Chemical Nature and Biosynthesis of Gibberellins
Chemical Nature of Gibberellins:- Gibberellins are tetracyclic diterpenes with an ent-gibberellane ring structure.
3. Biosynthesis of Gibberellins:- Gibberellins are synthesized inside the plastids of immature seeds, young leaves and even the roots. They are synthesized from acetate units of acetyl coenzyme A by the mevalonic pathway. The following steps are involved in the biosynthesis of gibberellins:
> Synthesis of gibberellins begins with acetate molecule. Acetate is esterified with coenzyme (CoA) to form three acetyl coenzyme A (acetyl Co A) molecules which undergoes a series of condensing reactions to β-hydroxyl-β-methyl glutaryl CoA (BOG-CoA). Then BOG-Co A is reduced in two successive NADPH-requiring steps to form mevalonic acid.
> Mevalonic acid is then phosphorylated by mevalonic acid kinase ( mevalonate kinase) in the presence of 2ATP molecules to form mevalonic acid pyrophosphate.
> Then decarboxylation of mevalonic acid pyrophosphate in the presence of ATP which yields Isopentenyl pyrophosphate (IpPP)
> IpPP is converted into dimethylallyl pyrophosphate (DMAPP) which is an isomer of IpPP, by enzyme IpPP isomerase.
> One molecule of dimethylallyl pyrophosphate then serve as an acceptor of one IpPP molecule with elimation of pyrophosphate and formation of one molecule of di-isoprenoid alcohol pyrophosphate or gereniol pyrophosphate(GPP).
> GPP accepts a molecule of IpPP to form farnesol pyrophosphate which also accepts another IpPP molecule to form geranyl geraniol pyrophosphate (GGPP).
> Then geranyl gereniol pyrophosphate (GGPP) is folded on various ways and then converted into a partially cyclized compound, copalyl pyrophosphate (CPP in the presence of ent- copalyl diphosphate synthase. Then it is finally transformed into a fully cyclic compound, ent-kaurene by ent- kaurene synthase.
> ent-kaurene is oxidized step-wise at C-19 to form ent-Kaurenol, ent-kaurenal and entKaurenoic acid. The latter is hydroxylated to ent-7α-hydroxy Kaurenoic acid.
> Now the contraction of β-ring and β-hydroxylation occurs. The conversion of ent-7α-hydroxy Kaurenoic acid to a 20 carbon GA12 -aldehyde involves loss of 6β-hydrogen, a shift of 7, 8 bond to 6, 8-positionand loss of a proton from the extruded C-7.
> GA12-aldehyde is converted into GA12 by ent- kaurene acid oxidase (KAO).
> Loss of one carbon must occur to give rise to C-19 GAs such as GA3.