Chapter 1: Respiratory Chains in Archaea: From Minimal Systems to Supercomplexes

Summary

Oxygen respiration originated in evolution within the prokaryotic kingdom and was acquired by eukaryotes later via endosymbiosis. Prokaryotes are divided into the domains of Bacteria and Archaea. The latter represent the so called third phylogenetic domain and are not only characterized by their unusual extremophilic life styles, but also by a variety of molecular pecularities including their bioenergetic systems. Though the prinicple of chemiosmotic energy conservation holds also for Archea, some primary energy transducing reactions are restricted to this domain, as for example methoangenesis and rhodopsin dependent phototrophic energentics. Whereas the majority of known Archaea can thrive anaerobically, also a significant number of species is capable of obligate or facultative aerobic growth. The present chapter highlights the essential and archetypical differences with respect to orthodox pro- and eukaryotic respiratory systems. In particular the following aspects are addressed: 1. Several archaeal genomes have been completely analyzed, but protein chemical verification of the genomic data on electron transport systems lacks behing; 2. According to present knowledge, aerobic Archaea can oxide NADH by a type-II NADH dehyrogenase while an equivalent to complex-I has not been verified; 3. In several reactions the function of NAD is replaced by ferredoxins; 4. Other auxiliary cofactors are small copper proteins which may replace the function of c-type cytochromes, as well as unusual thiopheno-benzoquinones which act as membrane residing pools for reducing equivalents. Further, most a-type cytochromes contain an archetypical heme-A_S which also replaces heme-b in several respiratory complexes of thermoacidophilic Archaea; 5. Archaeal complex-II equivalents contain modified S3 FeS-clusters as well as unusual cysteine-rich motifs in their small subunits which host a novel [2Fe2S] cluster and, in addition, can be devoid of their typical membrane anchors; 6. A novel type of Rieske/cytochrome complex, equivalent to respiratory complex-III, has been detected in thermoacidophilic Archaea; 7. Minimal respiratory chains can consist simply of a quinol oxidase and are contrasted by supercomplexes comprising a genetic and structural fusion of a whole respiratory chain into one unique protein assembly; 8. A novel regulatory adaptation toacidic environments is proposed based on the unique properties of a Cu_A containing terminal oxidase from an extreme thermoacidophile; 9. A revised view on the structural basis for proton pumping capabilities of terminal oxidases is provided. 10. The Conclusions paragraph addresses the necessary experimental approaches required to investigate the unsolved questions on archaeal respiratory systems.