Tryptophan synthase

Tryptophan synthase
Tryptophan synthase.Tryptophan synthase, {PDB: 1WQ5}
Gene code:	[1]ID#: GI:61680225
Structure:	[2]1WQ5
Recent publications:	[3]The amino acid sequence of the A protein (alpha subunit) of the tryptophan synthetase of Escherichia coli.

Tryptophan synthase (EC 4.2.1.20), 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 α₂β₂-tetramer. The two α-subunits are readily detached from the β₂-dimer, and the two β-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 α-subunit has a molecular mass of 29500: each β-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 α-subunits, while the β₂-dimer catalyses the condensation of indole and serine to produce tryptophan. However, the α₂β₂-tetramer is 30–100 times more active than the isolated subunits, and does not release free indole during the reaction, since this intermediate is passed between the active sites through a tunnel within the protein, in a process known as substrate channeling.^[1]

See also

• Synthase
• Lyase

References

• Conn, E. E.; & Stumpf, P. K. (1976). Outlines of Biochemistry (4th ed.). New York: Wiley. ISBN 0-471-01772-8.