The synthesis of Vitamin E and other tocochromanols in plants

The synthesis of tocochromanols by plants is of major importance in human nutrition as the biosynthesis is responsible for vitamin E. There are two major classes of tocochromanols each of which have four different forms:

1. Tocopherols - a, b, g, d
2. Tocotrienols - a, b, g, d

Of these it is a-tocopherol that has the most vitamin E activity; a-tocotrienol, b-tocopherol also have vitamin E activity, but at about 25 to 50% of that of a-tocopherols. g-tocopherol has even less (~15%) vitamin E activity. The others have 3% or less activity.

Structure of Tocochromanol

All tocochromanols are amphipathic in nature. The creation of the polar head of the molecule can be traced back to the metabolism of amino acids, whereas the tail of the tocochromonals cis created from phytyl-diphosphate (tocopherols) or geranylgeranyl diphosphate (tocotrienols). The four main types of each tocochromanol are a result of differing quantities or positions of methyl groups on the aromatic ring.

The use of vitamin E in the body

Although all tocochromanols have biological activity, from a nutritional point of view it is the a-tocopherol that are of the greatest importance as the body is able to make more efficient use of this form of vitamin E. Once in the body vitamin E can act as an antioxidant and therefore has a role to play in the reduction of free radicals; this helps to protect against damage to DNA, and in turn helps to protect against the risk of getting many cancers.

The use of vitamin E in plants

In plants the elimination of vitamin E (tocochromanols) is not lethal. It has been shown to play a role in the protection of seed storage lipids.

The next section on the Synthesis of vitamins in plants takes a look at the tocochromanol pathway.

The Tocochromanol (Vitamin E) Pathway

There are two main pathways that are involved in vitamin biosynthesis in the plastid, these being the carotenoid and tocochromanol pathways. The Carotenoid pathway is responsible for the production of b-carotene and vitamin A and has been discussed elsewhere in this section on biosynthesis of vitamins in plants. The biosynthesis tocochromanol pathway results in tocopherols and tocotrienols (vitamin E). This is an important vitamin in animals where it plays an essential role in antioxidation, in plants it has been shown to be involved in seed storage lipids.

An overview of the tocochromanol pathway

As mentioned previously in this section of plant biology advice the tocochromanols have an amphipathic structure with a polar head and a tail. the tail can be created from either geranylgeranyl diphosphate (tocotrienols) or phytyl-diphosphate (tocopherols), whereas the head is formed from amino acid derivatives.

The tocochromanol pathway should therefore be broken into two halves:
1. Bipolar head formation
2. Tail formation

The main process of the synthesis of the head involves the enzyme p-hydroxyphenylpyruvate dioxidase, which catalyzes the formation of homogentistic acid from p-hydroxyphenylpyruvate.

Geranylgeranyl diphosphate or phytyl-diphosphate is then fused with the homogentistic acid by homogentisate prenyl transferases to create either 2-methyl-6-phytylplastoquinol (tocopherols) or 2-methyl-6-geranylgeranylplastoquinol (tocotrienols). This results in the first true intermediates in the tocochromanol (vitamin E) pathway.

The tocopherol pathway

As it is the a-tocopherol that is responsible for the most vitamin activity in mammals it is discussed a little further here. Once the homogentistic acid has been fused to produce 2-methyl-6-phytylplastoquinol it undergoes three prenyltransferase catalyzed reactions to produce a-tocopherol. The way that a-tocopherol differs from the other tocopherol is the number of prenyltransferase steps it is subjected to or the use of the S-adenosyl methionine substrate in the first step.

DellaPenna and Pogson (2006). Vitamin Synthesis in Plants: Tocopherols and Carotenoids. Annual revue plant biology. 57: 711 to 738
Grusak and DellaPenna (1999). Improving the nutrient composition of plants to enhance human nutrition and health. Annu. Rev. Plant Physiol. Plant Mol. Biol. 50:133–61
Schneider (2005). Chemistry and biology of vitamin E. Nutr. Food Res. 49:7–30

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