Catalase is an antioxidant enzyme that breaks down hydrogen peroxide (H2O2) into molecular oxygen and water. In all monofunctional catalases the pathway that H2O2 takes to the catalytic centre is via the `main channel'. However, the structure of this channel differs in large-subunit and small-subunit catalases. In large-subunit catalases the channel is 15 Å longer and consists of two distinct parts, including a hydrophobic lower region near the heme and a hydrophilic upper region where multiple H2O2 routes are possible. Conserved glutamic acid and threonine residues are located near the intersection of these two regions. Mutations of these two residues in the Scytalidium thermophilum catalase had no significant effect on catalase activity. However, the secondary phenol oxidase activity was markedly altered, with kcat and kcat/Km values that were significantly increased in the five variants E484A, E484I, T188D, T188I and T188F. These variants also showed a lower affinity for inhibitors of oxidase activity than the wild-type enzyme and a higher affinity for phenolic substrates. Oxidation of heme b to heme d did not occur in most of the studied variants. Structural changes in solvent-chain integrity and channel architecture were also observed. In summary, modification of the main-channel gate glutamic acid and threonine residues has a greater influence on the secondary activity of the catalase enzyme, and the oxidation of heme b to heme d is predominantly inhibited by their conversion to aliphatic and aromatic residues.

中文翻译：

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研究主通道中的门残基如何影响嗜热镰刀菌过氧化氢酶的催化活性。


过氧化氢酶是一种抗氧化酶，可将过氧化氢 (H2O2) 分解成分子氧和水。在所有单功能过氧化氢酶中，H2O2 到达催化中心的途径是通过“主通道”。然而，该通道的结构在大亚基和小亚基过氧化氢酶中不同。在大亚基过氧化氢酶中，通道长 15 Å，由两个不同的部分组成，包括血红素附近的疏水性下部区域和可能存在多个 H2O2 路径的亲水性上部区域。保守的谷氨酸和苏氨酸残基位于这两个区域的交叉点附近。嗜热镰刀菌过氧化氢酶中这两个残基的突变对过氧化氢酶活性没有显着影响。然而，次级酚氧化酶活性显着改变，五个变体 E484A、E484I、T188D、T188I 和 T188F 中的 kcat 和 kcat/Km 值显着增加。这些变体还表现出比野生型酶对氧化酶活性抑制剂更低的亲和力以及对酚类底物更高的亲和力。在大多数研究的变体中，血红素 b 氧化为血红素 d 并未发生。还观察到溶剂链完整性和通道结构的结构变化。综上所述，主通道门谷氨酸和苏氨酸残基的修饰对过氧化氢酶的次级活性影响较大，血红素b向血红素d的氧化主要通过其向脂肪族和芳香族残基的转化而受到抑制。