Zwahlen Jacque, Kolappan Subramaniapillai, Zhou Rong, Kisker Caroline, Tonge Peter J
Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, USA.
Biochemistry. 2007 Jan 30;46(4):954-64. doi: 10.1021/bi060852x.
MbtI (rv2386c) from Mycobacterium tuberculosis catalyzes the initial transformation in mycobactin biosynthesis by converting chorismate to salicylate. We report here the structure of MbtI at 2.5 A resolution and demonstrate that isochorismate is a kinetically competent intermediate in the synthesis of salicylate from chorismate. At pH values below 7.5 isochorismate is the dominant product while above this pH value the enzyme converts chorismate to salicylate without the accumulation of isochorismate in solution. The salicylate and isochorismate synthase activities of MbtI are Mg2+-dependent, and in the absence of Mg2+ MbtI has a promiscuous chorismate mutase activity similar to that of the isochorismate pyruvate lyase, PchB, from Pseudomonas aeruginosa. MbtI is part of a larger family of chorismate-binding enzymes descended from a common ancestor (the MST family), that includes the isochorismate synthases and anthranilate synthases. The lack of active site residues unique to pyruvate eliminating members of this family, combined with the observed chorismate mutase activity, suggests that MbtI may exploit a sigmatropic pyruvate elimination mechanism similar to that proposed for PchB. Using a combination of structural, kinetic, and sequence based studies we propose a mechanism for MbtI applicable to all members of the MST enzyme family.
结核分枝杆菌的MbtI（rv2386c）通过将分支酸转化为水杨酸酯催化分枝杆菌酸生物合成中的初始转化反应。我们在此报告了分辨率为2.5埃的MbtI结构，并证明异分支酸是从分支酸合成水杨酸酯过程中的动力学活性中间体。在pH值低于7.5时，异分支酸是主要产物，而在该pH值以上，该酶将分支酸转化为水杨酸酯，溶液中不会积累异分支酸。MbtI的水杨酸酯和异分支酸合酶活性依赖于Mg2+，并且在没有Mg2+的情况下，MbtI具有与铜绿假单胞菌的异分支酸丙酮酸裂解酶PchB类似的混杂分支酸变位酶活性。MbtI是一个源自共同祖先的较大分支酸结合酶家族（MST家族）的一部分，该家族包括异分支酸合酶和邻氨基苯甲酸合酶。该家族中消除丙酮酸的成员缺乏独特的活性位点残基，再加上观察到的分支酸变位酶活性，表明MbtI可能利用了类似于为PchB提出的σ迁移丙酮酸消除机制。通过结合结构、动力学和基于序列的研究，我们提出了一种适用于MST酶家族所有成员的MbtI机制。
