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PubMed2006

The quinone binding site in Escherichia coli succinate dehydrogenase is required for electron transfer to the heme b.

We have examined the role of the quinone-binding (Q(P)) site of Escherichia coli succinate:ubiquinone oxidoreductase (succinate dehydrogenase) in heme reduction and reoxidation during enzyme turnover. The SdhCDAB electron transfer pathway leads from a cytosolically localized flavin adenine dinucleotide cofactor to a Q(P) site located within the membrane-intrinsic domain of the enzyme. The Q(P) site is sandwiched between the [3Fe-4S] cluster of the SdhB subunit and the heme b(556) that is coordinated by His residues from the SdhC and SdhD subunits. The intercenter distances between the cluster, heme, and Q(P) site are all within the theoretical 14 A limit proposed for kinetically competent intercenter electron transfer. Using EPR spectroscopy, we have demonstrated that the Q(P) site of SdhCDAB stabilized a ubisemiquinone radical intermediate during enzyme turnover. Potentiometric titrations indicate that this species has an E(m,8) of approximately 60 mV and a stability constant (K(STAB)) of approximately 1.0. Mutants of the following conserved Q(P) site residues, SdhC-S27, SdhC-R31, and SdhD-D82, have severe consequences on enzyme function. Mutation of the conserved SdhD-Y83 suggested to hydrogen bond to the ubiquinone cofactor had a less severe but still significant effect on function. In addition to loss of overall catalysis, these mutants also affect the rate of succinate-dependent heme reduction, indicating that the Q(P) site is an essential stepping stone on the electron transfer pathway from the [3Fe-4S] cluster to the heme. Furthermore, the mutations result in the elimination of EPR-visible ubisemiquinone during potentiometric titrations. Overall, these results demonstrate the importance of a functional, semiquinone-stabilizing Q(P) site for the observation of rapid succinate-dependent heme reduction.

Binding Sites↗
PubMed1986

Purification and properties of a cytochrome b560-d complex, a terminal oxidase of the aerobic respiratory chain of Photobacterium phosphoreum.

A cytochrome b560-d complex, a terminal oxidase in the respiratory chain of Photobacterium phosphoreum grown under aerobic conditions, was purified to near homogeneity. The purified oxidase complex is composed of equimolar amounts of two polypeptides with molecular weights of 41,000 and 54,000, as determined by gel electrophoresis in the presence of sodium dodecyl sulfate. It contains 10.2 nmol of protoheme and 22.5 nmol of iron/mg of protein. The enzyme is a "cytochrome bd-type oxidase," showing absorption peaks at 560 and 625 nm in its reduced minus oxidized difference spectrum at 77K. This oxidase combined with CO, and its CO difference spectrum at room temperature in the Soret region showed a peak at 418 nm and a trough at 434 nm. In addition, a trough at 560 nm (cytochrome b), and a trough at 620 nm and a peak at 639 nm (cytochrome d) were observed in the CO-binding spectrum. This cytochrome b560-d complex catalyzed the oxidation of ubiquinol-1 and ascorbate in the presence of N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride or phenazine methosulfate. The oxidase activity required phospholipids and was inhibited by the respiratory inhibitors, KCN and NaN3, and the divalent cation, ZnSO4. Formation of a membrane potential by the cytochrome b560-d complex reconstituted into liposomes was observed with the fluorescent dye, 3,3'-dipropylthiodicarbocyanine iodide, on the addition of ubiquinol-1, showing that the enzyme provided a coupling site for oxidative phosphorylation.

Amino Acids↗
PubMed1996

Non-invasive neurochemical analysis of focal excitotoxic lesions in models of neurodegenerative illness using spectroscopic imaging.

Water-suppressed chemical shift magnetic resonance imaging was used to detect neurochemical alterations in vivo in neurotoxin-induced rat models of Huntington's and Parkinson's disease. The toxins were: N-methyl-4-phenylpyridinium (MPP+), aminooxyacetic acid (AOAA), 3-nitropropionic acid (3-NP), malonate, and azide. Local or systemic injection of these compounds caused secondary excitotoxic lesions by selective inhibition of mitochondrial respiration that gave rise to elevated lactate concentrations in the striatum. In addition, decreased N-acetylaspartate (NAA) concentrations were noted at the lesion site over time. Measurements of lactate washout kinetics demonstrated that t1/2 followed the order: 3-NP approximately MPP+ >> AOAA approximately malonate, which parallels the expected lifetimes of the neurotoxins based on their mechanisms of action. Further increases in lactate were also caused by intravenous infusion of glucose. At least part of the excitotoxicity is mediated through indirect glutamate pathways because lactate production and lesion size were diminished using unilateral decortectomies (blockade of glutamatergic input) or glutamate antagonists (MK-801). Lesion size and lactate were also diminished by energy repletion with ubiquinone and nicotinamide. Lactate measurements determined by magnetic resonance agreed with biochemical measurements made using freeze clamp techniques. Lesion size as measured with MR, although larger by 30%, agreed well with lesion size determined histologically. These experiments provide evidence for impairment of intracellular energy metabolism leading to indirect excitotoxicity for all the compounds mentioned before and demonstrate the feasibility of small-volume metabolite imaging for in vivo neurochemical analysis.

Aging↗