Principles Of Carotenoid Biosynthesis

C40 carotenoids are synthesized by head-to-head condensation of two molecules of C20 geranylgeranyl pyrophosphate catalyzed by the enzyme phytoene synthase (Figure 7.3A). A very similar condensation reaction from C15 farnesyl pyrophosphate leads to the formation of C30 carotenoids. In the next steps, the conjugated double bond system of C30 and C40 carotenes is sequentially extended, each time integrating one of the isolated double bonds. Different phytoene desaturases exist with respect to the catalyzed number of desaturation steps. The enzyme Pds from plants inserts only two double bonds symmetrically at positions 11 and 11' (Figure 7.3B). The bacterial phytoene desaturase from Rhodobacter capsulatus, CrtIRc, carries out a 3-step desaturation with an additional

2 x farnesyl pyrophosphate

2 x geranylgeranyl pyrophosphate

Phytoene

Phytoene

Pds: (-carotene

I__CrtlR.c. : Neurosporene

Pds: (-carotene

I__CrtlR.c. : Neurosporene

_I CrtlEU. : Lycopene

I____Al-1: 3,4-didehydrolycopene

Figure 7.3 Principles of the biosynthesis of acyclic carotenoids with different polyene chains. (A) Establishment of the basic C30 and C40 carbon chain. (B) Extension of the conjugated double bond system. Double bonds in bold are introduced by the individual phytoene desaturases as indicated by the arrows yielding the specified carotenes.

double bond at position 7. The most abundant bacterial phytoene desaturase like CrtIEu catalyzes a 4-step desaturation with additional double bonds at position 7 and 7'. The enzyme Al-1 from the fungus Neurospora crassa catalyzes a 5-step desaturation with an additional double bond at position 3. These individual desaturation reactions lead to the formation of the products C-carotene, neurosporene, lycopene, and 3,4-didehydrolycopene, respectively.

Lycopene can be modified at the terminal double bonds by addition of water, resulting in 1-HO derivatives which can be methylated to 1-CH3O carotenoids (Figure 7.4). Desaturation at position 3,4 requires the presence of the 1-HO or 1-CH3O group. These types of carotenoids are typical for photosynthetic bacteria. Finally, a 2-keto group can be introduced under aerobic growth. Another addition to the 1,2 double bond of lycopene is involved in the synthesis of C45 and C50 carotenoids. This chain elongation at C-2 proceeds via addition of one or two dimethyl allyl cation. Then, the molecule is stabilized by abstraction of a proton from C-17 and C17'.

Cyclization of lycopene ends to ionone groups involves protonation of the 1,2 double bond and the addition of the resulting carbo cation to the 5,6 double bond (Figure 7.4). Stabilization occurred by proton abstraction from either C-6 or C-4 yields or s-rings, respectively. The possible individual substitutions of a ^-ionone end group are summarized in Figure 7.4. A cyclic carotenoid may carry a 3-HO, 4-keto, and/or 5,6-epoxy moiety. It should be pointed out that only the combinations of 3-HO and 4-keto, or 3-HO and 5,6-epoxy are known. Details on the carotenogenic pathway can be found in many reviews (1,21,22).

Elongation

Elongation

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