title: The caa3 terminal oxidase of the thermohalophilic bacterium Rhodothermus marinus: a HiPIP:oxygen oxidoreductase lacking the key glutamate of the D-channel.
abstract: The respiratory chain of the thermohalophilic bacterium Rhodothermus marinus contains a novel complex III and a high potential iron-sulfur protein (HiPIP) as the main electron shuttle (Pereira et al., Biochemistry 38 (1999) 1268-1275 and 1276-1283). In this paper, one of the terminal oxidases expressed in this bacterium is extensively characterised. It is a caa3-type oxidase, isolated with four subunits (apparent molecular masses of 42, 19 and 15 kDa and a C-haem containing subunit of 35 kDa), which has haems of the A(s) type. This oxidase is capable of using TMPD and horse heart cytochrome c as substrates, but has a higher turnover with HiPIP, being the first example of a HiPIP:oxygen oxidoreductase. The oxidase has unusually low reduction potentials of 260 (haem C), 255 (haem A) and 180 mV (haem A3). Subunit I of R. marinus caa3 oxidase has an overall significant homology with the subunits I of the COX type oxidases, namely the metal binding sites and most residues considered to be functionally important for proton uptake and pumping (K- and D-channels). However, a major difference is present: the putative essential glutamate (E278 in Paraccocus denitrificans) of the D-channel is missing in the R. marinus oxidase. Homology modelling of the R. marinus oxidase shows that the phenol group of a tyrosine residue may occupy a similar spatial position as the glutamate carboxyl, in relation to the binuclear centre. Moreover, sequence comparisons reveal that several enzymes lacking that glutamate have a conserved substitution pattern in helix VI: -YSHPXV- instead of -XGHPEV-. These observations are discussed in terms of the mechanisms for proton uptake and it is suggested that, in these enzymes, tyrosine may play the role of the glutamate in the proton channel.
authors.affiliation: Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Oeiras, Portugal.
authors.forename: M M
authors.initials: MM
authors.lastname: Pereira
authors.forename: M
authors.initials: M
authors.lastname: Santana
authors.forename: C M
authors.initials: CM
authors.lastname: Soares
authors.forename: J
authors.initials: J
authors.lastname: Mendes
authors.forename: J N
authors.initials: JN
authors.lastname: Carita
authors.forename: A S
authors.initials: AS
authors.lastname: Fernandes
authors.forename: M
authors.initials: M
authors.lastname: Saraste
authors.forename: M A
authors.initials: MA
authors.lastname: Carrondo
authors.forename: M
authors.initials: M
authors.lastname: Teixeira
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D003574:Cytochrome c Group / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry / Q000378:metabolism*; D018698:Glutamic Acid / Q000737:chemistry; D006088:Gram-Negative Aerobic Bacteria / Q000201:enzymology*; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins; D016415:Sequence Alignment; D013053:Spectrophotometry
chemical_list: D001426:Bacterial Proteins; D003574:Cytochrome c Group; D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; C007943:high potential iron-sulfur protein; D018698:Glutamic Acid; D010089:Oxidoreductases, N-Demethylating; C022726:tetramethylphenylenediamine oxidase; D003576:Electron Transport Complex IV