IV- Oxygen reductases

Oxygen reductases are the last complexes of aerobic respiratory chains, catalysing the reduction of dioxygen to water. Most of these enzymes belong to the superfamily of haem-copper oxygen reductases (Figure 5), which are characterised by having in their subunit I a low-spin haem and a binuclear centre harbouring a high-spin haem and a copper ion, being able to couple oxygen reduction to proton translocation. These enzymes are able to oxidise peripheral or periplasmatic electron donors (such as cytochromes, HiPIPs, or copper proteins), or membrane-bound electron donors (quinols). The former have in their subunit II a mixed valence dinuclear copper centre (Cu A ), which is absent in the latter. The cbb 3 oxidases are reported to be cytochrome oxidases, which instead of the subunit II of the other haem-copper oxidases, have one monohaemic and one dihaemic subunit.

In order to perform their function of reducing oxygen to water, and also to pump protons, these enzymes must possess proton conducting channels. Based on the amino acid residues forming these channels, on amino acid sequence comparisons, and on specific characteristics of subunit II, three families were established for haem-copper oxygen reductases, named A (which includes the subfamilies A1 and A2), B and C (Pereira et al. 2001) . This classification is supported by the kinetic and ligand binding properties of the binuclear centre (Pereira 2003) .

Cytochrome bd is a quinol:oxygen oxidoreductase present in many prokaryotic respiratory chains that does not belong to the haem-copper oxygen reductases superfamily. Cytochrome bd is a two subunits protein complex, containing a low-spin B type haem and a catalytic centre composed by two high-spin haems, one of the B and the other of the D type (Junemann 1997) . No structure of this type of oxygen reductases has yet been solved, and only a preliminary characterisation of functional amino acid residues has been performed. Cytochromes bd do not pump proton, but are electrogenic.

In several eukaryotes, there is a quinol:oxygen oxidoreductase, the so called alternative oxidase, which contains a di-iron centre (Berthold et al. 2000; Berthold et al. 2002) . The membrane attachment of this enzyme has been also proposed to occur through amphipathic helices (Joseph-Horne et al. 2000) .