Molecular and catalytic properties of the aldehyde dehydrogenase of Gluconacetobacter diazotrophicus, a quinoheme protein containing pyrroloquinoline quinone, cytochrome b, and cytochrome c.

Several aldehyde dehydrogenase (ALDH) complexes have been purified from the membranes of acetic acid bacteria. The enzyme structures and the chemical nature of the prosthetic groups associated with these enzymes remain a matter of debate. We report here on the molecular and catalytic properties of the membrane-bound ALDH complex of the diazotrophic bacterium Gluconacetobacter diazotrophicus. The purified ALDH complex is a heterodimer comprising two subunits of 79.7 and 50 kDa, respectively. Reversed-phase high-pressure liquid chromatography (HPLC) and electron paramagnetic resonance spectroscopy led us to demonstrate, for the first time, the unequivocal presence of a pyrroloquinoline quinone prosthetic group associated with an ALDH complex from acetic acid bacteria. In addition, heme b was detected by UV-visible light (UV-Vis) spectroscopy and confirmed by reversed-phase HPLC. The smaller subunit bears three cytochromes c. Aliphatic aldehydes, but not formaldehyde, were suitable substrates. Using ferricyanide as an electron acceptor, the enzyme showed an optimum pH of 3.5 that shifted to pH 7.0 when phenazine methosulfate plus 2,6-dichlorophenolindophenol were the electron acceptors. Acetaldehyde did not reduce measurable levels of the cytochrome b and c centers; however, the dithionite-reduced hemes were conveniently oxidized by ubiquinone-1; this finding suggests that cytochrome b and the cytochromes c constitute an intramolecular redox sequence that delivers electrons to the membrane ubiquinone.

The membrane-bound quinohemoprotein alcohol dehydrogenase from Gluconacetobacter diazotrophicus PAL5 carries a [2Fe-2S] cluster.

Gluconacetobacter diazotrophicus stands out among the acetic acid bacteria as it fixes dinitrogen and is a true endophyte. It has a set of constitutive enzymes to oxidize ethanol and acetaldehyde which is upregulated during N(2)-dependent growth. The membrane-bound alcohol dehydrogenase (ADH) is a heterodimer (subunit I approximately 72 kDa, subunit II approximately 44 kDa) and constitutes an important component of this organism. ADH of Ga. diazotrophicus is a typical quinohemoprotein with one pyrroloquinoline quinone (PQQ) and four c-type cytochromes. For the first time, a [2Fe-2S] cluster has been identified by EPR spectroscopy in this type of enzyme. This finding is supported by quantitative chemical analysis, revealing 5.90 +/- 0.15 Fe and 2.06 +/- 0.10 acid-labile sulfurs per ADH heterodimer. The X-band EPR spectrum of ADH (as isolated in the presence of dioxygen, 20 K) showed three broad resonances at g 2.007, 1.941, and 1.920 (g(av) 1.956), as well as an intense narrow line centered at g = 2.0034. The latter signal, which was still detected at 100 K, was attributed to the PQQ semiquinone radical (PQQ(sq)). The broad resonances observed at lower temperature were assigned to the [2Fe-2S] cluster in the one-electron reduced state. The oxidation-reduction potentials E(m) (pH 6.0 vs SHE) of the four c-type cytochromes were estimated to E(m1) = -64 (+/-2) mV, E(m2) = -8 (+/-2) mV, E(m3) = +185 (+/-15) mV, and E(m4) = +210 (+/-10) mV (spectroelectrochemistry), E(mFeS) = -250 (+/-5) mV for the [2Fe-2S] cluster, and E(mPQQ) = -210 (+/-5) mV for the PQQ/PQQH(2) couple (EPR spectroscopy). We propose a model for the membrane-bound ADH of Ga. diazotrophicus showing hypothetical intra- and intermolecular electron pathways. Subunit I binds the PQQ cofactor, the [2Fe-2S] cluster, and one c-type cytochrome. Subunit II harbors three c-type cytochromes, thus providing an efficient electron transfer route to quinones located in the cytoplasmic membrane.

Inhibitory properties of ruthenium amine complexes on mitochondrial calcium uptake.

The recent finding that the inhibition of Ca2+-stimulated respiration by ruthenium red is mainly due to a binuclear ruthenium complex (Ru360) present in the commercial samples of the classical inhibitor ruthenium red (Ying et. al., 1991), showed that this complex is the more potent and specific inhibitor of the mitochondrial calcium uniporter. This work was aimed to provide insights into the mechanism by which Ru360 and other ruthenium-related compounds inhibits calcium uptake. Ruthenium red and a synthesized analog (Rrphen) were compared with Ru360. The inhibition by this binuclear complex was noncompetitive, with a Ki of 9.89 nM. The number of specific binding sites for Ru360 was 6.2 pmol/mg protein. Ruthenium red and Ru360 were mutually exclusive inhibitors. Bound La3+ was not displaced by Ru360. Rrphen was the least effective for inhibiting calcium uptake. The results support the notion of a specific binding site in the uniporter for the polycationic complexes and a negative charged region from the phospholipids in the membrane, closely associated with the uniporter inhibitor-binding site.