Monzingo A F, Breksa A, Ernst S, Appling D R, Robertus J D
Department of Chemistry and Biochemistry, University of Texas at Austin, 78712, USA.
Protein Sci. 2000 Jul;9(7):1374-81. doi: 10.1110/ps.9.7.1374.
Eucaryotes possess one or more NADP-dependent methylene-THF dehydrogenases as part of multifunctional enzymes. In addition, yeast expresses an unusual monofunctional NAD-dependent enzyme, yMTD. We report X-ray structures for the apoenzyme and its complex with NAD+ at 2.8 and 3.0 A resolution, respectively. The protein fold resembles that seen for the human and Escherichia coli dehydrogenase/cyclohydrolase bifunctional enzymes. The enzyme has two prominent domains, with the active site cleft between them. yMTD has a noncanonical NAD-binding domain that has two inserted strands compared with the NADP-binding domains of the bifunctional enzymes. This insert precludes yMTD from dimerizing in the same way as the bifunctional enzymes. yMTD functions as a dimer, but the mode of dimerization is novel. It does not appear that the difference in dimerization accounts for the difference in cofactor specificity or for the loss of cyclohydrolase activity. These functional differences are probably accounted for by minor differences within the tertiary structure of the active site of the monomeric protein.
真核生物拥有一种或多种依赖NADP的亚甲基四氢叶酸脱氢酶，作为多功能酶的一部分。此外，酵母表达一种不寻常的单功能依赖NAD的酶，即yMTD。我们分别报告了该脱辅基酶及其与NAD+复合物的X射线结构，分辨率分别为2.8 Å和3.0 Å。该蛋白质折叠结构与人类和大肠杆菌的脱氢酶/环水解酶双功能酶相似。该酶有两个突出的结构域，它们之间有活性位点裂缝。yMTD有一个非典型的NAD结合结构域，与双功能酶的NADP结合结构域相比，有两条插入链。这种插入阻止yMTD以与双功能酶相同的方式二聚化。yMTD以二聚体形式发挥作用，但二聚化模式是新颖的。似乎二聚化的差异并不能解释辅因子特异性的差异或环水解酶活性的丧失。这些功能差异可能是由单体蛋白质活性位点三级结构内的微小差异造成的。
