## 4 Specific Prenylquinones
### Ulbiquinone
#### 4.1.1 Ubiquinone Biosynthesis and Distribution  
Ubiquinone is a prerylquinone composed of an isoprene chain attached at the carbon-2 of a 1,4-benzoquinone (Figure 6). UQ is a prerylquinone that probably originated from proteobacteria. There is a theory that proteobacteria gives rise to mitochondria via an endosymbiotic process with other organisms. Therefore, UQ can be found in \(\alpha\)-, \(\beta\)- and \(\gamma\)-proteobacteria as well as in several eukaryotic subcellular compartments [45; 147].  
Figure 6: Chemical structures of ubiquinone and atovaquone. (**a**) Chemical structures of UQ, \(n\) indicates variable number of isoprenic units contained on the prerylquinone side chain. (**b**) The antiparasitic drug atovaquone.  
The aromatic precursor of UQ, p-hydroxybenzoate, is formed from chorismate via the shikimate pathway in most bacteria and plants, while in eukaryotes, p-hydroxybenzoate is provided by tyrosine [45]. The conversion of chorismate to p-hydroxybenzoate is catalyzed by the enzyme chorismate-pyruvate lyase, encoded by the ubiC gene [45]. In the biosynthetic pathway in most organisms, p-hydroxybenzoate is preynylated by the p-hydroxybenzoate polyprenyltransferase enzyme (UbiA/Coq2), followed by modification of the prenylquinone head group (Figure 7). These modifications are performed by Ubi/Coq, but the enzymatic reaction order can differ among species. However, in all organisms, decarboxylation, hydroxylation and SAM-dependent methylation occur.  
Although p-hydroxybenzoate is the classic aromatic precursor for UQ biosynthesis, a recent study in yeast and _E. coli_ showed complete incorporation of [phenyl-\({}^{13}\)C\({}_{6}\)] aminobenzoic acid (pABA) into the prenylquinone [45]. This new pathway has not been elucidated to date because, in all organisms in which [phenyl-\({}^{13}\)C\({}_{6}\)] pABA incorporation into UQ occurs, the intermediate that undergoes deamination remains unknown. This discovery suggests that the mechanisms of action of sulfonamide and other classic antifoldes could be involved with inhibition of UQ biosynthesis [148]. Whether pABA is the UQ aromatic precursor in other organisms has not been studied to date [148; 149; 150; 151].  
The final biosynthetic enzymes in eukaryotes appear to be located on the membrane, suggesting that after the biosynthesis of UQ, there exists some type of intracellular transporter [152; 153; 154]. The main function of UQ is electron transport between complexes I or II and complex III of the mammalian internal mitochondrial membrane. However, many other enzymes that modify the redox state of UQ are known. During photosynthesis, UQ can also act on the PSII of organisms such as _R. rubrum_[45]. Finally,  
Figure 7: Scheme of ubiquinone biosynthesis. UQ biosynthesis from 4-hydroxybenzoate. In the figure depicting the metabolic pathways, the transformation of the cofactor SAM to S-adenosyl-t-homocysteine (SAH) for methylation is indicated. UbiC. Chorismate-pyruvate lyase, EC: 4.1.3.40; UbiA/Coq2. EC: 4-Hydroxybenzoate polyprenyltransferase, 2.5.1.39; UbiD. 4-Hydroxy-3-polyphenylbenzoate decarboxylase, EC: 4.1.1.98; Coq7/UbiI. 2-Polyphenylphenol 6-hydroxyylase, EC: 1.14.13.240; UbiH/Coq6. UQ biosynthesis monooxygenase/2-ocaprenyl-6-methoxyphenol hydroxylase, EC: 1.14.13.-; UbiE/Coq5. 2-methoxy-6-polyphenyl-1,4-benzoquinol methylase, EC: 2.1.1.201; UbiF/Coq7. 3-Demethoxyubiquol 3-hydroxylase, EC: 1.14.99.60; UbiG/Coq3. Polyprenyldihydroxygenzoate methyltransferase/3-demethylubiquol, 3-O-methyltransferasearse/2-polyphenyl-6-hydroxyphenyl cellulase/3-demethylubiquinone-9 3-methyltransferase, EC: 2.1.1.114/2.1.64/2.1.222/. Data from the Kyoto Encyclopedia of Genes and Genomes database [87].  
among other functions of UQ, antioxidant activity, inhibition of lipid peroxidation and regeneration oftocopherol [45; 57; 58; 59] have been described. The function of the existence of a predominant and specific UQ homolog in each species and/or cell compartment is unclear.