Abstract:

Lignin is a major component of lignocellulose, accounting for 15%-30% on a dry weight basis, with an annual yield estimated to be 20 billion tonnes. Lignin is a heterogenous aromatic polymer of phenylpropanoids linked by various C-C and C-O bonds. It is an integral component of the secondary cell wall from terrestrial plants, providing plants with rigidness and fending off microbial pathogens. The abundance and renewability of lignin has recently attracted ample interest in valorizing this readily available polymer. However, the complex nature of lignin presents a significant challenge for lignin breakdown and utilization, and at present the majority of lignin is simply burned as a fuel. Among the different methods, biological utilization of lignin has emerged as a highly attractive approach, since it proceeds under mild conditions and is generally considered environmentally friendly, especially considering that environmental sustainability is trending worldwide. This review comprises three major sections. First, we will summarize key enzymes that Nature has created to break down lignin, including laccase, manganese peroxidase, lignin peroxidase, dye-decolorizing peroxidase, and versatile peroxidase etc. Relevant enzymes and their catalytic mechanisms will also be briefly discussed. Second, we will review key reactions in priming and processing lignin derived aromatics before they enter microbial metabolic pathways: O-demethylation, hydroxylation, decarboxylation, and ring opening, as well as representative enzymes and their catalytic mechanisms. Finally, we will present engineering efforts toward biological valorization of lignin and lignin derived aromatics, which is largely driven by synthetic biology approaches. Biological valorization of lignin is undoubtedly a field full of potential, however its realization still faces several major hurdles, such as low conversion efficiency and long processing time. Nevertheless, as synthetic biology is developing rapidly, harnessing the power of genetic and metabolic engineering to improve the efficiency of lignin breakdown and utilization, microbial tolerance to toxic aromatics, and redox balance will certainly be a promising path forward, paving the way for industrial application in the near future.

Key words: lignin, biological degradation, biological utilization

摘要：

木质素是木质纤维素的一个主要成分，按干重计约占15%-30%，全球年产量在200亿吨。木质素是由苯丙烷单元通过多种不同的碳-碳和碳-氧键构成的一类芳香族高聚化合物，是高等陆生植物次生细胞壁的主要成分，赋予了植物的刚性和保护植物体免受微生物的入侵。由于木质素产量巨大、可再生，近些年全球对木质素利用的兴趣持续升高。但是木质素的成分复杂，无论是其降解还是后续的利用都充满了挑战，因此目前多用作燃料。在众多木质素降解利用的方法中，生物法反应条件温和、绿色环保，近些年在绿色可持续发展的大背景下受到广泛关注。本文首先总结了自然界中催化木质素降解的关键酶：漆酶、锰过氧化物酶、木质素过氧化物酶、染料脱色过氧化物酶、多功能过氧化物酶等，并简要介绍了其催化机制。其次，本文总结了生物利用木质素类芳香族化合物过程中涉及的四个主要反应：O-脱甲基、脱羧、羟基化和双加氧酶介导的开环反应，相关的酶和催化机制。最后，本文简要介绍了利用合成生物学手段构建细胞工厂实现木质素高值利用的案例。木质素的生物降解和利用是一个极具潜力的领域，但同时也存在诸多的挑战，例如转化效率低、反应时间长等。但本文作者相信随着合成生物学的迅猛发展，利用高效基因编辑和代谢工程改造提高关键酶的反应速率和代谢通路的效率、提高底盘细胞对有毒芳香族化合物的低抗能力、维持还原力的平衡等将有效提高木质素生物降解利用的效率，其工业应用也许在不久的将来就可能会实现。

关键词: 木质素, 生物降解, 生物利用