page_title: Tryptophan synthase
section_title: Enzyme mechanism
hierarchical_section_title: Tryptophan synthase / Enzyme mechanism
caption_attribution_description: English: Active Site of Tryptophan Synthase
context_page_description: Tryptophan synthase or tryptophan synthetase is an enzyme that catalyses the final two steps in the biosynthesis of tryptophan. It is commonly found in Eubacteria, Archaebacteria, Protista, Fungi, and Plantae. However, it is absent from Animalia. It is typically found as an α2β2 tetramer. The α subunits catalyze the reversible formation of indole and glyceraldehyde-3-phosphate from indole-3-glycerol phosphate. The β subunits catalyze the irreversible condensation of indole and serine to form tryptophan in a pyridoxal phosphate dependent reaction. Each α active site is connected to a β active site by a 25 angstrom long hydrophobic channel contained within the enzyme. This facilitates the diffusion of indole formed at α active sites directly to β active sites in a process known as substrate channeling. The active sites of tryptophan synthase are allosterically coupled.
context_section_description: α subunit reaction: The α subunit catalyzes the formation of indole and G3P from a retro-aldol cleavage of IGP. The αGlu49 and αAsp60 are thought to be directly involved in the catalysis as shown. The rate limiting step is the isomerization of IGP. See image 2.
  β subunit reaction: The β subunit catalyzes the β-replacement reaction in which indole and serine condense to form tryptophan in a PLP dependent reaction. The βLys87, βGlu109, and βSer377 are thought to be directly involved in the catalysis as shown. Again, the exact mechanism has not been conclusively determined. See image 2.
  Net reaction: See image 3.
caption: Image 1: Active sites for α and β subunits showing hypothesized catalytic residues
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