Characterization of chimeric heme-copper respiratory oxidases using subunits I of Escherichia coli cytochrome b o and Halobacterium salinarium cytochrome aa3

Biochemistry, regulation and genomics of haem biosynthesis in prokaryotes

Haems are involved in many cellular processes in prokaryotes and eukaryotes. The biosynthetic pathway leading to haem formation is, with few exceptions, well-conserved, and is controlled in accordance with cellular function. Here, we review the biosynthesis of haem and its regulation in prokaryotes. In addition, we focus on a modification of haem for cytochrome c biogenesis, a complex process that entails both transport between cellular compartments and a specific thioether linkage between the haem moiety and the apoprotein. Finally, a whole genome analysis from 63 prokaryotes indicates intriguing exceptions to the universality of the haem biosynthetic pathway and helps define new frontiers for future study.

Structural analyses of the deduced amino acid sequences of a novel type heme–copper terminal oxidase, cytochromeaco3, from alkalophilicBacillusYN-2000

Cytochrome aco3from a facultatively alkalophilic bacterium, Bacillus YN-2000, was found to be alkaline- and heat-tolerant. To better understand the structural features of Bacillus YN-2000 cytochrome aco3, the gene encoding this enzyme was cloned and sequenced. Nucleotide sequence analyses of the region neighboring the acoI (subunit I) gene revealed that the acoII (subunit II) and acoIII (subunit III) genes were concomitantly clustered upstream and downstream of the acoI gene, respectively, forming an operon with transcriptional polarity. The deduced amino acid sequence of subunit I was highly similar to that of cytochrome caa3from thermophilic bacterium Bacillus PS3 in which the heme a3could be replaced with heme o. Furthermore, a marked paucity of basic amino acid residues was found in the cytochrome c-binding subunit II, which might be a result of the adaptation to a highly alkaline external milieu.Key words: cytochrome c oxidase, alkalophile, thermostability, heme o, Bacilli.

Redundancy of aerobic respiratory chains in bacteria? Routes, reasons and regulation

Bacteria are the most remarkable organisms in the biosphere, surviving and growing in environments that support no other life forms. Underlying this ability is a flexible metabolism controlled by a multitude of environmental sensors and regulators of gene expression. It is not surprising, therefore, that bacterial respiration is complex and highly adaptable: virtually all bacteria have multiple, branched pathways for electron transfer from numerous low-potential reductants to several terminal electron acceptors. Such pathways, particularly those involved in anaerobic respiration, may involve periplasmic components, but the respiratory apparatus is largely membrane-bound and organized such that electron flow is coupled to proton (or sodium ion) transport, generating a protonmotive force. It has long been supposed that the multiplicity of pathways serves to provide flexibility in the face of environmental stresses, but the existence of apparently redundant pathways for electrons to a single acceptor, say dioxygen, is harder to explain. Clues have come from studying the expression of oxidases in response to growth conditions, the phenotypes of mutants lacking one or more oxidases, and biochemical characterization of individual oxidases. Terminal oxidases that share the essential properties of substrate (cytochrome c or quinol) oxidation, dioxygen reduction and, in some cases, proton translocation, differ in subunit architecture and complement of redox centres. Perhaps more significantly, they differ in their affinities for oxidant and reductant, mode of regulation, and inhibitor sensitivity; these differences to some extent rationalize the presence of multiple oxidases. However, intriguing requirements for particular functions in certain physiological functions remain unexplained. For example, a large body of evidence demonstrates that cytochrome bd is essential for growth and survival under certain conditions. In this review, the physiological basis of the many phenotypes of Cyd-mutants is explored, particularly the requirement for this oxidase in diazotrophy, growth at low protonmotive force, survival in the stationary phase, and resistance to oxidative stress and Fe(III) chelators.

Probing a role of subunit IV of the Escherichia coli bo-type ubiquinol oxidase by deletion and cross-linking analyses

Subunit IV of the Escherichia coli bo-type ubiquinol oxidase is a 12-kDa membrane protein encoded by the cyoD gene and is conserved in the bacterial heme-copper terminal oxidases. To probe the functional role of subunit IV, we carried out deletion analysis and chemical cross-linking experiments with a homobifunctional and cleavable reagent. Spectroscopic properties of the mutant oxidases suggest that the C-terminal two-third (Val45 to His109) containing helices II and III is essential for the functional expression of the oxidase complex and for the CuB binding to the heme-copper binuclear center in subunit I. Cross-linking studies indicate that subunit IV is in close vicinity to subunit III. Based on these observations, we propose that subunit IV is present in a cleft formed by subunits I and III and assists the CuB binding to subunit I during biosynthesis or assembly of the oxidase complex.