5 min). This is the first indication of a significant difference in the oxidation state of the PQQ prosthetic group in the catalytic sites of the active and inactive ADHs, respectively. The pH-dependence profiles for the ferric reductase activities of the ADHa and ADHi complexes were compared (Fig. 4a). The ADHa complex showed its maximal activity at pH 6.0 with find more small shoulders in the acidic and alkaline sides of the curve. On the other hand,
ADHi showed the maximal response at pH 4.5 without secondary responses in the alkaline and acidic sides of the slope. ADH possesses multiple cytochrome c centers, which are potentially reactive sites from which electrons can be withdrawn by the ferricyanide electron acceptor. The distinct optimal pH seen for ADHi suggests that ferricyanide reacts at a single site, other than that
preferentially used in the active and fully reduced ADHa. Thus, the pH profile of ADHi must be attributed to the electron donor activity of the cytochrome c in subunit I, which is based on the pH-dependence profiles previously obtained for the catalytic activity of the dissociated and partially reconstituted subunit complexes of the trimeric ADH complex of G. suboxydans (Matsushita et al., 1996) where the complex formed by subunits I and III (SI-SIII complex) showed a distinctive acidic optimal pH and very low activity, such as was shown by our inactive enzyme. In this regard, it must be remembered that SI bears the catalytic site and one of each, PQQ and cytochrome INK 128 solubility dmso c, whereas SII contains three cytochromes c and that in trimeric ADHs SIII Oxalosuccinic acid does not seem to have a role in the catalytic process (Matsushita et al., 1994). On the other hand, our ADHa (Fig. 4a) and the native ADH complex from G. suboxydans exhibit their maximal response at mild alkaline conditions (Matsushita et al., 1989). The heme c components of the ADHi complex were redox titrated at pH 6.0. Titration was
monitored from 500 to 600 nm, following the change of the α-band maximum at 553 nm (reference wavelength set at 540 nm, dual wavelength mode). The best fit of the redox titration data for our enzyme (Fig. 4b) revealed the presence of four potentials at Em1 = −34 mV (20%), Em2 = +90 mV (18%), Em3 = +215 (26%), and Em4 = +270 mV (36%) (vs. SHE). These values are significantly more positive than the mid-potential values obtained previously for its active counterpart (10): −64 mV (31%), −8 mV (18%), +185 mV (30%), and +210 mV (13%) (vs. SHE, pH 6.0). ADH quinohemoproteins are complex enzymes carrying several redox prosthetic groups. Notably, the four cytochrome c centers are redox-dependent chromogenic groups amenable for the assessment of electron transfer kinetics within the ADH complex. Accordingly, the rate of intramolecular electron transfer evoked by ethanol was measured in both ADHi and ADHa.