Tryptophan desmolase

The conversion of indole to tryptophan has been much more extensively studied. This is brought about by direct reaction of indole and serine under the influence of the enzyme tryptophan desmolase (better named tryptophan synthetase) (302, 853, 854) which requires p5U-idoxal phosphate as a coenzyme (890). The enzyme has been obtained in the cell-free state (890) and partially purified (965) and its genetics in Neurospora studied in detail (966). Zinc appears to play some part in tryptophan desmolase formation or function (628). 

Yanofsky, C. 1952. The effect of gene changes on tryptophan desmolase formation. Proc. Nal. Acad. Sci. U.S.A. 38, 215-226. 

L-Tryptophan synthase, tryptophan desmolase, L-serine hydro-lyase (adding imhleglycerol-phos-phate) (EC 4.2.1.20) the enzyme catalysing the synthesis of L-tryptophan from L-serine and indole 3-glycerol phosphate. T.s. from E. coli (M, 149,000) and other prokaryotes has 02 subunit composition. The enzyme separates easily into monomeric subunit a (also called protein B, M, 29,000) and dimeric subunit 02 (also called protein B Af, of dimer 90,000) when eluted from DEAE cellulose with a sodium chloride gradient. The separated subunits catalyse partial reactions of L-tryptophan synthesis ... 

The final step in the biosynthesis of tryptophan is the condensation of indole with serine, catalyzed by tryptophan desmolase (tryptophan S3rnthetase). This enzyme requires pyridoxal phosphate (III). 

Tryptophan Degradation. Tryptophan desmolase catalyzes an essentially irreversible reaction. The formation of indole from tryptophan is catalyzed by a very similar but quite distinct enzyme, tryptophanase. ... 

Siibstitution. A modification of the jS-elimination reaction may be substitution of a new group. This has been proposed as the mechanism of action of tryptophan desmolase, in which indole is substituted for the OH of serine. This reaction was found to proceed to a measurable extent in model reactions, in spite of the competing 8-elimination reactions of both serine and tryptophan and other side reactions of indole compounds. Additional substitution reactions of biological significance are the formation of cystathionine from homocysteine and serine and the formation of (Sf-methylcysteine from methyl mercaptan and serine. These reactions are catalyzed by enzymes that require pyridoxal phosphate as a cofactor. 

The enzyme which catalyzes the formation of tryptophan from indole and serine has been named tryptophan desmolase. A cell-free preparation has been obtained from Neurospora mycelium which catalyzes this reaction. Pyridoxal phosphate has been found to be a necessary cofactor. The enzyme has been partially purified by Yanofsky. He found the optimum activity to be at pH 7.8, and confirmed the necessity of pyridoxal phosphate as a coenzyme. Effective inhibitors are Co++, Zn++, CN, hydroxylamine, and tryptophan. 

Yanofsky, C. (1952) Tryptophan desmolase of Neurospora— partial purification and properties. J. Biol. Chem. 194, 279-286. 

The final step in the formation of tryptophan is the condensation of indole with serine (Fig. 15, reaction 4). This reaction was demonstrated by Tatum and Bonner ( S4) in Neurospora. The enzyme catalyzing this reaction has been prepared by a number of investigators 2S5-2S7). It was first named tryptophan desmolase 288), and the name indole-serine ligase has also been suggested 286). At present tr3rptophan synthetase is favored. 

When we come to tryptophan, which lies after shikimic acid, anthranilic acid and indole are found to be intermediates in the synthesis. The condensation of indole with serine to give tryptophan is brought about by the action of a specific enzyme, indole-ligase or tiyptophan desmolase, whose coenzyme is pytidoxal phosphate (this enzyme is not to be confused with tryptophanase which splits tryptophan into indole and pyrwoic acid). 

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