Hatfield Ronald, Ralph John, Grabber John H
USDA-Agricultural Research Service, US Dairy Forage Research Center, Madison, WI 53706, USA.
Planta. 2008 Nov;228(6):919-28. doi: 10.1007/s00425-008-0791-4. Epub 2008 Jul 25.
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.
禾本科木质素与其他类型木质素的区别在于，其含有相对大量的对香豆酸（pCA），该酸酰化木质素亚基的C-9位。就成熟玉米（Zea mays L.）茎而言，pCA占二氧六环可溶性酶解木质素的15 - 18%。pCA的主要部分特异性地附着于紫丁香基残基上。对分离出的玉米细胞壁过氧化物酶的研究表明，在过氧化氢存在下，pCA很容易发生自由基偶联，而芥子醇自由基偶联则进行得较慢。对玉米细胞壁过氧化物酶的分析未发现能像在其他植物中那样导致优先掺入芥子醇的特定酶。向含有过氧化物酶、芥子醇和过氧化氢的反应混合物中添加阿魏酸乙酯、对香豆酸甲酯或芥子醇对香豆酸共轭物，可使芥子醇自由基偶联速率提高10 - 20倍。基于光谱分析，似乎pCA和阿魏酸自由基迅速形成，但该自由基很容易转移到芥子醇上。新形成的芥子醇自由基会发生偶联和交叉偶联反应。然而，芥子醇自由基不会与pCA自由基发生交叉偶联。只要过氧化氢有限，pCA就保持未偶联状态。阿魏酸在自由基转移方面具有类似的反应模式，不过它们似乎比pCA在反应混合物中更容易发生交叉偶联。pCA的作用可能是在内部提供一种自由基转移机制，以优化芥子醇自由基偶联到正在生长的木质素聚合物中。一些pCA与芥子醇的附着确保了自由基转移机制定位于芥子醇正在掺入木质素的区域。
