Title: A Thermally Induced Reversible Conformational Transition of the Tryptophan Synthase Subunit Probed by the Spectroscopic Properties of Pyridoxal Phosphate and by Enzymatic Activity
Abstract: A reversible thermally induced conformational transition of the β subunit of tryptophan synthase from Salmonella typhimurium has been detected by use of the pyridoxal 5′-phosphate coenzyme as a spectroscopic probe. Increasing the temperature converts the major form of pyridoxal 5′-phosphate bound to the β subunit from a ketoenamine species with at 410 nm to a enolimine species with at 336 nm (T = 43°C) and results in loss of the circular dichroism signal at 410 nm and of fluorescence emission at 510 nm. The results indicate that increasing the temperature favors a conformer of the enzyme that binds pyridoxal 5′-phosphate in a more nonpolar environment and leads to loss of asymmetric pyridoxal 5′-phosphate binding. The internal aldimine between pyridoxal 5′-phosphate and the -amino group of lysine 87 is not disrupted by increased temperature because sodium borohydride treatment of the enzyme at either 15 or 60°C results in covalent attachment of [4′-3H]pyridoxal 5′-phosphate. The thermal transition of the β subunit below 60°C produces reversible thermal inactivation (T = 52°C) and occurs at a much lower temperature than the major reversible unfolding at 80°C (Remeta, D. P., Miles, E. W., and Ginsburg, A.(1995) Pure Appl. Chem. 67, 1859-1866). Our new results indicate that the 410 nm absorbing species of pyridoxal 5′-phosphate is the catalytically active form of the cofactor in the β subunit and that the low temperature reversible conformational transition disturbs the active site and causes loss of catalytic activity. A reversible thermally induced conformational transition of the β subunit of tryptophan synthase from Salmonella typhimurium has been detected by use of the pyridoxal 5′-phosphate coenzyme as a spectroscopic probe. Increasing the temperature converts the major form of pyridoxal 5′-phosphate bound to the β subunit from a ketoenamine species with at 410 nm to a enolimine species with at 336 nm (T = 43°C) and results in loss of the circular dichroism signal at 410 nm and of fluorescence emission at 510 nm. The results indicate that increasing the temperature favors a conformer of the enzyme that binds pyridoxal 5′-phosphate in a more nonpolar environment and leads to loss of asymmetric pyridoxal 5′-phosphate binding. The internal aldimine between pyridoxal 5′-phosphate and the -amino group of lysine 87 is not disrupted by increased temperature because sodium borohydride treatment of the enzyme at either 15 or 60°C results in covalent attachment of [4′-3H]pyridoxal 5′-phosphate. The thermal transition of the β subunit below 60°C produces reversible thermal inactivation (T = 52°C) and occurs at a much lower temperature than the major reversible unfolding at 80°C (Remeta, D. P., Miles, E. W., and Ginsburg, A.(1995) Pure Appl. Chem. 67, 1859-1866). Our new results indicate that the 410 nm absorbing species of pyridoxal 5′-phosphate is the catalytically active form of the cofactor in the β subunit and that the low temperature reversible conformational transition disturbs the active site and causes loss of catalytic activity.