Tryptophan synthase

Tryptophan synthase, also known as tryptophan synthetase, is an enzyme found in plants and bacteria, but not in animals, which catalyses the final step in the biosynthesis of tryptophan.

The enzyme isolated from E. coli is an &alpha;2&beta;2-tetramer. The two &alpha;-subunits are readily detached from the &beta;2-dimer, and the two &beta;-subunits can be separated by treatment with 4 M urea solution. The association of the subunits is promoted by the presence of pyridoxal phosphate (PLP) and serine. Each &alpha;-subunit has a molecular mass of 29500: each &beta;-subunit has a molecular mass of 45000 with one PLP binding site.



The different subunits catalyse separate steps in the reactions, as shown in the diagram. Indole 3-glycerolphosphate is converted into indole and glyceraldehyde 3-phosphate by the &alpha;-subunits, while the &beta;2-dimer catalyses the condensation of indole and serine to produce tryptophan. However, the &alpha;2&beta;2-tetramer is 30–100 times more active than the isolated subunits, and does not release free indole during the reaction.