Beckham Gregg T, Johnson Christopher W, Karp Eric M, Salvachúa Davinia, Vardon Derek R
National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80403, United States.
National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80403, United States.
Curr Opin Biotechnol. 2016 Dec;42:40-53. doi: 10.1016/j.copbio.2016.02.030. Epub 2016 Mar 11.
Lignin is a primary component of lignocellulosic biomass that is an underutilized feedstock in the growing biofuels industry. Despite the fact that lignin depolymerization has long been studied, the intrinsic heterogeneity of lignin typically leads to heterogeneous streams of aromatic compounds, which in turn present significant technical challenges when attempting to produce lignin-derived chemicals where purity is often a concern. In Nature, microorganisms often encounter this same problem during biomass turnover wherein powerful oxidative enzymes produce heterogeneous slates of aromatics compounds. Some microbes have evolved metabolic pathways to convert these aromatic species via 'upper pathways' into central intermediates, which can then be funneled through 'lower pathways' into central carbon metabolism in a process we dubbed 'biological funneling'. This funneling approach offers a direct, biological solution to overcome heterogeneity problems in lignin valorization for the modern biorefinery. Coupled to targeted separations and downstream chemical catalysis, this concept offers the ability to produce a wide range of molecules from lignin. This perspective describes research opportunities and challenges ahead for this new field of research, which holds significant promise towards a biorefinery concept wherein polysaccharides and lignin are treated as equally valuable feedstocks. In particular, we discuss tailoring the lignin substrate for microbial utilization, host selection for biological funneling, ligninolytic enzyme-microbe synergy, metabolic engineering, expanding substrate specificity for biological funneling, and process integration, each of which presents key challenges. Ultimately, for biological solutions to lignin valorization to be viable, multiple questions in each of these areas will need to be addressed, making biological lignin valorization a multidisciplinary, co-design problem.
木质素是木质纤维素生物质的主要成分，而木质纤维素生物质是不断发展的生物燃料行业中未得到充分利用的原料。尽管长期以来一直在研究木质素解聚，但木质素固有的异质性通常会导致芳香族化合物的异质流，这反过来在试图生产通常关注纯度的木质素衍生化学品时带来重大技术挑战。在自然界中，微生物在生物质周转过程中经常遇到同样的问题，即强大的氧化酶会产生芳香族化合物的异质混合物。一些微生物已经进化出代谢途径，通过“上游途径”将这些芳香族物质转化为中心中间体，然后在我们称为“生物漏斗效应”的过程中通过“下游途径”导入中心碳代谢。这种漏斗效应方法为克服现代生物精炼厂中木质素增值过程中的异质性问题提供了一种直接的生物学解决方案。与有针对性的分离和下游化学催化相结合，这一概念提供了从木质素生产多种分子的能力。本观点描述了这一新兴研究领域未来的研究机会和挑战，该领域对生物精炼概念具有重大前景，即多糖和木质素被视为同等有价值的原料。特别是，我们讨论了为微生物利用定制木质素底物、生物漏斗效应的宿主选择、木质素分解酶 - 微生物协同作用、代谢工程、扩大生物漏斗效应的底物特异性以及过程整合，每一项都带来了关键挑战。最终，要使木质素增值的生物学解决方案可行，这些领域中的每一个都需要解决多个问题，从而使生物木质素增值成为一个多学科的协同设计问题。
