The terminal cytochrome claa3 of the respiratory chain of Thermus thermophilus has been isolated and purified to homogeneity by a novel procedure. The two subunit proteins (55 and 33 kDa) have been characterized chemically. Computer searches with partial amino acid sequences obtained from both subunits show that the larger subunit belongs to the cytochrome oxidase subunit I protein family while the smaller covalently hemebinding subunit is not a cytochrome c1 but appears to be a fused protein between cytochrome c and cytochrome oxidase subunit 11.With respect to the 16-S rRNA-derived phylogeny of procaryotes, the results show that the genetic information for an 02-reacting cytochrome oxidase (EC 1.9.3.1) existed already in early eubacteria.Respiratory chains of many bacteria contain terminal 02-reducing oxidases with similar spectroscopic and functional properties but much simpler subunit structure than the mitochondrial cytochrome-c oxidase (EC 1.9.3.1) [I -31. Thus the enzyme from the plasma membrane of Paracoccus denitrificans can either be isolated as a functional (02-reducing and H+-pumping) two-or three-subunit complex homologous to the mtDNA-coded subunits I, I1 and eventually 111 of the mitochondrial enzyme [4, 51. From these data it had been inferred that subunits I and 11 represent a minimum catalytic structure capable of binding the canonical four metal centers (heme a, heme u3, CuA and CuB) and of catalysing the reaction 4Cytc" + O2 + (4 + n)H' = 4Cytc3+ + 2H2O + nH:where i and o refer to the inner and outer sites of the membrane. Furthermore, the sequence analyses with the Paracoccus enzyme have suggested that this bacterium is a descendant from those ancient bacteria from which eucaryotic mitochondria also originated under the selection pressure of a rising oxygen atmosphere [6]. The origin of more distantly related aerobic bacterial lines is, however, assumed to fall beyond the earths' O2 horizon and, consequently, it has been argued that O2 respiration evolved several times independently [7].Recently a terminal claa3 oxidase from the strictly aerobic bacterium Thermus thermophilus has been described [8, 91
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