diazotrophicus showed significant differences in the endogenous reduction levels of the cytochromes c. While the cytochromes appeared fully reduced in this website ADHa (Gómez-Manzo et al., 2008), the endogenous reduction levels in ADHi

were low (trace a, Fig. 2). Dithionite (trace b, Fig. 2) but not ethanol (not shown) caused a dramatic increase in the reduction levels of ADHi. To assess the number of cytochromes c that participate in the intramolecular electron transfer that takes place in the ADHi complex, the enzyme ‘as prepared’ was carefully titrated to its full reduced state with a 100 mM dithionite in 100 mM potassium phosphate buffer at pH 6.0 (not shown) and then, successively oxidized with the hydrosoluble quinone-2 (Q2) (trace c in Fig. 2). The data showed that roughly 90% of the ferrocytochrome c content of the enzyme was oxidized as revealed by the major decrease in wavelength signals at 419, 523, and 553 nm. Although catalysis by the ADHi enzyme was severely limited, the intramolecular electron transfer sequence

from the cytochromes c centers to the Q2 electron acceptor is not impaired. The presence of PQQ in ADHi was confirmed by EPR (Fig. 3a) and fluorescence spectroscopy (not shown), as well as by HPLC analysis (Fig. 3b). The intensity of the signal shown by ADHi (as purified) in EPR was rather low (not shown) as compared to that obtained for the ‘as purified’ ADHa complex of Ga. diazotrophicus (Gómez-Manzo Anti-infection Compound Library et al., 2010); however, after addition of dithionite to sample and recording the EPR spectrum of ADHi, a more intense signal was obtained (Fig. 3a). This suggested that the PQQ prosthetic group in ADHi is mainly in

its oxidized state, which is in contrast to the ADHa complex where PQQ was detected in its semiquinone form. Recently, we demonstrated the presence of a new prosthetic Fossariinae group: [2Fe-2S] in subunit I of ADHa (Gómez-Manzo et al., 2010). The determination of the acid-labile sulfurs by the method of Beinert (1983) showed the presence of 2.02 ± 0.1 sulfur atoms per ADHi heterodimer, which is similar to the amount of sulfur previously determined in the active ADH heterodimer (Gómez-Manzo et al., 2010). However, the EPR spectrum of the purified ADHi ‘as prepared’ showed no signal corresponding to the iron-sulfur cluster (not shown). As this latter form is a diamagnetic species, we conclude that this cluster in ADHi must be in the oxidized form. The redox state of the PQQ in ADHi was further analyzed by HPLC. To this purpose, PQQ was extracted from the purified ADHa and ADHi complexes by a methanol-ethanol mixture. For ADHa, a single peak with a retention time of 4.5 min was obtained, whereas the PQQ extracted from ADHi produced a single peak with a retention time of 6.8 min (Fig. 3b). Commercial PQQ (Sigma; PQQH2) showed a retention time of 4.1 min that shifted to 6.8 min after oxidation with ammonium peroxydisulfate (Fig. 3c). This result is indicative that PQQ in ADHi is present in its oxidized state (retention time 6.