[9][10] The α subunits catalyze the reversible formation of indole and glyceraldehyde-3-phosphate (G3P) from indole-3-glycerol phosphate (IGP).

The β subunits catalyze the irreversible condensation of indole and serine to form tryptophan in a pyridoxal phosphate (PLP) dependent reaction.

[12] The α and β active sites are separated by a 25 Ångstrom long hydrophobic channel contained within the enzyme allowing for the diffusion of indole.

If the channel did not exist, the indole formed at an α active site would quickly diffuse away and be lost to the cell as it is hydrophobic and can easily cross membranes.

Tryptophan synthase is commonly found in Eubacteria, Archaebacteria, Protista, Fungi, and Plantae.

[18] However, it is thought that bacteria have alternate mechanisms to produce amino acids which might make this approach less effective.

As such, the inhibition of tryptophan synthase along with other PLP-enzymes in amino acid metabolism has the potential to help solve medical problems.

[19] Inhibition of tryptophan synthase and other PLP-enzymes in amino acid metabolism has been suggested for: It is thought that early in evolution the trpB2 gene was duplicated.

The other copy remained outside as trpB2o, and fulfilled an existing role or played a new one such as acting as a salvage protein for indole.

The advantage of the indole salvage protein declined and the TrpB gene was lost.