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.