Tryptophan synthase: a two-component enzyme from pea plants (Pisum sativum cv. Alaska)

Abstract

Evidence is presented showing that tryptophan synthase (L-serine hydro-lyase (adding indole), EC. 4.2.1.20) from pea is composed of two components, A and B, analogous to the protein subunits reported for bacteria, algae, and tobacco enzyme. Separation of the two components was achieved by gel filtration on Sephadex G-100. In contrast with enzyme from tobacco the two components could not be separated by ammonium sulfate precipitation. However, the A component was removed by heating at 70 °C for 5 min. The B component, alone, catalyzed the condensation of L-serine plus indole to tryptophan (reaction 2) at a significant level. A combination of the A and B components was necessary for the formation of tryptophan from indole-glycerol phosphate (reaction 1); and conversion of indole-glycerol phosphate to indole (reaction 3). Reaction 3 could only be demonstrated with partially purified enzyme extract, and the reaction proceeded well in the absence of added pyridoxal phosphate but was increased by its presence. The relative rates of the reactions were in the ratio of 30:100:1.5 for reactions 1, 2, and 3, which is in agreement with studies with microbial TSase. However, in contrast, some free indole was isolated from reaction 1. The reversal of reaction 3 could not be detected. It is suggested that reaction 1 is probably the one of physiological significance as is believed to be the case in microorganisms.