A potential role for sinapyl p-coumarate as a radical transfer mechanism in grass lignin formation
A potential role for sinapyl p-coumarate as a radical transfer mechanism in grass lignin formation
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DOI：
10.1007/s00425-008-0791-4
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发表时间：
2008-11-01
期刊：
影响因子：
4.3
通讯作者：
Grabber, John H.
中科院分区：
文献类型：
--
作者：
Hatfield, Ronald;Ralph, John;Grabber, John H.
草木质素通过含有相对大量的酰化木质素亚基的C-9位置的对香豆酸（pCA）而与其它木质素类型区分开。在成熟玉米（Zea mays L.）茎中，pCA构成15-18%的二氧六环可溶性酶木质素。pCA的主要部分特异性地连接于双甘氨酰残基。与分离的玉米壁过氧化物酶的研究表明，在过氧化氢的存在下，pCA容易进行自由基偶联，而芥子醇自由基偶联进行得更慢。玉米壁过氧化物酶的分析并没有揭示特定的酶，这将导致在其他植物中看到的芥子醇的首选掺入。添加阿魏酸乙酯，甲基对香豆酸，或芥子对香豆酸共轭物的反应混合物含有过氧化物酶，芥子醇，和过氧化氢刺激芥子醇自由基偶联率的10-20倍。基于光谱分析，似乎pCA和阿魏酸根形成迅速，但自由基很容易转移到芥子醇。新形成的芥子醇自由基进行偶联和交叉偶联反应。然而，芥子醇自由基不与pCA自由基交叉偶联。只要过氧化氢是限制性的，pCA就保持未偶联。阿魏酸在自由基转移方面具有类似的反应模式，尽管它们似乎比pCA更容易在反应混合物中交叉偶联。pCA的作用可能是在内部提供自由基转移机制，用于优化芥子醇到生长的木质素聚合物中的自由基偶联。一些pCA与芥子醇的连接确保了在芥子醇被并入木质素的区域中自由基转移机制的定位。
Grass lignins are differentiated from other lignin types by containing relatively large amounts of p-coumaric acid (pCA) acylating the C-9 position of lignin subunits. In the case of a mature corn (Zea mays L.) stems, pCA constitutes 15-18% of a dioxane soluble enzyme lignin. The major portion of the pCA is specifically attached to syringyl residues. Studies with isolated corn wall peroxidases show that pCA readily undergoes radical coupling in the presence of hydrogen peroxide, whereas sinapyl alcohol radical coupling proceeds more slowly. Analysis of corn wall peroxidases did not reveal specific enzymes that would lead to the preferred incorporation of sinapyl alcohol as seen in other plants. The addition of ethyl ferulate, methyl p-coumarate, or sinapyl p-coumarate conjugates to a reaction mixture containing peroxidase, sinapyl alcohol, and hydrogen peroxide stimulated the rate of sinapyl alcohol radical coupling by 10-20-fold. Based on spectral analysis it appears that pCA and ferulate radicals form rapidly, but the radical is readily transferred to sinapyl alcohol. The newly formed sinapyl alcohol radicals undergo coupling and cross-coupling reactions. However, sinapyl alcohol radicals do not cross-couple with pCA radicals. As long as hydrogen peroxide is limiting pCA remains uncoupled. Ferulates have similar reaction patterns in terms of radical transfer though they appear to cross-couple in the reaction mixture more readily then pCA. The role of pCA may be to internally provide a radical transfer mechanism for optimizing radical coupling of sinapyl alcohol into the growing lignin polymer. Attachment of some pCA to sinapyl alcohol ensures localization of the radical transfer mechanism in areas where sinapyl alcohol is being incorporated into lignin.