The behaviour of the Bz-methylindoles as substrates and inhibitors for Neurospora crassa tryptophan synthase

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Abstract
A partially purified preparation of tryptophan synthase from N. crassa has been shown to catalyse the conversion of all 4 Bz-methylindoles into the corresponding methyl-tryptophans. The 4 Bz-methylindoles are much less effective substrates than indole itself, the 7- and 4-methyl compounds being the most effective, followed by the 6- and 5-methyl compounds, in descending order of effectiveness. The equilibrium constants, Km, for the dissociation of the enzyme-substrate complexes with indole and 4- and 7-methylindoles have been evaluated. The low affinity of the enzyme for 4-methylindole is ascribed to shielding of the 3-position by the 4-methyl group. The 5- and 6-methylindoles inhibit the conversion of indole into tryptophan by the enzyme and the equilibrium constants, Kj, for the dissociation of the enzyme-inhibitor complexes have been evaluated.