Has Sulfolobus an Archaic Respiratory System? Structure, Function and Genes of its Components

Günter Schäfer*, Stefan Anemüller, Ralf Moll, Michael Gleissner, Christian L. Schmidt

*Corresponding author for this work
20 Citations (Scopus)


The electron transport system of Sulfolobus acidocaldarius has been shown to act as a respiration driven proton pump. Essentially it is composed of three functional units: the NADH-and substrate dehydrogenases, caldariella quinone as a pool of bound hydrogen, and one or two terminal oxidases catalyzing its reoxidation by molecular oxygen. The latter systems contain only FeS proteins, b- and a-type cytochromes; c-type cytochromes are absent. The cytochrome aa3 from Sulfolobus is the first heme-a containing terminal oxidase shown to act as a quinol oxidase. Its redox centers were investigated by redox potentiometry, EPR- and Raman-resonance spectroscopy, revealing a typical binuclear heme-a/Cu center, however displaying unusual structural interaction between the formyl substituents and the protein environment; the redox potentials of low and high spin centers were titrated. A cytochrome a587 atypical for any other species emerged to contain a low spin, low potential heme-a center, mimicing the function of a b-type cytochrome and likely to be the product of the SOX-C gene from the SOX operon (Lübben et al., 1992). The aa3 oxidase when reconstituted into liposomes is shown to generate a proton motive force. Though Sulfolobus contains no equivalent to the bc1 complex of classical respiratory chains, in addition to the caldariella quinol oxidase a Rieske-type FeS protein could be detected in its plasma membrane; it was isolated and characterized by EPR, strongly suggesting a participation in respiratory electron transport. Its redox potential displays significant pH dependence revealing two distinct pKs. It remains to be established whether it interacts with the aa3 oxidase or other electron carriers. A cytochrome b562 appears to be present constitutively in small amount, while cytochrome-b558 is the major heme-b containing compound in the membrane of Sulfolobus. A previously envisaged function as an alternate terminal oxidase is still questionable. Recently conditions were found allowing to modulate its expression dramatically, leading to almost complete suppression of cytochrome-b558. Based on differential spectroscopy and redox titrations a tentative scheme of the electron transport system from Sulfolobus is proposed. Sequence comparisons are discussed with regard to the question, as to whether it resembles a primitive “archaic” precursor form of more complex “modern” respiratory systems, or whether it evolved by aquisition and adaptation of “foreign” genes.

Original languageEnglish
JournalSystematic and Applied Microbiology
Issue number4
Pages (from-to)544-555
Number of pages12
Publication statusPublished - 1993

Research Areas and Centers

  • Academic Focus: Center for Infection and Inflammation Research (ZIEL)


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