IAA-synthase, an enzyme complex from Arabidopsis thaliana catalyzing the formation of indole-3-acetic acid from (S)-tryptophan

An enzyme complex was isolated from Arabidopsis thaliana that catalyzes the entire pathway of biosynthesis of the major plant growth hormone, indole-3-acetic acid (IAA), from (S)-tryptophan. The 160-180 kDa, soluble complex catalyzes a strictly O2-dependent reaction which requires no further added factors and is stereospecific for the substrate (S)-tryptophan (app. Km = 120 microM). H2(18)O labeling proved that both oxygen atoms of IAA were delivered via H2O. This, as well as immunological evidence for the presence of a nitrilase-like protein in the complex, suggests the reaction to proceed via the intermediate indole-3-acetonitrile. IAA-synthase forms a tight metabolite channel committed to IAA production and occurs in shoots, roots and cell cultures of A. thaliana.