Kamb A, Finer-Moore J, Calvert A H, Stroud R M
Department of Biochemistry and Biophysics, University of California, San Francisco 94143.
Biochemistry. 1992 Oct 20;31(41):9883-90. doi: 10.1021/bi00156a005.
Thymidylate synthase (TS) catalyzes the final step in the de novo synthesis of thymidine. In vivo TS binds a polyglutamyl cofactor, polyglutamyl methylenetetrahydrofolate (CH2-H4folate), which serves as a carbon donor. Glutamate residues on the cofactor contribute as much as 3.7 kcal to the interaction between the cofactor, substrate, and enzyme. Because many ligand/receptor interactions appear to be driven largely by hydrophobic forces, it is surprising that the addition of hydrophilic, soluble groups such as glutamates increases the affinity of the cofactor for TS. The structure of a polyglutamyl cofactor analog bound in ternary complex with deoxyuridine monophosphate (dUMP) and Escherichia coli TS reveals how the polyglutamyl moiety is positioned in TS and accounts in a qualitative way for the binding contributions of the different individual glutamate residues. The polyglutamyl moiety is not rigidly fixed by its interaction with the protein except for the first glutamate residue nearest the p-aminobenzoic acid ring of folate. Each additional glutamate is progressively more disordered than the previous one in the chain. The position of the second and third glutamate residues on the protein surface suggests that the polyglutamyl binding site could be utilized by a new family of inhibitors that might fill the binding area more effectively than polyglutamate.
胸苷酸合成酶（TS）催化胸苷从头合成的最后一步。在体内，TS结合一种多聚谷氨酰辅因子，即多聚谷氨酰亚甲基四氢叶酸（CH2-H4叶酸），它作为碳供体。辅因子上的谷氨酸残基对辅因子、底物和酶之间的相互作用贡献高达3.7千卡。由于许多配体/受体相互作用似乎主要由疏水作用力驱动，所以令人惊讶的是，添加亲水性、可溶性基团如谷氨酸会增加辅因子对TS的亲和力。一种与单磷酸脱氧尿苷（dUMP）和大肠杆菌TS形成三元复合物的多聚谷氨酰辅因子类似物的结构揭示了多聚谷氨酰部分在TS中的定位方式，并定性地解释了不同单个谷氨酸残基的结合贡献。除了最靠近叶酸对氨基苯甲酸环的第一个谷氨酸残基外，多聚谷氨酰部分与其与蛋白质的相互作用并非刚性固定。在链中，每一个额外的谷氨酸都比前一个谷氨酸更加无序。蛋白质表面上第二个和第三个谷氨酸残基的位置表明，多聚谷氨酰结合位点可能被一类新的抑制剂利用，这类抑制剂可能比多聚谷氨酸更有效地占据结合区域。
