Cytochrome aa3 from Sulfolobus acidocaldarius. A single-subunit, quinol-oxidizing archaebacterial terminal oxidase.

The thermoacidophilic archaebacterium Sulfolobus acidocaldarius (DSM 369) extrudes protons when expending respiratory energy [Moll, R. & Schäfer, G. (1988) FEBS Lett. 232, 359-363]. Cytochromes of the membrane electron-transport systems are assumed to represent the proton pumps. Only a- and b-type cytochromes can be found; no c-type cytochromes are present. Of the two terminal oxidases [Anemüller, S. & Schäfer, G. (1989) FEBS Lett. 244, 451-455] one shows an absorption band at 604-605 nm, typical of cytochromes of the aa3 type. This hemoprotein has been solubilized from the membrane and purified to homogeneity. It exhibits distinct differences from known aa3-type oxidases. (a) It consists of a single polypeptide subunit of 38-40 kDa apparent molecular mass with two heme-a molecules and two copper ions. (b) In the oxidized state, absorption maxima are found at 421 nm and 597 nm, and in the reduced state at 439 nm and 601 nm; CO difference spectra suggest one heme to be a heme-a3 centre. (c) The redox potentials of the heme centres are +220 mV and +370 mV, respectively. (d) A high-spin heme signal at g = 6 is present in EPR spectra, which is more prominent than the low-spin heme signal at g = 3, the former already being present in the oxidized state. A signal at g = 2.1 may be due to one of the copper ions and is superimposed upon a minor free radical signal at g = 2. (e) Caldariella quinone was also isolated from the plasma membrane of Sulfolobus. Its redox midpoint potential at pH 6.5 was determined to be +100 (+/- 5) mV; spectral properties have also been determined. (f) The isolated aa3 preparation does not oxidize cytochrome c; however, it oxidizes N,N,N',N'-tetramethyl-1,4-phenylenediamine dihydrochloride as an artificial single-electron donor as well as reduced caldariella quinone, which is assumed to represent the natural substrate. The reaction is cyanide-sensitive and the product of oxygen reduction is water. (g) On the basis of the results obtained a novel type of cytochrome aa3 is postulated in this paper which oxidizes reduced quinones; its ability to act as a proton pump remains to be shown.