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).[1] The formation of the two rings has been shown to proceed by a cyclization–rearrangement mechanism to produce the observed stereochemistry.[2]
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),[3][4] bound to the outer chloroplastic membrane.
Finally, ent-kaurenoic acid is oxidized further to giberellin A12, catalyzed by a second cytochrome P450 enzyme (from the CYP88A subfamily), ent-kaurenoic acid oxidase (EC 1.14.13.79).[5][6]
Bioactive gibberellins
While it is believed that all gibberellins are derived from giberellin A12, many of the details of the biosynthetic pathway have yet to be elucidated. One important step is the oxidative removal of carbon-20 in giberellin A12, catalyzed by gibberellin 20-oxidase, a soluble 2-oxoglutarate-dependent dioxygenase.[7]
References
- ↑ EC 4.2.3.19 – ent-kaurene synthase. In IUBMB Enzyme Nomenclature, <http://www.chem.qmul.ac.uk/iubmb/enzyme/EC4/2/3/19.html>. (accessed 18 September 2009).
- ↑ Coates, Robert M.; Cavender, Patricia L. Stereochemistry of the Enzymic Cyclization of Copalyl Pyrophosphate to Kaurene in Enzyme Preparations from Marah macrocarpus. J. Am. Chem. Soc. 1980, 102 (20), 6358–59. DOI: 10.1021/ja00540a040.
- ↑ EC 1.14.13.78 – ent-kaurene oxidase. In IUBMB Enzyme Nomenclature, <http://www.chem.qmul.ac.uk/iubmb/enzyme/EC1/14/13/78.html>. (accessed 5 September 2009).
- ↑ Helliwell, Chris A.; Sheldon, Candice C.; Olive, Mark R.; Walker, Amanda R.; Zeevaart, Jan A. D.; Peacock, W. James; Dennis, Elizabeth S. Cloning of the Arabidopsisent-kaurene oxidase gene GA3. Proc. Natl. Acad. Sci. USA 1998, 95 (15), 9019–24, <http://www.pnas.org/content/95/15/9019.full>.
- ↑ EC 1.14.13.79 – ent-kaurenoic acid oxidase. In IUBMB Enzyme Nomenclature, <http://www.chem.qmul.ac.uk/iubmb/enzyme/EC1/14/13/79.html>. (accessed 5 September 2009).
- ↑ Helliwell, Chris A.; Chandler, Peter M.; Poole, Andrew; Dennis, Elizabeth S.; Peacock, W. James The CYP88A cytochrome P450, ent-kaurenoic acid oxidase, catalyzes three steps of the gibberellin biosynthesis pathway. Proc. Natl. Acad. Sci. USA 2001, 98 (4), 2065–70, <http://www.pnas.org/content/98/4/2065.full>.
- ↑ Lange, Theodor; Graebe, Peter Expression cloning of a gibberellin 20-oxidase, a multifunctional enzyme involved in gibberellin biosynthesis. Proc. Natl. Acad. Sci. USA 1994, 91 (18), 8552–56, <http://www.pnas.org/content/91/18/8552.full.pdf+html>.
| Error creating thumbnail: Unable to save thumbnail to destination |
This page is currently licensed under the Creative Commons Attribution 3.0 Unported license and any later versions of that license. |
