Heme‐Copper Oxidases and Their Electron Donors in Cyanobacterial Respiratory Electron Transport

Abstract

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.

Transcription Profiling of the Model Cyanobacterium Synechococcus sp. Strain PCC 7002 by Next-Gen (SOLiD™) Sequencing of cDNA

“…Because light is not continuously available, all phototrophic organisms must switch to another metabolic mode at night to produce the maintenance energy to support basic cell functions. Cyanobacteria have a complete respiratory electron transport chain that allows respiration with oxygen as terminal electron acceptor (Peschek et al, 2004; Bernroitner et al, 2008). However, in many natural environments, oxygen is rapidly consumed in the dark by cyanobacteria or other organisms (Stal, 1995; Steunou et al, 2008), and thus the local environmental conditions may quickly become anoxic.…”

MarR-type Transcriptional Regulator ChlR Activates Expression of Tetrapyrrole Biosynthesis Genes in Response to Low-oxygen Conditions in Cyanobacteria

“…They perform oxygenic photosynthesis like plants by extracting electrons from water with generation of oxygen as a byproduct. Cyanobacteria also use oxygen for respiration to produce energy in the dark (2). The oxygen levels greatly change in the field environment from hyperoxic to almost anaerobic conditions (3,4).…”

“…Protoporphyrin IX is converted to heme by the insertion of ferrous ion catalyzed by ferrochelatase. Bilin biosynthesis starts from the oxygen-dependent cleavage of heme catalyzed by heme oxygenase (HO) 2 . Photosynthetic organisms, including cyanobacteria, should have elaborate mechanisms to regulate the biosynthetic pathways of Chl and bilin to avoid photooxidative damage caused by the accumulation of free pigments and their intermediates.…”

Effects of Allelochemicals from Ficus microcarpaon Chlorella pyrenoidosa

“…(Bernroitner et al 2008;Zamocky et al 2008). CAT also plays an important role in protecting cells from oxidative damage, by catalyzing the H 2 O 2 into H 2 O and O 2 (Bernroitner et al 2008;Zamocky et al 2008). In this study, the decrease of SOD activity in C. pyrenoidosa may directly reflect the degradation of cellular detoxification to O 2 − • leaking mainly from the energy centers.…”

“…SOD can specially catalyze the conversion of O 2 − • into H 2 O 2 and O 2 , acting as the first line of defense against the potential toxicity of superoxide radical. (Bernroitner et al 2008;Zamocky et al 2008). CAT also plays an important role in protecting cells from oxidative damage, by catalyzing the H 2 O 2 into H 2 O and O 2 (Bernroitner et al 2008;Zamocky et al 2008).…”