Gibberellin biosynthesis

Gibberellin biosynthesis is a primary metabolic pathway that occurs in all higher plants but not, apparently, in other organisms. The main products are gibberillins, functionalized diterpenes which have important hormonal roles in processes from seed germination to stem growth to reproduction. A number of species also produce diterpenes based on the kaurane skeleton from the same biosynthetic pathway.

The pathway can be divided into two portions: the synthesis of giberellin A₁₂ from geranylgeranyl pyrophosphate, and the functionalization of giberellin A₁₂ into giberellin hormones such as giberellic acid.

Synthesis of the giberellin skeleton

The giberellin skeleton, in the form of giberellin A₁₂, is synthesized from geranylgeranyl pyrophosphate in eight steps catalyzed by three different enzymes (although there are four distinct enzymic activities).

The first committed step in giberellin and kaurane biosynthesis is the the partial cyclization of geranylgeranyl pyrophosphate to ent-copalyl pyrophosphate, catalyzed by ent-copalyl diphosphate synthase (EC 5.5.1.12). This reaction forms the opposite enantiomer, (−)-copalyl pyrophosphate, from that used in other diterpene biosyntheses, isolating the production of giberellin hormones from other metabolic pathways.

The cyclization process is completed by the transformation of ent-copalyl pyrophosphate to ent-kaurene, catalyzed by ent-kaurene synthase (EC 4.2.3.19).

In those cases which have been studied, ent-copalyl diphosphate synthase and ent-kaurene synthase activities are shown by the same protein, which is often simply referred to as kaurene synthase. It is a cytosolic enzyme widely distributed among different plant tissue types.

While some kaurane terpenes may be synthesized directly from ent-kaurene, the next major step is the oxidation of ent-kaurene to ent-kaurenoic acid, catalyzed by a cytochrome P450 enzyme, ent-kaurene oxidase (EC 1.14.13.78), bound to the outer chloroplastic membrane.

Finally, ent-kaurenoic acid is oxidized further to giberellin A₁₂, catalyzed by a second cytochrome P450 enzyme, ent-kaurenoic acid oxidase (EC 1.14.13.79).