Effects of Pressure and Osmolytes on the Allosteric Equilibria of Salmonella typhimurium Tryptophan Synthase

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• 作者：Robert S. Phillips ; Alexandre Kim Wang ; Stephane Marchal ; Reinhard Lange
• 刊名：Biochemistry
• 出版年：2012
• 出版时间：November 20, 2012
• 年：2012
• 卷：51
• 期：46
• 页码：9354-9363
• 全文大小：483K
• 年卷期：v.51,no.46(November 20, 2012)
• ISSN：1520-4995

文摘

Osmolytes are common constituents of bacteria that may be produced or accumulate at high concentrations, up to 1 M, when cells are subjected to stresses like ionic strength and temperature. However, the effects of osmolytes on the allosteric properties of bacterial enzymes have rarely been examined. We have studied the effects of osmolytes and hydrostatic pressure on the allosteric equilibria of Salmonella typhimurium tryptophan (Trp) synthase. Trp synthase is a well-studied multienzyme complex with activity tightly regulated by allosteric interactions between the 伪- and 尾-subunits. Trp synthase activity is affected by a wide range of physical parameters, including monovalent cations, pH, ligands, solvents, and hydrostatic pressure. Osmolytes, including betaine, taurine, sucrose, and polyethylene glycol, activate Trp synthase 2鈥?-fold in the absence of monovalent cations, indicating that osmolytes can stabilize the active closed conformation. However, in the presence of monovalent cations, osmolytes have only minor effects on activity and allosteric equilibria, but 1 M betaine stabilizes the Trp synthase鈥揝er鈥搃ndoline complex against apparent pressure-induced subunit dissociation. Na⁺ and K⁺ are more effective at shifting the 伪-aminoacrylate鈥搃ndoline quinonoid equilibrium toward the quinonoid side, with a K_Q of 8鈥?0, than NH₄⁺(K_Q 2). Furthermore, pressure-jump experiments show that the mechanism of indoline reaction to form a quinonoid complex may be different for the NH₄⁺ enzyme than the Na⁺ and K⁺ forms. These results show that osmolytes have subtle but significant effects on the allosteric properties of Trp synthase, and these effects may be important in vivo.