Matthews R G, Baugh C M
Biochemistry. 1980 May 13;19(10):2040-5. doi: 10.1021/bi00551a005.
Dihydrofolate and dihydropteroylpolyglutamates inhibit pig liver methylenetetrahydrofolate reductase. In all cases the inhibition is linearly competitive with respect to methylenetetrahydrofolate. The Ki values decrease with each additional glutamyl residue from one to six, from a value of 6.5 microM for dihydrofolate to 0.013 microM for dihydropteroylhexaglutamate. Dihydropteroylheptaglutamate has a Ki of 0.065 microM. These data indicate a free energy of binding of approximately 0.75 kcal/mol for each of the five terminal glutamyl residues in dihydropteroylhexaglutamate. Methylenetetrahydropteroylpolyglutamates are substrates for the enzyme, and the increased free energy of binding is reflected in increased values for Vmax/Km with polyglutamate substrates. Vmax is increased 1.76-fold on going from the mono- to the diglutamate substrate; additional glutamyl residues lead to decreases in Km values for methylenetetrahydropteroylpolyglutamates. Our results suggest that the in vivo activity of methylenetetrahydrofolate reductase may also be sensitive to fluctuations in the ratio of methylenetetrahydropteroylpolyglutamates to dihydropteroylpolyglutamates and that this ratio may be important in determining the relative fluxes of methylenetetrahydropteroylpolyglutamates into the pathways leading to thymidylate biosynthesis and methionine regeneration.
二氢叶酸和二氢蝶酰多聚谷氨酸抑制猪肝亚甲基四氢叶酸还原酶。在所有情况下，这种抑制作用相对于亚甲基四氢叶酸呈线性竞争关系。从一到六个额外的谷氨酰残基，抑制常数（Ki值）逐渐降低，从二氢叶酸的6.5微摩尔降至二氢蝶酰六聚谷氨酸的0.013微摩尔。二氢蝶酰七聚谷氨酸的Ki值为0.065微摩尔。这些数据表明，二氢蝶酰六聚谷氨酸中五个末端谷氨酰残基每个的结合自由能约为0.75千卡/摩尔。亚甲基四氢蝶酰多聚谷氨酸是该酶的底物，结合自由能的增加反映在多聚谷氨酸底物的Vmax/Km值增加。从单谷氨酸底物到双谷氨酸底物，Vmax增加了1.76倍；额外的谷氨酰残基导致亚甲基四氢蝶酰多聚谷氨酸的Km值降低。我们的结果表明，亚甲基四氢叶酸还原酶的体内活性可能也对亚甲基四氢蝶酰多聚谷氨酸与二氢蝶酰多聚谷氨酸比例的波动敏感，并且该比例在确定亚甲基四氢蝶酰多聚谷氨酸进入胸苷酸生物合成和甲硫氨酸再生途径的相对通量方面可能很重要。
