A molecular design that stabilizes active state in bacterial allosteric L-lactate dehydrogenases
作者:K. Arai、J. Ichikawa、S. Nonaka、A. Miyanaga、H. Uchikoba、S. Fushinobu、H. Taguchi
DOI:10.1093/jb/mvr100
日期:2011.11.1
l-Lactate dehydrogenase (l-LDH) of Lactobacillus casei (LCLDH) is a typical bacterial allosteric l-LDH that requires fructose 1,6-bisphosphate (FBP) for its enzyme activity. A mutant LCLDH was designed to introduce an inter-subunit salt bridge network at the Q-axis subunit interface, mimicking Lactobacillus pentosus non-allosteric l-LDH (LPLDH). The mutant LCLDH exhibited high catalytic activity with hyperbolic pyruvate saturation curves independently of FBP, and virtually the equivalent Km and Vm values at pH 5.0 to those of the fully activated wild-type enzyme with FBP, although the Km value was slightly improved with FBP or Mn2+ at pH 7.0. The mutant enzyme exhibited a markedly higher apparent denaturating temperature (T1/2) than the wild-type enzyme in the presence of FBP, but showed an even lower T1/2 without FBP, where it exhibited higher activation enthalpy of inactivation (ΔH‡). This result is consistent with the fact that the active state is more unstable than the inactive state in allosteric equilibrium of LCLDH. The LPLDH-like network appears to be conserved in many bacterial non-allosteric l-LDHs and dimeric l-malate dehydrogenases, and thus to be a key for the functional divergence of bacterial l-LDHs during evolution.
干酪乳杆菌(LCLDH)的l-乳酸脱氢酶(l-LDH)是典型的细菌异位l-LDH,其酶活性需要1,6-二磷酸果糖(FBP)。我们设计了一种突变体 LCLDH,在 Q 轴亚基界面上引入了一个亚基间盐桥网络,以模仿戊糖乳杆菌非异位l-LDH(LPLDH)。突变体 LCLDH 表现出很高的催化活性,丙酮酸饱和曲线呈双曲线状,与 FBP 无关,在 pH 5.0 时的 Km 值和 Vm 值几乎与 FBP 完全激活的野生型酶相当,但在 pH 7.0 时,Km 值在 FBP 或 Mn2+ 的作用下略有提高。在有 FBP 的情况下,突变体酶的表观变性温度(T1/2)明显高于野生型酶,但在没有 FBP 的情况下,突变体酶的表观变性温度(T1/2)甚至更低,它表现出更高的活化失活焓(ΔH‡)。这一结果与 LCLDH 异位平衡中活性状态比非活性状态更不稳定这一事实相一致。LPLDH 样网络似乎在许多细菌非异位l-LDHs和二聚l-苹果酸脱氢酶中是保守的,因此是细菌l-LDHs在进化过程中功能分化的关键。