本研究通过使用密度泛函理论 B3LYP 方法和量子化学簇方法来研究锰过氧化氢酶(MnCat) KatB 催化 H2O2 发生歧化反应的机理. 与此前分子水平上的 Thermus thermophilus MnCat (TTC) 的研究相比, 本研究在 KatB高分辨率 X 射线晶体结构的基础上使用更现代的方法论和更大的化学模型作为研究对象. 使用大模型 (Large) 来研究KatB 的反应路径时, 第一个底物H2O2 通过 μ-η1:η1 配位方式发生 O—O 键的均裂, 它的能垒为 10.9 kcal/mol. 在O—O 键断裂的中间体中, 处于 Mn2(III,III) 氧化态的双锰簇形成了两个分别与双锰离子配位的末端氢氧根离子. 两个Mn(III)—OH– 中的一个末端氢氧根离子和第二配位层的一个酪氨酸通过氢键作用来稳定位于活性中心的第二层的第二个底物H2O2. 这个 H2O2 与金属未配位, 它首先经过一个能垒为 9.5 kcal/mol 的 PCET 过程被氧化为 HO2?. 随后,体系切换到三重态, 这个未配位的 HO2? 取代了与 Mn(II) 末端配位的产物水分子, 随后被氧化为 O2 的过程是自发进行的. 与TTC 类似的 μ?η2 -OH–/O2– 型过渡态的能垒很高.

The mechanism of the H2O2 disproportionation catalyzed by the manganese catalase (MnCat) KatB was studied using the hybrid density functional theory B3LYP and the quantum chemical cluster approach. Compared to the previous mechanistic study at the molecular level for the Thermus thermophilus MnCat (TTC), more modern methodology was used and larger models with increasing sizes were employed with the help of the high resolution X-ray structure. In the reaction pathway suggested for KatB using the Large chemical model, the O—O homolysis of the first substrate H2O2 occurs through a μ-η1:η1 coordination mode and requires a barrier of 10.9 kcal/mol. In the intermediate state of the bond cleavage, two hydroxides were formed as terminal ligands of the di-manganese cluster at the Mn2(III,III) oxidation state. One of the two Mn(III)—OH– moieties and a second-sphere tyrosine stabilize the second substrate H2O2 in the second-sphere of the active site via hydrogen bonding interactions. The H2O2, unbound to the metals, is first oxidized into HO2? through a proton-coupled electron transfer (PCET) step with a barrier of 9.5 kcal/mol. After the system switches to the triplet surface, the uncoordinated HO2? replaces the product water terminally bound to the Mn(II) and is then oxidized into O2 spontaneously. Transition states with structural similarities to those obtained for TTC, where μ?η2 -OH–/O2– groups play important roles, were found to be higher in energy.