branch point enzymes,(phenylalanine ammonium lyase):shikkimic acid pathwaychalcone synthase...
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Branch Point Enzymes:
NAVEED UL MUSHTAQDEPT. : Bio-RESOURCE
UNIVERSITY OF KASHMIR
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DEFINITION: Branch point enzymes are those enzymes
which connect two pathways and yield more than one products from one metabolite.
Schematic representation of a metabolic branch point.The numbers represent chemical compounds,whereas the letters represent enzymes that catalyse the conversion indicated by nearby arrow.
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Contd. The Pathways of secondary metabolites are
inherently linked with key processes (e.g;Photosynthesis and respiration) and products of primary metabolism (e.g; sugars , fatty acids, and amino acids)
The major classes of secondary compounds in plants are the : saponins ,terpenoids , sterols, phenols ,phenylpropanoids, alkaloids ,flavonoids and tannins (Mabry et al,1970; Edwards and Gatehouse, 1999)
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Contd. The first stage in forming a secondary
metabolite involves the action of a branch point enzyme , which directs a certain amount of the primary metabolic flux into secondary metabolism.
Common examples of branch point enzymes include : PAL , Chalcone synthase , GGPPS ,IFS, Squalene synthase and Sesquiterpene cyclase.
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PAL(Phenylalanine Ammonium lyase):
The PAL in plant secondary is perhaps the most studied in plant secondary metabolism.
PAL is situated at a branch point between primary and secondary metabolism , so the reaction it catalysis is an important regulatory step in the formation of many phenolic compounds.
Pal catalyses the formation of trans-cinnamic acid by deammination of phenylalanine(First described by Koukol and conn.) in phenyl propanoid pathway.
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Shikkimic acid pathway
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CHALCONE SYNTHASE : The flavonoids consist of various groups of
plant metabolites, which include :Chalcones,aurones,flavonones,isoflavonoids,flavones,flavanols,and anthocyanins.
The first comitted step of the flavonoid pathway is catalysed by chalcone synthase.
CHS, a dimeric polyketide synthase with each subunit at about 42kDa,has no co-factor requirements.
Both chalcones can be converted into aurones,a subclass of flavonoids found in certain plant species.
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Phenylpropanoid pathway
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GGPPS(Geranyl Geranyl pyrophosphosynthase):
A pivotal enzyme in monoterpene metabolism is GGPPS, a member of the short chain prenyltransferase family catalysing the Head-to-Tail condensation of 1 IPP and 1 DMAPP molecule to rise to GPP.
Supply of GPP is critical for terpenoid yield,therefore,studies on the regulation of genes in GPP biosynthesis assume central importance.
GGPPS is a central intermediate in the synthesis of plastidic isoprenoids such as Monoterpenoids and Carotenoids.
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MEVALONIC ACID/MEP PATHWAY:
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OTHER BRANCH POINT ENZYMES:
IFS(Isoflavone synthase),catalyses the first comitted step of Isoflavone biosynthesis, a branch of phenyl propanoid pathway.
In the main trunk of the isoprenoid pathway the enzymes : Squalene synthase (Porter and Spurgeon,1981) and Sesquiterpene cyclase (Cane,1981) are the first branch point enzymes for sterol and sesquiterpenoid biosynthesis competing for the last common intermediate between the two branches.
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PHENYLPROPANOID PATHWAY
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