Beatrix G. Schlarb-Ridley

Terminal oxidases of cyanobacteria

The respiratory chain of cyanobacteria appears to be branched rather than linear; furthermore, respiratory and photosynthetic electron-transfer chains co-exist in the thylakoid membrane and even share components. This review will focus on the three types of terminal respiratory oxidases identified so far on a genetic level in cyanobacteria: aa3-type cytochrome c oxidase, cytochrome bd-quinol oxidase and the alternative respiratory terminal oxidase. We summarize here their genetic, biochemical and biophysical characterization to date and discuss their interactions with electron donors as well as their physiological roles.

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. (2002) Biochemistry 41, 3279-3285]. To investigate the relative contributions of the reaction partners to these differences, the reactions of turnip cytochrome f with P. laminosum plastocyanin and P. laminosum cytochrome f with pea plastocyanin were examined. Exchanging one of the plant reaction partners with the corresponding cyanobacterial protein nearly abolished electron transfer at low ionic strength but increased the rate at high ionic strength. This increase was larger for P. laminosum cytochrome f than for P. laminosumplastocyanin. To identify molecular features of P. laminosum cytochrome f that contribute to the increase, the effect of mutations in the N-terminal heme-shielding peptide on the reaction with P. laminosum plastocyanin was determined. Phenylalanine-3 was converted to valine and tryptophan-4 to phenylalanine or leucine. The mutations lowered the rate constant at 0.1 M ionic strength by factors of 0.71 for F4V, 0.42 for W4F, and 0.63 for W4L while introducing little change in the shape of the ionic strength dependence curve. When the N-terminal tetrapeptide (sequence YPFW) was converted into that found in the chloroplast of Chlamydomonas reinhardtii (YPVF), the reaction was slowed further (factor of 0.26). The N-terminal heme-shielding peptide was found to be responsible for 75% of the kinetic differences between cytochrome f from chloroplasts and the cyanobacterium when electrostatic interactions were eliminated.

Cytochrome c6A 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 haem group of cytochrome c 6A acts as an electron sink, allowing rapid resolution of a radical intermediate formed during reoxidation of cytochrome c 6A. Our model is consistent with previously published data on mutant plants, and the likely evolution of the protein.

Cytochrome c 6A: 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. One salient difference between the haem pockets is that a valine residue in cytochrome c(6A) replaces a highly conserved glutamine residue in cytochrome c(6). This difference has been probed using site-directed mutagenesis, X-ray crystallography and protein film voltammetry studies. It has been found that the stereochemistry of the glutamine residue within the haem pocket has a destabilizing effect and is responsible for tuning the haems midpoint potential by over 100 mV. This large effect may have contributed to the evolution of a new biological function for cytochrome c(6A).