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.