Thylakoid terminal oxidases are essential for the cyanobacterium Synechocystis sp. PCC 6803 to survive rapidly changing light intensities.

Cyanobacteria perform photosynthesis and respiration in the thylakoid membrane, suggesting that the two processes are interlinked. However, the role of the respiratory electron transfer chain under natural environmental conditions has not been established. Through targeted gene disruption, mutants of Synechocystis sp.

The role of a disulfide bridge in the stability and folding kinetics of Arabidopsis thaliana cytochrome c(6A).

Cytochrome c(6A) is a eukaryotic member of the Class I cytochrome c family possessing a high structural homology with photosynthetic cytochrome c(6) from cyanobacteria, but structurally and functionally distinct through the presence of a disulfide bond and a heme mid-point redox potential of +71mV (vs normal hydrogen electrode). The disulfide bond is part of a loop insertion peptide that forms a cap-like structure on top of the core α-helical fold. We have investigated the contribution of the disulfide bond to thermodynamic stability and (un)folding kinetics in cytochrome c(6A) from Arabidopsis thaliana by making comparison with a photosynthetic cytochrome c(6) from Phormidium laminosum and through a mutant in which the Cys residues have been replaced with Ser residues (C67/73S).

Structural and kinetic studies of imidazole binding to two members of the cytochrome c (6) family reveal an important role for a conserved heme pocket residue.

The amino acid at position 51 in the cytochrome c(6) family is responsible for modulating over 100 mV of heme midpoint redox potential. As part of the present work, the X-ray structure of the imidazole adduct of the photosynthetic cytochrome c(6) Q51V variant from Phormidium laminosum has been determined. The structure reveals the axial Met ligand is dissociated from the heme iron but remains inside the heme pocket and the Ω-loop housing the Met ligand is stabilized through polar interactions with the imidazole and heme propionate-6.

Cytochrome c6A: discovery, structure and properties responsible for its low haem redox potential.

Cytochrome c(6A) is a unique dithio-cytochrome of green algae and plants. It has a very similar core structure to that of bacterial and algal cytochromes c(6), but is unable to fulfil the same function of transferring electrons from cytochrome f to Photosystem I. A key feature of cytochrome c(6A) is that its haem midpoint potential is more than 200 mV below that of cytochrome c(6) (E(m) approximately +340 mV) despite both cytochromes having histidine and methionine residues as axial haem-iron ligands.

Modulation of heme redox potential in the cytochrome c6 family.

Cytochrome c6A is a unique dithio-cytochrome of green algae and plants. It has a very similar core structure to that of bacterial and algal cytochromes c6 but is unable to fulfill the same function of transferring electrons from cytochrome f to photosystem I. A key feature is that its heme midpoint potential is more than 200 mV below that of cytochrome c6 despite having His and Met as axial heme-iron ligands.

Structure of cytochrome c6A, a novel dithio-cytochrome of Arabidopsis thaliana, and its reactivity with plastocyanin: implications for function.

Cytochrome c6A is a unique dithio-cytochrome present in land plants and some green algae. Its sequence and occurrence in the thylakoid lumen suggest that it is derived from cytochrome c6, which functions in photosynthetic electron transfer between the cytochrome b6f complex and photosystem I. Its known properties, however, and a strong indication that the disulfide group is not purely structural, indicate that it has a different, unidentified function.

Cytochrome c(6A) is a funnel for thiol oxidation in the thylakoid lumen.

Cytochrome c(6A) is a dithio-cytochrome recently discovered in land plants and green algae, and believed to be derived from the well-known cytochrome c(6). The function of cytochrome c(6A) is unclear. We propose that it catalyses the formation of disulphide bridges in thylakoid lumen proteins in a single-step disulphide exchange reaction, with subsequent transfer of the reducing equivalents to plastocyanin.

The Unfinished Story of Cytochrome f.

Cytochrome f is a unique, integral membrane protein. The background to its discovery by Robert Hill (1899-1991) and Ronald Scarisbrick over 60 years ago and the influence of David Keilin (1887-1963) and Frederick Gowland Hopkins (1861-1947) are discussed. The development of methods for isolating cytochrome f is outlined, emphasizing the remarkable achievement of Hill and Scarisbrick at a time when few if any membrane proteins had been isolated, and the importance of the discovery of a natural proteolysis in Brassica spp.

The novel cytochrome c6 of chloroplasts: a case of evolutionary bricolage?

Cytochrome c6 has long been known as a redox carrier of the thylakoid lumen of cyanobacteria and some eukaryotic algae that can substitute for plastocyanin in electron transfer. Until recently, it was widely accepted that land plants lack a cytochrome c6. However, a homologue of the protein has now been identified in several plant species together with an additional isoform in the green alga Chlamydomonas reinhardtii.

Implications of the effects of viscosity, macromolecular crowding, and temperature for the transient interaction between cytochrome f and plastocyanin from the cyanobacterium Phormidium laminosum.

The reaction between cytochrome f and plastocyanin is a central feature of the photosynthetic electron-transport system of all oxygenic organisms. We have studied the reaction in solution to understand how the very weak binding between the two proteins from Phormidium laminosum can nevertheless lead to fast rates of electron transfer. In a previous publication [Schlarb-Ridley, B.

Redox properties of Arabidopsis cytochrome c6 are independent of the loop extension specific to higher plants.

Cytochrome c6 (cytc6) from Arabidopsis differs from the cyanobacterial and algal homologues in several redox properties. It is possible that these differences might be due to the presence of a 12 amino acid residue loop extension common to higher plant cytc6 proteins. However, homology modelling suggests this is not the case.

Role of charges on cytochrome f from the cyanobacterium Phormidium laminosum in its interaction with plastocyanin.

The role of charge on the surface of cytochrome f from the cyanobacterium Phormidium laminosum in the reaction with plastocyanin was investigated in vitro using site-directed mutagenesis. Charge was neutralized at five acidic residues individually and introduced at a residue close to the interface between the two proteins. The effects on the kinetics of the reaction were measured using stopped-flow spectrophotometry, and the midpoint potentials of the mutant proteins were determined.

Relation between interface properties and kinetics of electron transfer in the interaction of cytochrome f and plastocyanin from plants and the cyanobacterium Phormidium laminosum.

Cytochrome f and plastocyanin from the cyanobacterium Phormidium laminosum react an order of magnitude faster than their counterparts from chloroplasts when long-range electrostatic interactions have been screened out by high salt concentration [Schlarb-Ridley, B. G., et al.

Role of electrostatics in the interaction between plastocyanin and photosystem I of the cyanobacterium Phormidium laminosum.

The interactions between photosystem I and five charge mutants of plastocyanin from the cyanobacterium Phormidium laminosum were investigated in vitro. The dependence of the overall rate constant of reaction, k2, on ionic strength was investigated using laser flash photolysis. The rate constant of the wild-type reaction increased with ionic strength, indicating repulsion between the reaction partners.

Role of electrostatics in the interaction between cytochrome f and plastocyanin of the cyanobacterium Phormidium laminosum.

The role of charged residues on the surface of plastocyanin from the cyanobacterium Phormidium laminosum in the reaction with soluble cytochrome f in vitro was studied using site-directed mutagenesis. The charge on each of five residues on the eastern face of plastocyanin was neutralized and/or inverted, and the effect of the mutation on midpoint potentials was determined. The dependence of the overall rate constant of reaction, k(2), on ionic strength was investigated using stopped-flow spectrophotometry.

The role of acidic residues of plastocyanin in its interaction with cytochrome ƒ.

The role of the acidic patches of spinach plastocyanin in the interaction with a soluble form of turnip cytochrome ƒ was studied by a combination of site-directed mutagenesis, NMR spectroscopy and kinetic analysis. The charge of the two 'eastern' patches, consisting of conserved acidic residues 42-45 and 59-61 respectively, was altered by incorporation of neutral or positively charged groups. Up to four negative charges were deleted in six different mutants and a further mutant, Q88E, provided an additional negative charge in the same region.