PARAMAGNETIC INTERMEDIATE IN REDUCTION OF OXYGEN BY REDUCED LACCASE
PARAMAGNETIC INTERMEDIATE IN REDUCTION OF OXYGEN BY REDUCED LACCASE
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DOI：
10.1016/0014-5793(76)81016-2
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发表时间：
1976-01-01
期刊：
影响因子：
3.5
通讯作者：
VANNGARD, T
中科院分区：
文献类型：
--
作者：
AASA, R;BRANDEN, R;VANNGARD, T
在乳糖酶的催化反应中，分子氧的四电子还原，像细胞色素c氧化酶一样，提出了很大的机理问题[1，2]。特别是对于乳糖酶，涉及四个连续的单电子步骤的机制似乎不太可能，因为酶的氧化还原位点的高氧化还原电位[3]将导致非常高的正自由能变化以形成超氧自由基。这一事实，再加上酶中存在一个协同的双电子受体，导致了这样的建议[2]，即还原乳糖酶与氧的反应包括连续的双电子步骤。先前已经提出了还原乳糖酶与氧反应中间体的光谱证据[4]。初步提出该物种是与2型Cu 2 '结合的HZ 02（或其离子之一）。然而，最近对树木和真菌乳糖酶的实验[5]表明，当完全还原的酶与氧气反应时，1型铜和双电子受体都迅速再氧化，而2型铜保持还原状态。如此形成的中间体具有与先前描述的类似的光学性质[4]。在77 K下的epr谱显示仅存在1型Cu”。由于O2是偶电子体系，三个电子已从还原酶位点转移，因此另一电子顺磁共振信号的缺失与动力学结果不一致。这促使了在较低温度下的epr研究，导致了顺磁性间的发现。
The four-electron reduction of molecular oxygen in the catalytic reaction of lactase, like that of cytochrome c oxidase, poses great mechanistic problems [1, 2]. With lactase, in particular, a mechanism involving four consecutive one-electron steps would appear unlikely, since the high oxidation-reduction potentials of the redox sites of the enzymes [3] would lead to a very high, positive free-energy change for the formation of the superoxide radical. This fact, together with the presence of a co-operative twoelectron acceptor in the enzyme, has led to the suggestion [2] that the reaction of reduced lactase with oxygen comprises consecutive two-electron steps. Spectroscopic evidence for an intermediate in the reaction of reduced lactase with oxygen has been presented earlier [4]. It was tentatively proposed that this species is HZ02 (or one of its ions) bound to Type 2 Cu2’. However, recent experiments [5] with tree as well as fungal lactase indicate, that when the fully reduced enzymes react with oxygen, both the Type 1 copper and the two-electron acceptor become rapidly re-oxidized, while the Type 2 copper remains reduced. The intermediate so formed has optical properties similar to that described previously [4]. The epr spectrum at 77 K showed only the presence of Type 1 Cu”. As 02 is an even-electron system and three electrons had been transferred from reduced enzyme-sites, the absence of another epr signal appeared inconsistent with the kinetic results. This prompted an epr study at lower temperatures, leading to the discovery of the paramagnetic inter-