Phosphoenolpyruvate PEP condenses with

1
Phosphoenolpyruvate (PEP) condenses with
erythrose 4-phosphate to form a 7-carbon
compound, 2-keto 3-deoxy arabinoheptulosonate,
DAHP.
2
DAHP cyclises to form 5-dehydroquinate.
5-dehydroquinate
Dehydration and reduction yield shikimate.
Quinate
Shikimate
3
Shikimate is phosphorylated and condensed with a
molecule of phosphoenolpyruvate.
Enolpyruvoyl shikimate 5-phosphate
4
Elimination of the phosphate results in formation
of chorismate.
Chorismate
5
Biosynthesis of PHE and TYR
Chorismate mutase converts chorismate to
prephenate.
6
Prephenate dehydratase forms the a-ketoacid
precursor of PHE.
7
Prephenate dehydrogenase forms the a-ketoacid
precursor of TYR.
8
TRP biosynthesis
Chorismate is converted to anthranilate. GLN is
the amino group donor.
Anthranilate synthase produces NH3 by hydrolysing
GLN. Addition of the NH3 to the ring results in
elimination of water.
9
Loss of a proton from C2 of the resulting
intermediate leads to elimination of
enolpyruvate.
This reaction is favoured because it leads to
formation of an aromatic ring, a highly stable
product.
10
Anthranilate reacts with PRPP to form
phosphoribosyl anthranilate.
11
The sugar ring opens. Reaction is similar to
PRPP ribose opening in HIS biosynthesis.
A complex reaction results in formation of
indole glycerol phosphate.
12
TRP synthase catalyses reaction of serine with
indole glycerol phosphate to form TRP and
glyceraldehyde 3-phosphate.
serine
glyceraldehyde 3-phosphate
The carbon atoms of serine replace those of
glycerol phosphate.