Kirk T K, Tien M, Kersten P J, Mozuch M D, Kalyanaraman B
Biochem J. 1986 May 15;236(1):279-87. doi: 10.1042/bj2360279.
This study examined the ligninase-catalysed degradation of lignin model compounds representing the arylglycerol beta-aryl ether substructure, which is the dominant one in the lignin polymer. Three dimeric model compounds were used, all methoxylated in the 3- and 4-positions of the arylglycerol ring (ring A) and having various substituents in the beta-ether-linked aromatic ring (ring B), so that competing reactions involving both rings could be compared. Studies of the products formed and the time courses of their formation showed that these model compounds are oxidized by ligninase (+ H2O2 + O2) in both ring A and ring B. The major consequence with all three model compounds is oxidation of ring A, leading primarily to cleavage between C(alpha) and C(beta) (C(alpha) being proximal to ring A), and to a lesser extent to the oxidation of the C(alpha)-hydroxy group to a carbonyl group. Such C(alpha)-oxidation deactivates ring A, leaving only ring B for attack. Studies with C(alpha)-carbonyl model compounds corresponding to the three basic model compounds revealed that oxidation of ring B leads in part to dealkoxylations (i.e. to cleavage of the glycerol beta-aryl ether bond and to demethoxylations), but that these are minor reactions in the model compounds most closely related to lignin. Evidence is also given that another consequence of oxidation of ring B in the C(alpha)-carbonyl model compounds is formation of unstable cyclohexadienone ketals, which can decompose with elimination of the beta-ether-linked aromatic ring. The mechanisms proposed for the observed reactions involve initial formation of aryl cation radicals in either ring A or ring B. The cation radical intermediate from one of the C(alpha)-carbonyl model compounds was identified by e.s.r. spectroscopy. The mechanisms are based on earlier studies showing that ligninase acts by oxidizing appropriately substituted aromatic nuclei to aryl cation radicals [Kersten, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 2609-2612; Hammel, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 8348-8353].
本研究考察了木质素酶催化降解代表芳基甘油β-芳基醚亚结构的木质素模型化合物，该亚结构是木质素聚合物中的主要亚结构。使用了三种二聚体模型化合物，它们在芳基甘油环（环A）的3位和4位均被甲氧基化，并且在β-醚键连接的芳环（环B）上具有各种取代基，以便比较涉及两个环的竞争反应。对所形成产物及其形成时间进程的研究表明，这些模型化合物在环A和环B中均被木质素酶（+ H2O2 + O2）氧化。所有三种模型化合物的主要结果是环A的氧化，主要导致C（α）和C（β）之间的裂解（C（α）靠近环A），并且在较小程度上导致C（α）-羟基氧化为羰基。这种C（α）-氧化使环A失活，仅留下环B可供攻击。对与三种基本模型化合物相对应的C（α）-羰基模型化合物的研究表明，环B的氧化部分导致脱烷氧基化（即甘油β-芳基醚键的裂解和脱甲氧基化），但在与木质素最密切相关的模型化合物中，这些是次要反应。还给出了证据表明，C（α）-羰基模型化合物中环B氧化的另一个结果是形成不稳定的环己二烯酮缩酮，其可通过消除β-醚键连接的芳环而分解。针对观察到的反应提出的机制涉及在环A或环B中最初形成芳基阳离子自由基。通过电子自旋共振光谱法鉴定了其中一种C（α）-羰基模型化合物的阳离子自由基中间体。这些机制基于早期的研究，表明木质素酶通过将适当取代的芳核氧化为芳基阳离子自由基而起作用[Kersten, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 2609 - 2612; Hammel, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 8348 - 8353]。
