ubiquinone and isoprene

Ubiquinone, known as coenzyme Q, was shown to be the part of the metabolic pathways by Crane et al. in 1957. Its function as a component of the mitochondrial respiratory chain is well established. However, ubiquinone has recently attracted increasing attention with regard to its function, in the reduced form, as an antioxidant. In ubiquinone synthesis the para-hydroxybenzoate ring (which is the derivative of tyrosine or phenylalanine) is condensed with a hydrophobic polyisoprenoid side chain, whose length varies from 6 to 10 isoprene units depending on the organism. para-Hydroxybenzoate (PHB) polyprenyltransferase that catalyzes the condensation of PHB with polyprenyl diphosphate has a broad substrate specificity. Most of the genes encoding (all-E)-prenyltransferases which synthesize polyisoprenoid chains, have been cloned. Their structure is either homo- or heterodimeric. Genes that encode prenyltransferases catalysing the transfer of the isoprenoid chain to para-hydroxybenzoate were also cloned in bacteria and yeast. To form ubiquinone, prenylated PHB undergoes several modifications such as hydroxylations, O-methylations, methylations and decarboxylation. In eukaryotes ubiquinones were found in the inner mitochondrial membrane and in other membranes such as the endoplasmic reticulum, Golgi vesicles, lysosomes and peroxisomes. Still, the subcellular site of their biosynthesis remains unclear. Considering the diversity of functions of ubiquinones, and their multistep biosynthesis, identification of factors regulating their cellular level remains an elusive task.

Topics: Alkyl and Aryl Transferases; Animals; Butadienes; Hemiterpenes; Humans; Molecular Structure; Pentanes; Prohibitins; Quinones; Ubiquinone

Ubiquinone (UQ) is a conserved polyprenylated lipid essential to cellular respiration. Two papers, one in this issue of Cell Chemical Biology (Hajj Chehade et al., 2019) and another in Molecular Cell (Lohman et al., 2019), identify lipid-binding proteins that play crucial roles in chaperoning UQ-intermediates.

Topics: Butadienes; Carrier Proteins; Eukaryota; Hemiterpenes; Lipids; Ubiquinone

The formation of a di-tert-alkyl nitroxide has been observed by Electron Spin Resonance during the exposure of coenzyme CoQ(10), in both the oxidized and reduced forms, to nitrogen dioxide (˙NO(2)) or to nitric oxide (˙NO) in the presence of oxygen. The same kind of nitroxide has been observed also with CoQ(1), CoQ(3) or with 1-phenyl-3-methyl-2-butene, chosen as model compounds. In all cases, the formation of the nitroxide may be justified only by admitting the involvement of the isoprenic chain of the coenzymes and in particular the addition of ˙NO(2) to the double bond. A mechanism which accounts for the formation of the nitroxide as well as the other compounds observed in the reactions is proposed and confirmed by a spectroscopic investigation (FT-IR, (1)H NMR, X-ray analysis) and by ESI-MS.

Topics: Butadienes; Hemiterpenes; Models, Molecular; Molecular Structure; Nitric Oxide; Nitrogen Dioxide; Oxidation-Reduction; Pentanes; Ubiquinone

NADH-ubiquinone oxidoreductase (Complex I) is located at the entrance of the mitochondrial electron transfer chain and transfers electrons from NADH to ubiquinone with 10 isoprene units (Q(10)) coupled with proton pumping. The composition of Complex I, the largest and most complex proton pump in the mitochondrial electron transfer system, especially the contents of Q(10) and phospholipids, has not been well established. An improved purification method including solubilization of mitochondrial membrane with deoxycholate followed by sucrose gradient centrifugation and anion-exchange column chromatography provided reproducibly a heme-free preparation containing 1 Q(10), 70 phosphorus atoms of phospholipids, 1 zinc ion, 1 FMN, 30 inorganic sulfur ions, and 30 iron atoms as the intrinsic constituents. The rotenone-sensitive enzymatic activity of the Complex I preparation was comparable to that of Complex I in the mitochondrial membrane. It has been proposed that Complex I has two Q(10) binding sites, one involved in the proton pump and the other functioning as a converter between one and two electron transfer pathways [Ohnishi, T., Johnson, J. J. E., Yano, T., LoBrutto, R., and Widger, R. W. (2005) FEBS Lett. 579, 500-506]. The existence of one molecule of Q(10) in the fully oxidized Complex I suggests that the affinity of Q(10) to one of the two Q(10) sites is greatly dependent on the oxidation state and/or the membrane potential and that the Q(10) in the present preparation functions as the converter of the electron transfer pathways which should be present in any oxidation state.

Topics: Animals; Butadienes; Catalysis; Cattle; Electron Transport Complex I; Flavin Mononucleotide; Hemiterpenes; Mitochondria, Heart; Models, Molecular; NAD; Oxidation-Reduction; Pentanes; Protein Conformation; Ubiquinone

Radical anions and cations of the biologically important coenzymes Q-6 and Q-10, which have 6 and 10 unsaturated isoprene units in their side chains, respectively, have been generated in various solvents, and the results compared with those obtained for Q-0, a ubiquinone with no isoprene units, and for decylubiquinone Q-2 which has a saturated side chain. Hyperfine splitting constants (hfsc) of methyl and methoxy protons of the substituents in the quinone ring, and beta and gamma protons of the side chain were measured by EPR and ENDOR spectroscopy for both the radical anions and cations of Q-0, Q-6 and Q-10, and for the radical anion of Q-2. The relative signs of the hfsc were determined by general TRIPLE resonance spectroscopy. TRIPLE induced EPR (TIE) spectra were used for identification of the primary and secondary radicals of Q-10. The temperature dependence of the hfsc of the beta protons of Q-2 was different from those of Q-6 and Q-10. Fully optimised structures of Q-3 and Q-7 were obtained by performing semiempirical PM3 molecular orbital (MO) calculations for both neutral molecules and radical anions, neutral radicals and radical cations. Partial optimisation of the molecules was carried out for the side chain in a planar conformation. The folded conformation always had the minimum energy. Folding was so complete in the Q-7 series that the end of the side chain came into contact with the quinone ring, and small hfsc were detected in the PM3 calculations.

Topics: Butadienes; Electron Spin Resonance Spectroscopy; Free Radicals; Hemiterpenes; Models, Molecular; Molecular Conformation; Molecular Structure; Pentanes; Ubiquinone

The conformation and partial electron spin density distribution of the reduced primary electron acceptor (QA-), a plastosemiquinone-9 (PQ-9-) anion radical, in photosystem II protein complexes from spinach as well as free PQ-9- in solution have been determined by EPR and 1H ENDOR spectroscopies. The data show that the conformation of the isoprenyl chain at C beta relative to the aromatic ring differs by 90 degrees for QA- in higher plant PSII versus both types of bacterial reaction centers, Rhodobacter sphaeroides and Rhodopseudomonas viridis [containing ubiquinone (UQ) or menaquinone (MQ) at QA site, respectively]. This conformational distinction between the QA- species in PSII vs bacterial RCs follows precisely the conformational preferences of the isolated semiquinone anion radicals free in solution; type II semiquinones like PQ-9- have the isoprenyl C beta C gamma bond coplanar with the aromatic ring, while type I semiquinones like UQ- and MQ- place the C beta C gamma bond perpendicular to the ring. This conformational difference originates from nonbonded repulsions between the isoprenyl chain and the C6 methyl group present in type I semiquinones, forcing the perpendicular conformation, but absent in type II semiquinones having the smaller H atom at C6. Thus, the QA binding site in both higher plant PSII and bacterial reaction centers accommodates the lower energy conformation of their native semiquinones observed in solution. The energy difference between ground (C beta C gamma bond perpendicular to the ring) and excited (C beta C gamma bond coplanar with the ring) conformations of UQ- and vitamin K1- radicals is estimated to be sufficiently large (ca. 6 kcal/mol) to produce greater than a 10-fold difference in populations of these conformations at room temperature. For PQ-9-, a similar number is estimated. We propose that the strong confornational preferences of type I and type II semiquinones has lead to the evolution of different reaction center protein structures surrounding the isoprenyl/quinone head junction of QA to accommodate the favored low energy conformers. This predicted difference in protein structures could explain the low effectiveness (high selectivities) observed in quinone replacement experiments for type II vs type I quinones seen in higher plant PSII and bacterial reaction centers, respectively.

Topics: Butadienes; Coenzymes; Electron Spin Resonance Spectroscopy; Electron Transport; Free Radicals; Hemiterpenes; Molecular Conformation; Pentanes; Photosynthetic Reaction Center Complex Proteins; Photosystem II Protein Complex; Plastoquinone; Rhodobacter sphaeroides; Rhodopseudomonas; Solutions; Spinacia oleracea; Ubiquinone

Isoprenoid compounds are found in all organisms. In Escherichia coli the isoprene pathway has three distinct branches: the modification of tRNA; the respiratory quinones ubiquinone and menaquinone; and the dolichols, which are long-chain alcohols involved in cell wall biosynthesis. Very little is known about procaryotic isoprene biosynthesis compared with what is known about eucaryote isoprene biosynthesis. This study approached some of the questions about isoprenoid biosynthesis and regulation in procaryotes by isolating and characterizing mutants in E. coli. Mutants were selected by determining their resistance to low levels of aminoglycoside antibiotics, which require an electron transport chain for uptake into bacterial cells. The mutants were characterized with regard to their phenotypes, map positions, enzymatic activities, and total ubiquinone content. In particular, the enzymes studied were isopentenyldiphosphate delta-isomerase (EC 5.3.3.2), farnesyldiphosphate synthetase (EC 2.5.1.1), and higher prenyl transferases.

Topics: Butadienes; Carbon-Carbon Double Bond Isomerases; Chromatography, High Pressure Liquid; Cosmids; Dimethylallyltranstransferase; Drug Resistance, Microbial; Escherichia coli; Genetic Complementation Test; Hemiterpenes; Intramolecular Lyases; Isomerases; Mutagens; Mutation; Pentanes; Ubiquinone

Recently discovered metabolites in urine have suggested a defect of isoprenoid metabolism in multiple sclerosis. Lymphocyte HMG-CoA reductase was found unaffected however, and so was lymphocyte biosynthesis of geraniol, farnesol and squalene from mevalonolactone. The level of dolichol in white matter of an MS brain was similar to that of a control sample. Serum ubiquinone, on the other hand, was decreased in multiple sclerosis. Ubiquinone in serum was both age-dependent and related to serum cholesterol. Active as well as stable MS displayed a decreased level of serum ubiquinone, and a reduced ubiquinone-cholesterol ratio. These results are compatible with a deficient ubiquinone biosynthesis in multiple sclerosis.

Topics: Adult; Brain Chemistry; Butadienes; Cholesterol; Diterpenes; Hemiterpenes; Humans; Hydroxymethylglutaryl CoA Reductases; Lymphocytes; Mevalonic Acid; Multiple Sclerosis; Pentanes; Ubiquinone

Topics: Animals; Butadienes; Cholesterol; Doxorubicin; Hemiterpenes; Hydroxymethylglutaryl CoA Reductases; In Vitro Techniques; Liver; Male; Microsomes, Liver; Pentanes; Rats; Ubiquinone