I- NADH:quinone oxidoreductase
Three distinct types of membrane-bound enzymes are able to oxidise NADH, transferring the electrons to quinones: Type I – rotenone-sensitive NADH dehydrogenase, or complex I (NDH-1); Type II – the so called alternative NADH dehydrogenase, or rotenone-insensitive NADH dehydrogenase (NDH-2); and Type III – the so called Na+ -translocating NADH:quinone oxidoreductases.
Type I
Complex I is the largest complex of the respiratory chains, and can be found in the three domains of life. It is responsible for the transfer of electrons from NADH to quinone, through a number of prosthetic groups, coupling electron transfer to proton (in some cases sodium) translocation across the membrane (Hatefi 1985). The activity of complex I is inhibited by rotenone, piericidin A, capsaicin and rolliniastatin-1, among other compounds (Yagi et al. 1998) .
At least two distinct types of NDH-1-like complexes can be considered, regarding the electron donor and its interacting subunits. The NDH-1, whose electron donor is NADH, is widespread among bacteria and eukarya. The bacterial enzymes generally consist of 14 subunits, named Nqo 1 – Nqo 14 (Nqo – N ADH: q uinone o xidoreductase). The F 420 H 2 dehydrogenases, which were described in the archaea Methanosarcina ( Ms. ) mazei (Baumer et al. 2000) and Archeoglobus fulgidus (Kunow et al. 1994) , are composed of eleven out of the fourteen subunits of NDH-1. This enzyme complex lacks the subunits responsible for the NADH dehydrogenase reaction (Nqo 1 – Nqo 3 ), containing two other subunits, FpoO and FpoF, where the oxidation of their electron donor, F 420 H 2 , takes place. Other organisms contain genes encoding eleven subunits of complex I (Nqo 4 – Nqo 14 ), but not those coding for Nqo 1-3 or FpoF. This suggests that more subtypes will be found having different substrate-interacting subunits, and/or different electron donors, as more biochemical data become available.
Type II
Alternative NADH dehydrogenases (NDH-2) are able to oxidise NADH and/or NADPH, and are resistant to complex I specific inhibitors such as rotenone and piericidin A. NDH-2 usually contain non-covalently bound FAD (de Vries et al. 1988; Yagi et al. 1993; Luttik et al. 1998; Small et al. 1998; Bjorklof et al. 2000) . The Trypanosoma brucei NDH-2 was the first example of an FMN containing NDH-2 (Fang et al. 2002) .
Type III
The so-called Na+ -translocating catalyses the oxidation of NADH, coupling Na+ translocation across the membrane with electron transfer to quinones. The Vibrio ( V. ) cholerae (Barquera et al. 2002) and V. alginolyticus (Hayashi et al. 1994) enzymes are typical examples of such NADH:quinone oxidoreductase.
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