Cyanobacterial cytochrome cM: Probing its role as electron donor for CuA of cytochrome c oxidase

Abstract

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.

Characterization ofProchlorococcusclades from iron-depleted oceanic regions

“…Cytochrome C m is thought to be involved in electron transfer from PSI to the cytochrome c oxidase complex during periods of stress (39). Although the exact conduits are unknown, the loss of two ferredoxins likely removes several paths for electrons to move away from or to photosynthetic reaction centers.…”

Cytochrome cMdownscales photosynthesis under photomixotrophy inSynechocystissp. PCC 6803

“…Nevertheless, its subcellular location is ambiguous. An early study localised CytM to the thylakoid and plasma membranes in 'purified' membrane fractions (Bernroitner et al 2009). However, cross contamination between membranes was not determined, which has been an issue in studies using similar separation techniques (Sonoda et al 1997, Schultze et al 2009.…”

“…However, cleavage does not seem to occur in vivo, as the protein extracted from various cyanobacterial species, including Synechocystis, Synechococcus elongatus PCC 6301 and Anabaena sp. PCC 7120, was found to be around 12 kDa (Cho et al 2000, Bernroitner et al 2009, implying that the hydrophobic N-terminus remains on the protein and serves as a membrane anchor.…”

“…It has been suggested that CytM may play a role in respiratory or photosynthetic electron transfer (Vermaas et al 2001, Bernroitner et al 2009. In Synechocystis, CytM was shown to reduce the Cu A center of Cox in vitro with similar efficiency as Cyt c 6 (Bernroitner et al 2009).…”

“…It has been suggested that CytM may play a role in respiratory or photosynthetic electron transfer (Vermaas et al 2001, Bernroitner et al 2009. In Synechocystis, CytM was shown to reduce the Cu A center of Cox in vitro with similar efficiency as Cyt c 6 (Bernroitner et al 2009). However, given the midpoint potential of CytM (+150 mV), electron transfer from Cyt b 6 f (+320 mV) to CytM would be energetically uphill (Cho et al 2000).…”