Abstract
A study has been made of the properties of a tryptophan synthase ( l-serine hydro-lyase (adding indole), EC 4.2.1.20) obtained from cultured cells of Nicotiana tabacum var. Wisconsin 38. The data presented show that the enzyme more closely resembles the two-component bacterial type of tryptophan synthase in contrast to the single-component fungal enzyme. 1. 1. K m values for the substrates indoleglycerol phosphate, indole, l-serine, and pyridoxal phosphate are 0.11 mM, 0.016 mM, 34.0 mM, and 1.3 μM, respectively. The pH optimum for Reaction 2 activity ( indole + l-serine → l-tryptophan ) of the enzyme is 8.2. 2. 2. The enzyme can be separated into two components by differential precipitation with (NH 4) 2SO 4. Component B alone catalyzes the conversion of indole to tryptophan and this activity is not stimulated by the addition of Component A. The combination of Components A and B is necessary for the conversion of indoleglycerol phosphate to tryptophan. Component A is heat-labile with a sedimentation constant of 3.1 S. Component B is heat-stable with a sedimentation constant of 5.5 S. 3. 3. Mixed complementation experiments with the A and B subunits of Escherichia coli and N. tabacum tryptophan synthase, as well as antibody neutralization experiments, indicate that the B protein has undergone less change than the A protein throughout the course of evolution.
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