3.2.3. Tocopherol: Vitamin E

Tocopherol or tocochromanols are, just as carotenoids, lipid soluble antioxidants. Eight different molecules of tocopherols and tocotrienols collectively represent Vitamin E (DellaPenna and Pogson 2006). Their basic structure is a double ring, and a hydrophobic polyprenyl side chain, products from the shikimate and MEP/DOXP pathways (Fritsche et al. 2017). The tocopherols have a fully saturated side chain and the tocotrienols have an unsaturated side chain. There are 4 tocopherols, an α, β, γ, and δ form, based on the number of methyl groups on the chromanol ring. For the tocotrienols, the same types exist. Vitamin E activity scavenges radicals and quenches reactive oxygen species (ROS). The vitamin E activity is highest by far for α-tocopherol, followed by β -tocopherol, α-tocotrienol, γ- tocopherol, and δ-tocopherol (see Figure 3.4). The most needed of all tocopherols in our diet is therefore for α-tocopherol, found in high concentrations in vegetable oils and other high fat sources such as seeds, nuts and grains (DellaPenna 2005; DellaPenna and Last 2006).  The improvement of crops for higher vitamin E has therefore become a target in plant breeding (Péter et al. 2015).

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Figure 3.4: Tocochromanol structures and their activities. Key differences in molecules are indicated in red ( from: DellaPenna and Pogson 2006).

Tocopherols occur in all kingdoms of life, but only photosynthetic organisms are able to synthesize them. Since the discovery of vitamin E in 1922 (Evans and Bishop 1922), the beneficial effects of vitamin E have been demonstrated in animals. The lipid radical scavenging activity of vitamin E species are well described. Usually, there is a mixture of tocopherols and tocotrienols occur as a mixture in our food. Vitamin E cannot be synthesized by humans, and it is an essential component of our nutrition.  Adequate uptake of vitamin E is important. Together with other phytonutrients it is an important compound that is thought to work preventive for chronic diseases and neurological disorders. In particular the ones that are related to oxidative stress, such as cancer, atherosclerosis, and cataracts (Fritsche et al. 2017).

Biosynthesis of tocopherol occurs in the plastids, except for the first steps that are initiated in the cytosol. The aromatic headgroup of tocopherol comes from the shikimate pathway (Glossary in Chapter 1 and §3.2.2 on flavonoids), p-hydroxyphenylpyruvate (HPP), derived from Tyrosine, is modified into homogentisic acid (HGA), by p-hydroxyphenylpyruvate dioxygenase (PDS1/HPPD) (Schenck and Maeda 2018) The polyprenyl side chain, phytyl diphosphate (PDP), is derived from the MEP/DOXP pathway (Glossary in Chapter 1 and §3.2.1 on flavonoids carotenoids). HGA and PDP are merged by HPT (homogentisate phytyl transferase (HPT), to 2-methyl-6- phytyl-1,4-benzoquinol (MPBQ). MPBQ is methylated to 2,3-dimethyl-6-phytyl-1,4-benzoquinone (DMPBQ). Tocopherol cyclase (TC) transforms both MPBQ and DMPBQ to γ- and δ-tocopherol, respectively. Finally, γ- tocopherol methyltransferase (γ-TMT) catalyzes the conversion of γ- and δ-tocopherol to β- and α-tocopherol, respectively (3.2.3. Tocopherol: Vitamin E-Figure 3.5).

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Figure 3.5: Tocopherol biosynthetic pathway. Abbreviations: Tyr-AT: Tyr aminotransferase
CHLP: chloroplastic geranylgeranyl diphosphate reductase; HPPD: HPP dioxygenase; HPT: HGA phytyl transferase;
MPBQ-MT: MPBQ methyltransferase; MT: Methyltransferase