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Cyanobacteria have evolved approximately 3x10(9) years ago from ancient phototrophic microorganisms that already lived on our planet Earth. By opening the era of an aerobic, oxygen-containing biosphere, they are the true pacemakers of geological and biological evolution. Cyanobacteria must have been among the first organisms to elaborate mechanisms for the detoxification of partially reduced oxygen species including (hydrogen) peroxide. Since there is still an suprising lack of knowledge on the type, role, and mechanism(s) of peroxide-degrading enzymes in these bacteria, all 44 fully or partially sequenced genomes for haem and non-haem catalases and peroxidases have been critically analysed based on well known structure-function relationships of the corresponding oxidoreductases. It is demonstrated that H(2)O(2)-dismutating enzymes are mainly represented by bifunctional (haem) catalase-peroxidases and (binuclear) manganese catalases, with the latter being almost exclusively found in diazotrophic species. Several strains even lack a gene that encodes an enzyme with catalase activity. Two groups of peroxidases are found. Genes encoding putative (primordial) haem peroxidases (with homology to corresponding mammalian enzymes) and vanadium-containing iodoperoxidases are found only in a few species, whereas genes encoding peroxiredoxins (1-Cys, 2-Cys, type II, and Q-type) are ubiquitous in cyanobacteria. In addition, approximately 70% contain NADPH-dependent glutathione peroxidase-like proteins. The occurrence and phylogeny of these enzymes is discussed, as well as the present knowledge of their physiological role(s).

Cyanobacteria are the paradigmatic organisms of oxygenic (plant-type) photosynthesis and aerobic respiration. Since there is still an amazing lack of knowledge on the role and mechanism of their respiratory electron transport, we have critically analyzed all fully or partially sequenced genomes for heme-copper oxidases and their (putative) electron donors cytochrome c(6), plastocyanin, and cytochrome c(M). Well-known structure-function relationships of the two branches of heme-copper oxidases, namely cytochrome c (aa(3)-type) oxidase (COX) and quinol (bo-type) oxidase (QOX), formed the base for a critical inspection of genes and ORFs found in cyanobacterial genomes. It is demonstrated that at least one operon encoding subunits I-III of COX is found in all cyanobacteria, whereas many non-N(2)-fixing species lack QOX. Sequence analysis suggests that both cyanobacterial terminal oxidases should be capable of both the four-electron reduction of dioxygen and proton pumping. All diazotrophic organisms have at least one operon that encodes QOX. In addition, the highly refined specialization in heterocyst forming Nostocales is reflected by the presence of two paralogs encoding COX. The majority of cyanobacterial genomes contain one gene or ORF for plastocyanin and cytochrome c(M), whereas 1-4 paralogs for cytochrome c(6) were found. These findings are discussed with respect to published data about the role of respiration in wild-type and mutated cyanobacterial strains in normal metabolism, stress adaptation, and nitrogen fixation. A model of the branched electron-transport pathways downstream of plastoquinol in cyanobacteria is presented.

It has been shown that efficient functioning of photosynthesis and respiration in the cyanobacterium Synechocystis PCC 6803 requires the presence of either cytochrome c6 or plastocyanin. In order to check whether the blue copper protein plastocyanin can act as electron donor to cytochrome c oxidase, we investigated the intermolecular electron transfer kinetics between plastocyanin and the soluble CuA domain (i.e. the donor binding and electron entry site) of subunit II of the aa3-type cytochrome c oxidase from Synechocystis. Both copper proteins were expressed heterologously in Escherichia coli. The forward and the reverse electron transfer reactions were studied yielding apparent bimolecular rate constants of (5.1+/-0.2) x 10(4) M(-1) s(-1) and (8.5+/-0.4) x 10(5) M(-1) s(-1), respectively (20 mM phosphate buffer, pH 7). This corresponds to an apparent equilibrium constant of 0.06 in the physiological direction (reduction of CuA), which is similar to Keq values calculated for the reaction between c-type cytochromes and the soluble fragments of other CuA domains. The potential physiological role of plastocyanin in cyanobacterial respiration is discussed.

Cytochrome c 6 is a soluble metalloprotein located in the periplasmic space and the thylakoid lumen of many cyanobacteria and is known to carry electrons from cytochrome b 6 f to photosystem I. The Cu A domain of cytochrome c oxidase, the terminal enzyme which catalyzes the four-electron reduction of molecular oxygen in the respiratory chains of mitochondria and many bacteria, also has a periplasmic location. In order to test whether cytochrome c 6 could also function as a donor for cytochrome c oxidase, we investigated the kinetics of the electron transfer between recombinant cytochrome c 6 (produced in high yield in Escherichia coli by coexpressing the maturation proteins encoded by the ccmA-H gene cluster) and the recombinant soluble Cu A domain (i.e., the donor binding and electron entry site) of subunit II of cytochrome c oxidase from Synechocystis PCC 6803. The forward and the reverse electron transfer reactions were studied by the stopped-flow technique and yielded apparent bimolecular rate constants of (3.3 % 0.3) Â 10 5 M À1 s À1 and (3.9 % 0.1) Â 10 6 M À1 s À1 , respectively, in 5 mM potassium phosphate buffer, pH 7, containing 20 mM potassium chloride and 25°C. This corresponds to an equilibrium constant K eq of 0.085 in the physiological direction (D r G 00 ¼ 6:1 kJ/mol). The reduction of the Cu A fragment by cytochrome c 6 is almost independent on ionic strength, which is in contrast to the reaction of the Cu A domain with horse heart cytochrome c, which decreases with increasing ionic strength. The findings are discussed with respect to the potential role of cytochrome c 6 as mobile electron carrier in both cyanobacterial electron transport pathways.

It is well known that efficient functioning of photosynthetic (PET) and respiratory electron transport (RET) in cyanobacteria requires the presence of either cytochrome c(6) (Cytc(6)) or plastocyanin (PC). By contrast, the interaction of an additional redox carrier, cytochrome c(M) (Cytc(M)), with either PET or RET is still under discussion. Here, we focus on the (putative) role of Cytc(M) in cyanobacterial respiration. It is demonstrated that genes encoding the main terminal oxidase (cytochrome c oxidase, COX) and cytochrome c(M) are found in all 44 totally or partially sequenced cyanobacteria (except one strain). In order to check whether Cytc(M) can act as electron donor to COX, we investigated the intermolecular electron transfer kinetics between Cytc(M) and the soluble Cu(A) domain (i.e. the donor binding and electron entry site) of subunit II of COX. Both proteins from Synechocystis PCC6803 were expressed heterologously in E. coli. The forward and the reverse electron transfer reactions were studied yielding apparent bimolecular rate constants of (2.4+/-0.1)x10(5) M(-1) s(-1) and (9.6+/-0.4)x10(3) M(-1) s(-1) (5 mM phosphate buffer, pH 7, 50 mM KCl). A comparative analysis with Cytc(6) and PC demonstrates that Cytc(M) functions as electron donor to Cu(A) as efficiently as Cytc(6) but more efficient than PC. Furthermore, we demonstrate the association of Cytc(M) with the cytoplasmic and thylakoid membrane fractions by immunobloting and discuss the potential role of Cytc(M) as electron donor for COX under stress conditions.

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Biochemistry Z 0250 Heme-Copper Oxidases and Their Electron Donors in Cyanobacterial Respiratory Electron Transport -[106 refs.]. -(BERNROITNER, M.; ZAMOCKY, M.; PAIRER, M.; FURTMUELLER, P. G.; PESCHEK, G. A.; OBINGER*, C.; Chem.