Involvement of a histidine residue in the interaction between membrane-anchoring protein (QPs) and succinate dehydrogenase in mitochondrial succinate-ubiquinone reductase

A study on the molecular mechanism of resistance to amicarthiazol in Xanthomonas campestris pv. citri

Three amicarthiazol-resistant mutants (Xuv10, Xuv20 and Xuv40) were obtained by UV induction and used in this study. Minimal inhibition concentrations (MICs) of amicarthiazol against the growth of mutants and wild-type isolate were 400 and 100 microg ml(-1) respectively. Inhibition by amicarthiazol of succinate dehydrogenase (SDH) activities of Xanthomonas campestris pv. citri (Hasse) Dye wild-type isolate (Xcc) and three resistant mutants derived from this isolate were assayed using triphenyltetrazolium chloride (TTC). The SDH activities of these mutants were significantly lower than that of Xcc. The complete nucleotide sequences of four subunits (SdhA, SdhB, SdhC and SdhD) of succinate-ubiquinone oxidoreductase (SQR) were cloned by polymerase chain reaction (PCR) amplification. An amino acid mutation (His229--> Leu229) in sdhB was found to confer resistance of X. campestris pv. citri to amicarthiazol. It is suggested that this mutation alters the SDH complex in some way that prevents binding of amicarthiazol.

Inactivation of succinate-ubiquinone reductase in substrate mixture

Succinate-ubiquinone reductase plays an important role in the respiratory chain. Previous work showed that preparation of succinate-ubiquinone reductase was relatively stable. Though the enzyme catalysis has been extensively studied, the inactivation of succinate-ubiquinone reductase has never been reported. In the present study, the kinetic theory of the substrate reaction of irreversible inhibition described by Tsou (Adv. Enzymol. Relat. Areas Mol. Biol. 61 (1988) 381-436) was applied to study the course of an unexpected slow inactivation of succinate-ubiquinone reductase in the substrate assay mixture containing different concentrations of substrates, succinate and 2,6-dichloroindophenol. The results showed that the inactivation of succinate-ubiquinone reductase in the substrate mixture is a first order reaction. The inactivation rate decreased with increasing concentration of succinate. The values of the micro rate constants for free and succinate bound enzyme were 0.22 +/- 0.01 and 0.052 +/- 0.002 min-1, respectively. Binding with 2-thenoyl-trifluroacetone, a inhibitor specially for the quinone binding site, slowed down the inactivation. However, the rate of inactivation did not change with increasing 2,6-dichloroindophenol concentration. The study showed that succinate-ubiquinone reductase was irreversibly inactivated in the substrate mixture. The results suggest that the inactivation was not due to dilution or dissociation of the enzyme, nor to complete usage of the substrate, inhibition of the yielded product or some possible trace component in the substrate mixture, nor to modification of the essential thiol group in the succinate binding site of succinate-ubiquinone reductase. The enzyme became more stable after binding with succinate.

A structural model for the membrane-integral domain of succinate: quinone oxidoreductases

Many succinate:quinone oxidoreductases in bacteria and mitochondria, i.e. succinate:quinone reductases and fumarate reductases, contain in the membrane anchor a cytochrome b whose structure and function is poorly understood. Based on biochemical data and polypeptide sequence information, we show that the anchors in different organisms are related despite an apparent diversity in polypeptide and heme composition. A general structural model for the membrane-integral domain of the anchors is proposed. It is an antiparallel four-helix bundle with a novel arrangement of hexa-coordinated protoheme IX. The structure can be applied to a larger group of membrane-integral cytochromes of b-type and has evolutionary and functional implications.

Genes encoding the same three subunits of respiratory complex II are present in the mitochondrial DNA of two phylogenetically distant eukaryotes

Although mitochondrial DNA is known to encode a limited number (<20) of the polypeptide components of respiratory complexes I, III, IV, and V, genes for components of complex II [succinate dehydrogenase (ubiquinone); succinate:ubiquinone oxidoreductase, EC 1.3.5.1] are conspicuously lacking in mitochondrial genomes so far characterized. Here we show that the same three subunits of complex II are encoded in the mitochondrial DNA of two phylogenetically distant eukaryotes, Porphyra purpurea (a photosynthetic red alga) and Reclinomonas americana (a heterotrophic zooflagellate). These complex II genes, sdh2, sdh3, and sdh4, are homologs, respectively, of Escherichia coli sdhB, sdhC, and sdhD. In E. coli, sdhB encodes the iron-sulfur subunit of succinate dehydrogenase (SDH), whereas sdhC and sdhD specify, respectively, apocytochrome b558 and a hydrophobic 13-kDa polypeptide, which together anchor SDH to the inner mitochondrial membrane. Amino acid sequence similarities indicate that sdh2, sdh3, and sdh4 were originally encoded in the protomitochondrial genome and have subsequently been transferred to the nuclear genome in most eukaryotes. The data presented here are consistent with the view that mitochondria constitute a monophyletic lineage.

Mitochondrial myopathy with succinate dehydrogenase and aconitase deficiency. Abnormalities of several iron-sulfur proteins

Recently, we described a patient with severe exercise intolerance and episodic myoglobinuria, associated with marked impairment of succinate oxidation and deficient activity of succinate dehydrogenase and aconitase in muscle mitochondria (1). We now report additional enzymatic and immunological characterization of mitochondria. In addition to severe deficiency of complex II, manifested by reduction of succinate dehydrogenase and succinate:coenzyme Q oxidoreductase activities to 12 and 22% of normal, respectively, complex III activity was reduced to 37% and rhodanese to 48% of normal. Furthermore, although complex I activity was not measured, immunoblot analysis of complex I showed deficiency of the 39-, 24-, 13-, and 9-kD peptides with lesser reductions of the 51- and 18-kD peptides. Immunoblots of complex III showed markedly reduced levels of the mature Rieske protein in mitochondria and elevated levels of its precursor in the cytosol, suggesting deficient uptake into mitochondria. Immunoreactive aconitase was also low. These data, together with the previous documentation of low amounts of the 30-kD iron-sulfur protein and the 13.5-kD subunit of complex II, compared to near normal levels of the 70-kD protein suggest a more generalized abnormality of the synthesis, import, processing, or assembly of a group of proteins containing iron-sulfur clusters.