### Isoprenoids  
#### 2.1.1 Biosynthesis of Isoprenoids  
Isoprenoids (also known as terpenoids) belong to secondary metabolites, which include carotenoids, sterols, polyprenyl alcohols, ubiquinone-10 (UQ), plastoquinone-9 (PQ), tocopherols, and others. The first stages of their biosynthetic pathways involve the formation of prenyl side chain precursors and a benzoquinone ring. Isopentenyl diphosphate (IPP) (Figure 1A) is the universal isoprene precursor for prenyl side chain synthesis of all isoprenoids. A benzoquinone ring originates from L-tyrosine (Figure 1B) or phenylalanine, both products of the shikimate pathway, which is a specialized pathway for the biosynthesis of aromatic compounds. In the second stage, the condensation of the ring and the prenyl side chain with subsequent modifications takes place [15]. The prenyl side chain is synthesized in the cytoplasm IPP, which is converted by isopentenyl diphosphate isomerase (IPPI) to dimethylallyl pyrophosphate (DMAPP), followed by the synthesis of numerous isoprenoids, including PQ and UQ (Figure 2). Information about the genes which encode the main enzymes of the isoprenoid biosynthesis in _Arabidopsis thaliana_ is given in Table 1.  
Figure 1: The chemical structures of the main precursors in isoprenoid synthesis. (**A**) Isopentenyl diphosphate; (**B**) L-tyrosine; (**C**) 4-hydroxybenzoic acid. Figure 2: Schematic overview of biosynthetic pathways of isoprenoid antioxidants. Metabolites: 4-HBA, 4-hydroxybenzoic acid; 4-HPPA, 4-hydroxyphenylpyruvate acid; DAHP, 3-deoxy-D-arabino-heptulosonate 7-phosphate; DMAPP, dimethylallylpyrophosphate; DMPBQ, dimethylply-phenyl-benzoquinone; GGPP, geranyligeranyl diphosphate; HGA, homogentisate acid; IPP, isoprenyl diphosphate; MEP, 2C-methyl-D-erythritid-4-phosphate; MPBQ, methyl-plylyl-benzquinone; MSBQ, methyl-solanesyl-benzoquinone; MVA, mevalonic acid; Phytyl-DP, phytyl diphosphate; PPDHB, polyprenyl-dihydroxybenzoate; PPHB, polyprenyl-hydroxybenzoate; PPPP, polyprenyl pyrophosphate; Solanesyl-DP, solanesyl diphosphate. Enzymes (colored): 4 CL8, peroxisomal 4-couramate CoA ligase; \(\beta\)LCY1, \(\beta\)-carotene cyclase; \(\beta\)-OHase, \(\beta\)-carotene hydroxylase; \(\varepsilon\)LCY, \(\varepsilon\)-carotene cyclase; \(\varepsilon\)OHase, \(\varepsilon\)-carotene hydroxylase; CoQ1 (SPS3), solanesyl diphosphate synthase;  
Coq3, Coq5, S-adenosyl-l-methionine (SAM)-dependent methyltransferaseases; Coq2 (PPT1), 4-hydroxybenzoate polytprenyl diphosphate transferase; Coq6, CoqF, flavin-dependent monooxygenenses; CRTISO, carotenoid isomerase; FPPS, famesyl diphosphate synthase; GGPPS, geranylanyl diphosphate synthase; GGPPS, geranylgeranyl diphosphate synthase; HPPD, 4-hydroxyphenylpyruvate dioxygenase; HPPR, 4-hydroxyphenylpyruvateateductase; HPT (VTE2), homogenistate phytyl transferase; HST, homogenistate solanesyl diphosphate transferase; IDI, isopentenyl diphosphate isomerase; IPPI, isopentenyl diphosphate isomerase; NXS, neoxanthin synthase; PDS, Phytoene synthase; SPS, solanesyl diphosphate synthases; TAT, tyrosine aminotransferase; VDE, violaxanthin de-epoxidase; VTE1, locoperol cyclase; VTE3, MPBQ/MSBQ methyl transferase; VTE4, \(\gamma\)-tocopherol methyltransferase; VTE5, phytol kinase; VTE6, phytol-phosphate kinase; ZDS, \(\zeta\)-carotene desaturase; ZE, zeaxanthin epoxidase. The enzymes of the cytoplasmic stages of isoprenoid synthesis are marked in purple, the stages and enzymes of the chloroplast isoprenoid synthesis are marked in turquoise, the stages oftocopherol synthesis are yellow-brown, the stages of plastoquinone synthesis are green, and the stages of carotenoid synthesis are red. The stages and enzymes of ubiquinone synthesis are marked in brown.