title: Reactions of O-acyl-L-serines with tryptophanase, tyrosine phenol-lyase, and tryptophan synthase.
abstract: The reactions of tryptophanase, tyrosine phenol-lyase, and tryptophan synthase with a new class of substrates, the O-acyl-L-serines, have been examined. A method for preparation of O-benzoyl-L-serine in high yield from tert.-butyloxycarbonyl (tBoc)-L-serine has been developed. Reaction of the cesium salt of tBoc-L-serine with benzyl bromide in dimethylformamide gives tBoc-L-serine benzyl ester in excellent yield. Acylation with benzoyl chloride and triethylamine in acetonitrile followed by hydrogenolysis with 10% palladium on carbon in trifluoroacetic acid gives O-benzoyl-L-serine, isolated as the hydrochloride salt. O-Benzoyl-L-serine is a good substrate for beta-elimination or beta-substitution reactions catalyzed by both tryptophanase and tyrosine phenol-lyase, with Vmax values 5- to 6-fold those of the physiological substrates and comparable to that of S-(o-nitrophenyl)-L-cysteine. Unexpectedly, O-acetyl-L-serine is a very poor substrate for these enzymes, with Vmax values about 5% of those of the physiological substrates. Both O-acyl-L-serines are poor substrates for tryptophan synthase, measured either by the synthesis of 5-fluoro-L-tryptophan from 5-fluoroindole and L-serine catalyzed by the intact alpha 2 beta 2 subunit or by the beta-elimination reaction catalyzed by the isolated beta 2 subunit. With all three enzymes, the elimination of benzoate appears to be irreversible. These results suggest that the binding energy from the aromatic ring of O-benzoyl-L-serine is used to lower the transition-state barrier for the elimination reactions catalyzed by tryptophanase and tyrosine phenol-lyase. Our findings support the suggestion (M. N. Kazarinoff and E. E. Snell (1980) J. Biol. Chem. 255, 6228-6233) that tryptophanase undergoes a conformational change during catalysis and suggest that tyrosine phenol-lyase also may undergo a conformational change during catalysis.
mesh_terms: D002384:Catalysis; D007700:Kinetics; D008190:Lyases / Q000378:metabolism*; D011487:Protein Conformation; D012694:Serine / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000378:metabolism; D014367:Tryptophan Synthase / Q000378:metabolism*; D014368:Tryptophanase / Q000378:metabolism*; D014447:Tyrosine Phenol-Lyase / Q000378:metabolism*
chemical_list: D012694:Serine; C052987:O-benzoylserine; D008190:Lyases; D014368:Tryptophanase; D014447:Tyrosine Phenol-Lyase; D014367:Tryptophan Synthase