Branch Point Enzymes:

NAVEED UL MUSHTAQDEPT. : Bio-RESOURCE

UNIVERSITY OF KASHMIR

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.

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)

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.

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.

Shikkimic acid pathway

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.

Phenylpropanoid pathway

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.

MEVALONIC ACID/MEP PATHWAY:

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.

PHENYLPROPANOID PATHWAY