, with concomitant processing to the mature form. Targeting to the periplasm and processing of the plastocyanin protein in E. coli appears to be dependent on components of the Sec apparatus, since the unprocessed precursor accumulates in the cytoplasm of a secA mutant. Expression of plastocyanin in E. coli is copper-inducible and apparently controlled at the level of transcription, leading to the conclusion that copper-regulated promoters exist in the regions flanking the gene and are recognized in a heterologous system. Possible implications for gene expression and protein targeting in the cyanobacterium are discussed.

Electron-transfer from cytochrome c to ascorbate oxidase and its type 2 copper-depleted derivatives

Rate constants have been determined for the electron-transfer reactions between reduced horse heart cytochrome c and resting cucumber ascorbate oxidase as functions of pH, ionic strength, and temperature. The second-order rate constant for the oxidation of reduced cytochrome c was determined to be k = 820 M-1 s-1 in 0.2 M phosphate buffer at pH 6.0 and 25 degrees C. The activation parameters were estimated to be delta H++ = 5 kJ mol-1 and delta S++ = -188 Jmol-1 K-1. The rate constants increased with decreasing buffer concentration, indicating that the electron-transfer from cytochrome c to ascorbate oxidase is realized by the local electrostatic interaction between them in spite of the reaction between positively charged proteins. Reactions of type 2 copper-depleted ascorbate oxidase whose type 3 coppers were in the reduced or oxidized form indicated that the type 1 copper site accepts an electron from cytochrome c. The reaction rate was remarkably increased with decreasing pH for both the native enzyme and derivatives. Further, on addition of hexametaphosphate anion the rate of the electron-transfer decreased because the association of both proteins to realize the electron-transfer was inhibited due to a change in distribution of the local charge on the protein surface(s).

The kinetics of reactions around the cytochrome bf complex studied in an isolated system

The kinetics of oxidation and reduction of P700, plastocyanin, cytochrome f and cytochrome b-563 were studied in a reconstituted system consisting of Photosystem I particles, cytochrome bf complex and plastocyanin, all derived from pea leaf chloroplasts. Decyl plastoquinol was the reductant of the bf complex. Turnovers of the system were initiated by laser flashes. The reaction between oxidised P700 and plastocyanin was non-homogeneous in that a second-order rate coefficient of c. 5×10(-7) M(-1) s(-1) applied to 80% of the P700(+) and c. 0.7×10(7) M(-1) s(-1) to the remainder. In the presence of bf complex, but without quinol, the electron transfer between cytochrome f and oxidised plastocyanin could be described by a second-order rate coefficient of c. 4×10(7) M(-1) s(-1) (forward), and c. 1.6×10(7) M(-1) s(-1) (reverse). The equilibrium coefficient was thus 2.5. Unexpectedly, there was little reduction of cytochrome f (+) or plastocyanin(+) by electrons from the Rieske centre. With added quinol, reduction of cytochrome b-563 occurred. Concomitantly, electrons appeared in the oxidised species. It was inferred that either the Rieske centre was not involved in the high-potential chain of electron transfer events, or that, only in the presence of quinol, electrons were quickly passed from the Rieske centre to cytochrome f (+). Additionally, the presence of quinol altered the equilibrium coefficient for the cyt f/PC interaction from 2.5 to c. 5. The reaction between quinol and the bf complex was describable by a second-order rate coefficient of about 3×10(6) M(-1) s(-1). The pattern of the redox reactions around the bf complex could be simulated in detail with a Q-cycle model as previously found for chloroplasts.

Crystal structure of chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation

BACKGROUND

Cytochrome f is the high potential electron acceptor of the chloroplast cytochrome b6f complex, and is the electron donor to plastocyanin. The 285-residue cytochrome f subunit is anchored in the thylakoid membrane of the chloroplast by a single membrane-spanning segment near the carboxyl terminus. A soluble redox-active 252-residue lumen-side polypeptide with native spectroscopic and redox properties, missing the membrane anchor and carboxyl terminus, was purified from turnip chloroplasts for structural studies.

RESULTS

The crystal structure of cytochrome f, determined to 2.3 A resolution, has several unexpected features. The 252-residue polypeptide is organized into one large and one small domain. The larger heme-binding domain is strikingly different from known structures of other c-type cytochromes and has the same fold as the type III domain of the animal protein, fibronectin. Cytochrome f binds heme with an unprecedented axial heme iron ligand: the amino terminus of the polypeptide.

CONCLUSION

The first atomic structure of a subunit of either the cytochrome b6f complex or of the related cytochrome bc1 complex has been obtained. The structure of cytochrome f allows prediction of the approximate docking site of plastocyanin and should allow systematic studies of the mechanism of intra- and inter-protein electron transfer between the cytochrome heme and plastocyanin copper, which are approximately isopotential. The unprecedented axial heme iron ligand also provides information on the sequence of events (i.e. cleavage of signal peptide and ligation of heme) associated with translocation of the cytochrome across the membrane and its subsequent folding.

Plastocyanin: structural and functional analysis

Plastocyanin is one of the best characterized of the photosynthetic electron transfer proteins. Since the determination of the structure of poplar plastocyanin in 1978, the structure of algal (Scenedesmus, Enteromorpha, Chlamydomonas) and plant (French bean) plastocyanins has been determined either by crystallographic or NMR methods, and the poplar structure has been refined to 1.33 A resolution. Despite the sequence divergence among plastocyanins of algae and vascular plants (e.g., 62% sequence identity between the Chlamydomonas and poplar proteins), the three-dimensional structures are remarkably conserved (e.g., 0.76 A rms deviation in the C alpha positions between the Chlamydomonas and poplar proteins). Structural features include a distorted tetrahedral copper binding site at one end of an eight-stranded antiparallel beta-barrel, a pronounced negative patch, and a flat hydrophobic surface. The copper site is optimized for its electron transfer function, and the negative and hydrophobic patches are proposed to be involved in recognition of physiological reaction partners. Chemical modification, cross-linking, and site-directed mutagenesis experiments have confirmed the importance of the negative and hydrophobic patches in binding interactions with cytochrome f and Photosystem I, and validated the model of two functionally significant electron transfer paths in plastocyanin. One putative electron transfer path is relatively short (approximately 4 A) and involves the solvent-exposed copper ligand His-87 in the hydrophobic patch, while the other is more lengthy (approximately 12-15 A) and involves the nearly conserved residue Tyr-83 in the negative patch.

Structural aspects of the cytochrome b6f complex; structure of the lumen-side domain of cytochrome f

The following findings concerning the structure of the cytochrome b6f complex and its component polypeptides, cyt b6, subunit IV and cytochrome f subunit are discussed: (1) Comparison of the amino acid sequences of 13 and 16 cytochrome b6 and subunit IV polypeptides, respectively, led to (a) reconsideration of the helix lengths and probable interface regions, (b) identification of two likely surface-seeking helices in cyt b6 and one in SU IV, and (c) documentation of a high degree of sequence invariance compared to the mitochondrial cytochrome. The extent of identity is particularly high (88% for conserved and pseudoconserved residues) in the segments of cyt b6 predicted to be extrinsic on the n-side of the membrane. (2) The intramembrane attractive forces between trans-membrane helices that normally stabilize the packing of integral membrane proteins are relatively weak. (3) The complex isolated in dimeric form has been visualized, along with isolated monomer, by electron microscopy. The isolated dimer is much more active than the monomer, is the major form of the complex isolated and purified from chloroplasts, and is inferred to be a functional form in the membrane. (4) The isolated cyt b6f complex contains one molecule of chlorophyll a. (5) The structure of the 252 residue lumen-side domain of cytochrome f isolated from turnip chloroplasts has been solved by X-ray diffraction analysis to a resolution of 2.3 A.

Use of laser flash photolysis time-resolved spectrophotometry to investigate interprotein and intraprotein electron transfer mechanisms

A description is given of the methodology developed in our laboratory for the application of laser flash photolysis to the elucidation of the kinetics and mechanism of electron transfer processes which occur intermolecularly between two protein molecules within a collisional complex, or intramolecularly between two redox centers within a single multisubunit or multidomain protein. This involves the use of flavin analogs, excited to their lowest triplet state by a laser flash, to initiate electron transfer, either by oxidation of a sacrificial donor followed by redox protein reduction via the flavin semiquinone, or by direct oxidation of a reduced redox protein by the flavin triplet. Time-resolved spectrophotometry is used to follow the course of the sequence of electron transfer events initiated by the laser flash. The application of this methodology to the following systems is described: cytochrome c/cytochrome c peroxidase; ferredoxin/ferredoxin NADP+ reductase; cytochrome c/plastocyanin; flavocytochrome b2; and sulfite oxidase.

Plastocyanin: Structure and function

The aim of this review is to analyze the current state of knowledge concerning the blue copper protein plastocyanin (PC) focusing on its interactions with its reaction partners cytochromef and P700. Plastocyanin is a 10 kD blue copper protein which is located in the lumen of the thylakoid where it functions as a mobileelectron carrier shuttling electrons from cytochromef to P700 in Photosystem I. PC is a typical β-barrel protein containing a single copper center which is ligated to two histidines, a methionine and a cysteine in a distorted tetrahedral geometry. PC has two potential binding sites for reaction partners. Site 1 consists of the H87 ligand to the copper and Site 2 consists of Y83 which is surrounded by two clusters of negative charges which are highly conserved in higher plant PCs.The interaction of PC with cytochromef has been studied extensively. It is electrostatic in nature with negative charges on PC interacting with positive charges on cytochromef. Evidence from cross-linking, chemical modification, kinetics and site-directed mutagenesis studies implicate Site 2 as the binding site for Cytf. The interaction is thought to occur in two stages: an initial diffusional approach guided by electrostatic interactions, followed by more precise docking to form a final electron transfer complex.Due to the multisubunit nature of the Photosystem I complex, the evidence is not as clear for the binding site for P700. However, a small positively-charged subunit (Subunit III) of Photosystem I has been implicated in PC binding. Also, both chemical modification and site-directed mutagenesis experiments have suggested that PC interacts with P700 via Site 1.

Transient kinetics of electron transfer from a variety of c-type cytochromes to plastocyanin

Plastocyanin (PC) and its physiological reaction partner cytochrome (cyt) f form a complex which is electrostatically stabilized by interactions between complementary localized charges. We have measured the kinetics of intracomplex electron transfer between several reduced cytochromes and PC using laser flash photolysis. With spinach cyt f and spinach PC, we obtain first-order rate constants, kforward = 2780 s-1 and kreverse = 1050 s-1, for the reversible reaction and a complex dissociation constant of about 23 microM at an ionic strength (I) of 5 mM. The observed rate constant increases by a factor of 2 between I = 5 and 40 mM and then decreases monotonically at higher ionic strengths. This indicates that the complex is not completely dissociated until I = 150 mM and that the proteins within the electrostatically most stable complex are not optimally oriented for electron transfer. Similar results were obtained with turnip cyt f and spinach PC, although in this case intracomplex electron transfer is about 4 times as fast. Horse cyt c also forms an electrostatically stabilized complex with PC, and yields a limiting rate constant for intracomplex electron transfer (1750 s-1) and a dissociation constant (10 microM) comparable to those for spinach cyt f. The ionic strength dependence shows that the complex is more readily dissociated (complete at I = 25 mM) than is that of cyt f and that rearrangement is not required for optimal electron transfer. Addition of polylysine results in 10-fold inhibition of the rate of electron transfer. Pseudomonas cyt c-551 is an acidic cytochrome which does not form a complex with PC.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The cloning and sequencing of Synechococcus sp. PCC 7002 petCA operon: Implications for the cytochrome c-553 binding domain of cytochrome f

The genes encoding the Rieske iron-sulfur protein and cytochrome f from a unicellular, naturally transformable, photoheterotrophic cyanobacterium, Synechococcus sp. PCC 7002, formerly Agmenellum quadruplicatum, have been isolated and sequenced. The two genes were found to be on a single operon, petCA.The Synechococcus sp. PCC 7002 iron-sulfur protein contains 181 amino acids, the conserved putative iron-binding domains CTHLGCV, residues 108-114, and CPCHGS, residues 128-133, no presequence and has a 73% sequence identity to the Nostoc PCC 7906 iron-sulfur protein. The 325 amino acid apocytochrome f sequence contains a 42 amino acid presequence, a CANCH heme binding domain, residues 20-24 from the presumed start of the mature protein, and a predicted hydrophobic membrane-spanning domain, residues 250-269. The mature cytochrome f sequence has a 71.5% sequence identity with Nostoc PCC 7906 cytochrome f and possesses a large (-14) negative charge and low calculated pI of 4.47 compared to higher plant chloroplast sequences. Nine separate domains showing differences in charged residues among cyanobacteria and plants have been identified and the possibility that these domains are involved in the ionic interactions with plastocyanin or cytochrome c-553 is discussed.

High-resolution solution structure of reduced French bean plastocyanin and comparison with the crystal structure of poplar plastocyanin

The three-dimensional solution structure of reduced (CuI) plastocyanin from French bean leaves has been determined by distance geometry and restrained molecular dynamics methods using constraints obtained from 1H n.m.r. (nuclear magnetic resonance) spectroscopy. A total of 1244 experimental constraints were used, including 1120 distance constraints, 103 dihedral angle constraints and 21 hydrogen bond constraints. Stereospecific assignments were made for 26 methylene groups and the methyls of 11 valines. Additional constraints on copper co-ordination were included in the restrained dynamics calculations. The structures are well defined with average atomic root-mean-square deviations from the mean of 0.45 A for all backbone heavy atoms and 1.08 A for side-chain heavy atoms. French bean plastocyanin adopts a beta-sandwich structure in solution that is similar to the X-ray structure of reduced poplar plastocyanin; the average atomic root-mean-square difference between 16 n.m.r. structures and the X-ray structure is 0.76 A for all backbone heavy atoms. The conformations of the side-chains that constitute the hydrophobic core of French bean plastocyanin are very well defined. Of 47 conserved residues that populate a single chi 1 angle in solution, 43 have the same rotamer in the X-ray structure. Many surface side-chains adopt highly preferred conformations in solution, although the 3J alpha beta coupling constants often indicate some degree of conformational averaging. Some surface side-chains are disordered in both the solution and crystal structures of plastocyanin. There is a striking correlation between measures of side-chain disorder in solution and side-chain temperature factors in the X-ray structure. Side-chains that form a distinctive acidic surface region, believed to be important in binding other electron transfer proteins, appear to be disordered. Fifty backbone amide protons form hydrogen bonds to carbonyls in more than 60% of the n.m.r. structures; 45 of these amide protons exchange slowly with solvent deuterons. Ten hydrogen bonds are formed between side-chain and backbone atoms, eight of which are correlated with decreased proton exchange. Of the 60 hydrogen bonds formed in French bean plastocyanin, 56 occur in the X-ray structure of the poplar protein; two of the missing hydrogen bonds are absent as a result of mutations. It appears that molecular dynamics refinement of highly constrained n.m.r. structures allows accurate prediction of the pattern of hydrogen bonding.

The use of monoclonal antibodies to study the structure and function of cytochrome f

Monoclonal antibodies (MAbs) were prepared against native cytochrome f (cyt f) isolated from turnip leaves. The two MAbs obtained, designated MAb-JB2 and MAb-ED4, were Western blot positive to purified turnip cytochrome f and also reacted with inside-out (ISO) but not right-side-out (RSO) spinach thylakoid membranes. MAb-ED4 reacted with a covalent adduct formed by crosslinking cyt f and plastocyanin (PC), whereas MAb-JB2 did not. In contrast, MAb-JB2 reacted with the isolated cyt b6/f complex but MAb-ED4 did not. These results indicate that MAb-JB2 binds to cyt f at or near the PC binding site on f, whereas MAb-ED4 binds to a portion of cyt f which is not exposed in the cyt b6/f complex. The location of the epitopes in the primary sequence of cyt f was determined by trypsin hydrolysis, HPLC separation of tryptic peptides, and ELISA identification of the purified peptides. The molecular weights of the purified peptides, determined by gel exclusion chromatography, were found to be 5040 and 3130 Da for MAb-JB2 and MAb-ED4, respectively. Amino acid sequencing showed that the first eight amino acids of the MAb-ED4 positive peptide were L-D-Q-P-L-T-S-N. These results suggest that the 3130-Da peptide has 28 amino acids extending from Leu 223 to Arg 250. This peptide is located on the N-terminal (lumen) side of the postulated membrane-spanning sequence. The first eight amino acids of the MAb-JB2-positive peptide were N-I-L-V-I-G-P-V. This sequence and the peptide molecular weight indicate that the epitope for MAb-JB2 is located within a 44-amino acid peptide extending from Asn 111 to Arg 154.

Reconstitution of mature plastocyanin from precursor apo-plastocyanin expressed in Escherichia coli

The precursor plastocyanin from Silene pratensis (white campion) has been expressed in Escherichia coli. The precursor protein was accumulated in insoluble aggregates and partially purified as an apo-protein. The purified precursor apo-plastocyanin was processed to the mature apo-plastocyanin by chloroplast extracts. N-terminal amino-acid sequencing indicated that the processed protein was identical to the N-terminal amino-acid residues of mature plastocyanin that was deduced from the nucleotide sequence. The copper could be incorporated into the apo-plastocyanin of mature size in vitro, but could not into the precursor apo-plastocyanin under the same conditions. Absorption spectra and reduction potential of the reconstituted mature plastocyanin were indistinguishable from those of the purified spinach plastocyanin. The electron transfer activities of the reconstituted plastocyanin with both the Photosystem I reaction center (P700) and cytochrome f were almost the same as those of the purified spinach plastocyanin.

The interaction of nitrotyrosine-83 plastocyanin with cytochromes f and c: pH dependence and the effect of an additional negative charge on plastocyanin

Spinach plastocyanin was selectively modified using tetranitromethane which incorporates a nitro group ortho to the hydroxyl group of tyrosine 83 (Anderson, G.P., Draheim, J.E. and Gross, E.L. (1985) Biochim. Biophys. Acta 810, 123-131). This tyrosine residue has been postulated to be part of the cytochrome f binding site on plastocyanin. Since the hydroxyl moiety of nitrotyrosine 83 is deprotonated above its pK of 8.3, it provides a useful modification for studying the effect of an extra negative charge on the interaction of plastocyanin with cytochrome f. No effect on cytochrome f oxidation was observed at pH 7 under conditions in which the hydroxyl moiety is protonated. However, the rate of cytochrome f oxidation increased at pH values greater than 8, reaching a maximum at pH 8.6 and decreasing at still higher pH values. The increase was half-maximal at pH 8.3 which is the pK for the hydroxyl moiety on nitrotyrosine 83. In contrast, the rate of cytochrome f oxidation for control plastocyanin was independent of pH from pH 7 to 8.6. These results show that increasing the negative charge on plastocyanin at Tyr-83 increases the ability to react with cytochrome f, supporting the hypothesis that cytochrome f interacts with plastocyanin at this location. In contrast, the reaction of Ntyr-83 plastocyanin with mammalian cytochrome c was independent of pH, suggesting that its mode of interaction with plastocyanin is different from that of cytochrome f. A comparison of the effects of Ntyr-83 modification of plastocyanin with the carboxyl- and amino-group modifications reported previously suggests that plastocyanin binds to cytochrome f in such a way that electrons could be donated to plastocyanin at either of its two binding sites.

Comparison of the kinetics of reduction and intramolecular electron transfer in electrostatic and covalent complexes of ferredoxin-NADP+ reductase and flavodoxin from Anabaena PCC 7119

The kinetics of reduction and intracomplex electron transfer in electrostatically stabilized and covalently crosslinked complexes between ferredoxin-NADP+ reductase (FNR) and flavodoxin (Fld) from the cyanobacterium Anabaena PCC 7119 were compared using laser flash photolysis. The second-order rate constant for reduction by 5-deazariboflavin semiquinone (dRfH) of FNR within the electrostatically stabilized complex at 10 mM ionic strength (4.0 X 10(8) M-1 s-1) was identical to that for free FNR. This suggests that the FAD cofactor of FNR is not sterically hindered upon complex formation. A lower limit of approximately 7000 s-1 was estimated for the first-order rate constant for intracomplex electron transfer from FNRred to Fldox under these conditions. In contrast, for the covalently crosslinked complex, a smaller second-order rate constant (2.1 X 10(8) M-1 s-1) was obtained for the reduction of FNR by dRfH within the complex, suggesting that some steric hindrance of the FAD cofactor of FNR occurs due to crosslinking. A limiting rate constant of 1000 s-1 for the intracomplex electron transfer reaction was obtained for the covalent complex, which was unaffected by changes in ionic strength. The substantially diminished limiting rate constant, relative to that of the electrostatic complex, may reflect either a suboptimal orientation of the redox cofactors within the covalent complex or a required structural reorganization preceding electron transfer which is not allowed once the proteins have been covalently linked. Thus, although the covalent complex is biochemically competent, it is not a quantitatively precise model for the catalytically relevant intermediate along the reaction pathway.