Tryptophan synthetase components

Tryptophan synthetase from E. coli contains two different types of polypeptides which are referred to as a and subunits. The physiologically functional tryptophan synthetase complex consists of two a and two y8 subunits and may be represented by the stylised illustration shown in Fig. 26. Upon dilution the complex dissociates to furnish the a subunits as monomers of mol. wt. 29 000 and a dimer consisting of two j8 subunits, mol. wt. 100000. The dimer contains one pyridoxal phosphate per polypeptide chain. These two components have been separated and used in the study of partial reactions. 

Fig. 26. A stylised model of tryptophan synthetase, 2/ 2 the two components, a protein and P2 protein, derived therefrom. The model shows an intimate interaction between the a and fi subunits at the pyridoxal-P binding sites.

Fig. 4. Diagram of the tryptophan operon plus two neighboring genes. The gene designations for both E. coli and S. typhimurium are given. The supX locus has not been identified in E. coli. Controlling elements PI, the promoter associated with the operator 0, the operator P2, the low-efficiency internal promoter. Enzymes AS-I, anthianilate synthetase component I PRT, phosphoribosyl transferase (AS-II, anthianilate synthetase component II) PRAI, phosphoribosyl anthianilate isomerase InGPS, indoleglycerol phosphate synthetase TS-a, tryptophan synthetase a-chain TS-)S, tryptophan synthetase j8-chain. RUM, region of unusual mutations in S. typhimurium.

Crawford, 1. P. and Yanofsky, C. (1958) On the separation of the tryptophan synthetase of Escherichia coli into two protein components. Proc. Nat. Acad. Sci. 44,1161-1170. 

However, the a and p2 sub-units were shown to combine when mixed and in combination as the fully associated L-tryptophan synthetase a 2 complex they catalysed a reaction which is the sum of (0 and (h) but in which indole (40)—a product of (0 and a substrate for ( )—was not detectable as a free intermediate. Thus indole (40), although it can enter the pathway between indoleglycerol-3-phos-phate (38) and L-tryptophan (3) either as a growth supplement or an accumulated product, may not be a true physiological intermediate. Yanofsky and his collaborators have observed that the full a/ 2 complex has an enhanced catalytic activity relative to the component a and P2 sub-units and that there is a concomitant gain in specificity for the reaction (ii). Thus the a 2a complex was 100 times more active than the a sub-unit in reaction (i) and 30 times more active than the 2 sub-unit in reaction (ii). In addition these workers observed that the intermediate 0. 2 complex was about one half as active per p2 sub-unit as the tetrameric complex and they concluded that each a unit contributed equally to each of the two identical active sites in the full a 2 enzyme. During the process of synthesis of the complex it has been assumed that the /S chains dimerise rapidly and then become associated with the a chains to give the tetrameric species. 

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