Indole glycerol-3-phosphate synthetase

In B. subtilis, the pathway from chorismate to tryptophan is feedback-inhibited by tryptophan, which suppresses anthranilate synthase activity. Mutant B. subtilis that lacks tryptophan synthetase can grow on minimal medium only when supplemented with exogenous tryptophan. Under these conditions, none of the intermediates in the tryptophan biosynthetic pathway from anthranilate to indole 3-glycerol phosphate are produced. However, when the bacteria have depleted the medium of tryptophan, the levels of those intermediates increase, even though there is no net production of tryptophan. Why ... 

Figure 5.41 Early steps of the proposed indole acetic acid biosynthesis pathways for Ara-bidopsis. CHO, chorismate ANA, anthranilate PANA, 5-phosphoribosylanthranilate CADP, l-(o-carboxyphenylamino)-l-deoxyribulose-5-phosphate IGP, indole-3-glycerol phosphate TRP, tryptophan. Enzymes ASA, anthranilate synthetase, suhunit a ASB, anthranilate synthetase, suhunit P PAT, phosphorihosylanthranUate transferase PAI, phosphoiibosylanthrani-late isomerase IGS, indole-3-glycerol-phosphate synthase TSA, tryptophan synthase, subunit a and TSB, tryptophan synthase, suhunit p.

During my stay at Western Reserve University I also continued my studies of tryptophan synthetase—but now with the enzyme from E. coU as well as the one from Neurospora. With Jody Stadler and Martin Rachmeler initial observations were made that suggested that the tryptophan synthetase reaction was more complex than we had imagined. The enzyme converted indole-3-glycerol phosphate to indole and indole -3-glycerol phosphate to tryptophan in addition to catalyzing the previously characterized reaction, indole -F l-serine —> L-tryptophan< - - >. 

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