The Synthesis of vitamins in plant photosynthetic tissue

Many of the compounds that are involved in plastidic isoprenoid synthesis have important human nutrition impacts. There are two major classes of lipid soluble photosynthetic antioxidants.

1. Carotenoids - over 700 known compounds, usually involved in colour and pigmentation, these include b-Carotene and other precursors of Vitamin A (Retinaldehyde).
2. Tocochromanol - 4 tocopherols and 4 tocotrienols - vitamin E

The identification of many of the core genes that are involved in the pathways responsible for the synthesis of vitamins in the plastids have been discovered by an integrated approach that that has involved a combination of genetic, genomic and molecular approaches.

Knowledge of plastid genes helps to improve human health.

The identification of these genes has enabled metabolic engineering to take place; this has led to changes in the vitamin A and vitamin E levels in many plants. This can have an enhanced effect upon human health. An example of this is Golden Rice, which contains an enhanced level of Carotenoids (vitamin A); this has helped many people get a sufficient quantity of Vitamin A in their diet, with out making any major changes in their diet (I will leave the arguments about the rights and wrongs of genetically modified foods to people with more passion and knowledge on the subject.

Synthesis of carotenoids and tocochromanol in the plastids

Both Carotenoids (vitamn A) and Tocochromanol synthesis occurs in the plastidic isoprenoid biosynthetic pathway. There are two distinct pathways that are involved in Isopetenylpyrophosphate production.

1. MEP - Methylerythritol 4-phosphate Pathway
2. Cytosolic Mevalonic acid pathway

The production of Isopetenylpyrophosphate in turn leads to the production of geranylgeranyl diphosphate, which is an intermediate in many plastidic isoprenoids such as tocochromanols and carotenoids.

The biosynthesis of carotenoids in plants

What are Carotenoids?

Carotenoids are C₄₀ tetraterpenoids that are derived from phytoene; the compound stucture may have a linear backbone or contain cyclic b-ionone or e-ionone rings. Carotenoids contain over 700 compounds that play a role in plant biosynthesis, these include many that are known to be beneficial to human health such as b-carotene and vitamin A, and the plant hormone abscisic acid.

The carotenoids have many roles to play in the plant, being involved in seed setting, light harvesting, photomorphogenesis, peroxidation of lipids and the assembly of the photosystem.

Carotenoid biosynthetic pathways

Some of the well known Carotenoid biosynthetic pathways are as follows:

1. Lutein Biosynthesis
2. b-carotenoid derived Xanthophyll biosynthesis
3. Violaxanthin and Neoxanthin pathway

The oxidation of Carotenoids plays a major role in their classification:

1. Oxidised: Carotenes. These include b-carotene and lycopene
2. Non-Oxygenised: Xanthophylls. This group includes lutein, violaxanthin and neoxanthin

The steps in the carotenoid biosynthesis pathway

The initial steps of the carotenoid pathway are summarized as follows

1. The condensation of two geranylgeranyl diphosphate by the enzyme phytoene synthase creates phytoene.
2. The enzymes phytoene desaturase and z-carotene desaturase add double bonds to form lycopene.

In higher plants carotenoid isomerase then acts to catalyse cis-trans-isomerisation of the lycopene to produce trans-lycopene.

Once trans-lycopene has been produced, two possible outcomes can occur:

1. If two cyclic bete rings are added (b,b branch) then b-carotene, zeaxanthin, neoxanthin, violaxanthin and antheraxanthin are created.
2. If a beta and an epsilon ring (b,e branch) are added to the cyclic end then a-carotene is created.

Further information about carotene pathways is given in the next section of plant plastid vitamin synthesis.