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Abstract: Plastocyanin and cytochrome c 6 are two small soluble electron carriers located in the intrathylacoidal space of cyanobacteria. Although their role as electron shuttle between the cytochrome b 6 f and photosystem I complexes in the photosynthetic pathway is well established, their participation in the respiratory electron transport chain as donors to the terminal oxidase is still under debate. Here, we present the first time-resolved analysis showing that both cytochrome c 6 and plastocyanin can be efficiently oxidized by the aa 3 type cytochrome c oxidase in Nostoc sp. PCC 7119. The apparent electron transfer rate constants are ca. 250 and 300s−1 for cytochrome c 6 and plastocyanin, respectively. These constants are 10 times higher than those obtained for the oxidation of horse cytochrome c by the oxidase, in spite of being a reaction thermodynamically more favourable. [Copyright &y& Elsevier]

Abstract: Cyanobacteria are significant contributors to global photosynthetic productivity, thus making it relevant to study how the different environmental stresses can alter their physiological activities. Here, we review the current research work on the response of cyanobacteria to different kinds of stress, mainly focusing on their response to metal stress as studied by using the modern proteomic tools. We also report a proteomic analysis of plastocyanin and cytochrome c 6 deletion mutants of the cyanobacterium Synechocystis sp. PCC 6803 grown under copper or iron deprivation, as compared to wild-type cells, so as to get a further understanding of the metal homeostasis in cyanobacteria and their response to changing environmental conditions. [Copyright &y& Elsevier]

Cu2+ binding is so far the best characterized property of the prion protein. This interaction has been mapped to the N-terminal domain of the prion protein where multiple His residues occur largely embedded within the repetitive PHGGGWGQ sequence known as octarepeats. When Cu2+ interaction is studied using a solution of full-length bovine prion protein containing six octarepeats at protein concentrations above 25 μM, a drastic increase in solution turbidity is observed due to the formation of insoluble cation–protein complexes that appear as bidimensional polymer meshes. These bidimensional meshes consist of a single layer of protein molecules crosslinked by Cu2+ cations. Polymer formation is a cooperative process that proceeds by nucleation of protein molecules with a Cu2+ site occupancy of above 2. These results support the hypothesis that the N-terminal domain of prion protein is a ligand binding module that promotes crosslinked assembly, and suggest the existence of inter-repeat Cu2+ sites. [Copyright &y& Elsevier]

In addition to the standard NADPH thioredoxin reductases (NTRs), plants hold a plastidic NTR (NTRC), with a thioredoxin module fused at the C-terminus. NTRC is an efficient reductant of 2-Cys peroxiredoxins (2-Cys Prxs). The interaction of NTRC and chloroplastic thioredoxin x with 2-Cys Prxs has been confirmed in vivo, by bimolecular fluorescence complementation (BiFC) assays, and in vitro, by isothermal titration calorimetry (ITC) experiments. In comparison with thioredoxin x , NTRC interacts with 2-Cys Prx with higher affinity, both the thioredoxin and NTR domains of NTRC contributing significantly to this interaction, as demonstrated by using the NTR and thioredoxin modules of the enzyme expressed separately. The presence of the thioredoxin domain seems to prevent the interaction of NTRC with thioredoxin x . [ABSTRACT FROM AUTHOR]

Abstract: Under nitroxidative stress, a minor fraction of cytochrome c can be modified by tyrosine nitration. Here we analyze the specific effect of nitration of tyrosines 46 and 48 on the dual role of cytochrome c in cell survival and cell death. Our findings reveal that nitration of these two solvent-exposed residues has a negligible effect on the rate of electron transfer from cytochrome c to cytochrome c oxidase, but impairs the ability of the heme protein to activate caspase-9 by assembling a non-functional apoptosome. It seems that cytochrome c nitration under cellular stress counteracts apoptosis in light of the small amount of modified protein. We conclude that other changes such as increased peroxidase activity prevail and allow the execution of apoptosis. [Copyright &y& Elsevier]

Cytochrome cM is a new c-class photosynthetic haem protein whose physiological role is still unknown. It has been proposed previously that cytochrome cM can replace cytochrome c6 and plastocyanin in transferring electrons between the two membrane complexes cytochrome b6–f and photosystem I in organisms growing under stress conditions. The experimental evidence herein provided allows us to discard such a hypothesis. We report a procedure to overexpress cytochrome cM from the cyanobacterium Synechocystis sp. PCC 6803 in Escherichia coli cells in mg quantities. This has allowed us to perform a comparative laser flash-induced kinetic analysis of photosystem I reduction by the three metalloproteins from Synechocystis. The bimolecular rate constant for the overall reaction is up to 100 times lower with cytochrome cM than with cytochrome c6 or plastocyanin. In addition, the redox potential value and surface electrostatic potential distribution of cytochrome cM are quite different from those of cytochrome c6 and plastocyanin. These findings strongly indicate that cytochrome cM cannot be recognised by and interact with the same redox partners as the other two metalloproteins. [Copyright &y& Elsevier]