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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.
Farnesyl diphosphate synthase (FPPS) is a key enzyme in isoprenoid biosynthesis which supplies sesquiterpene precursors for several classes of essential metabolites including sterols, dolichols, ubiquinones and carotenoids as well as substrates for farnesylation and geranylgeranylation of proteins. It catalyzes the sequential head-to-tail condensation of two molecules of isopentenyl diphosphate with dimethylallyl diphosphate. The enzyme is a homodimer of subunits, typically having two aspartate-rich motifs with two sets of substrate binding sites for an allylic diphosphate and isopentenyl diphosphate per homodimer. The synthase amino-acid residues at the 4th and 5th positions before the first aspartate rich motif mainly determine product specificity. Hypothetically, type I (eukaryotic) and type II (eubacterial) FPPSs evolved from archeal geranylgeranyl diphosphate synthase by substitutions in the chain length determination region. FPPS belongs to enzymes encoded by gene families. In plants this offers the possibility of differential regulation in response to environmental changes or to herbivore or pathogen attack.
The plant Solanum nigrum treated with the pathogen Phytophthora infestans-de- rived elicitor responded by elevated reactive oxygen species (ROS) production, lipid peroxidation and lipoxygenase (EC 1.13.11.12) activity in comparison with control plants indicating that oxidative stress took place. We demonstrate that these events are accompanied by a significant increase in plastoquinone (PQ) level. It is postulated that PQ may be associated with mechanisms maintaining a tightly controlled balance between the accumulation of ROS and antioxidant activity that determines the full ex­pression of effective defence.
The yeast Saccharomyces cerevisiae strain W303 synthesizes in the early logarith­mic phase of growth dolichols of 14-18 isoprene residues. The analysis of the polyisoprenoids present in the stationary phase revealed an additional family which proved to be also dolichols but of 19-24 isoprene residues, constituting 39% of the to­tal dolichols. The transfer of early logarithmic phase cells to a starvation medium lacking glucose or nitrogen resulted in the synthesis of the longer chain dolichols. The additional family of dolichols represented 13.8% and 10.3% of total dolichols in the glucose and nitrogen deficient media, respectively. The level of dolichols in yeast cells increased with the age of the cultures. Since both families of dolichols are present in stationary phase cells we postulate that the longer chain dolichols may be responsi­ble for the physico-chemical changes in cellular membranes allowing yeast cells to adapt to nutrient deficient conditions to maintain long-term viability.
Statins are inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the key enzyme of the sterol biosynthesis pathway. Statin therapy is commonly regarded as well tolerated. However, serious adverse effects have also been reported, especially during high-dose statin therapy. The aim of our study was to investigate the effect of statins on gene expression profiles in human hepatoma HepG2 cells using Affymetrix Human Genome U133 Plus 2.0 arrays. Expression of 102, 857 and 1091 genes was changed substantially in HepG2 cells treated with simvastatin, fluvastatin and atorvastatin, respectively. Pathway and gene ontology analysis showed that many of the genes with changed expression levels were involved in a broad range of metabolic processes. The presented data clearly indicate substantial differences between the tested statins.
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