Most mesophilic bacteria have a multimeric tryptophan synthase complex (EC?4.2.1.20) that forms?L-tryptophan from?L-serine and 1-C-(indol-3-yl)glycerol 3-phosphate via an indole intermediate. This intermediate, which is formed by the ¦Á subunits, is transferred in an internal tunnel to the ¦Â units, which convert it to tryptophan. In thermophilic organisms the high temperature enhances diffusion and causes the loss of indole. This enzyme, which does not combine with the ¦Á unit to form a complex, salvages the lost indole back to?L-tryptophan. It has a much lower Km for indole than the ¦Â subunit of EC?4.2.1.20.
