One crucial step in processing the recalcitrant lignocellulosic biomass is the fast hydrolysis of natural cellulose to fermentable sugars that can be subsequently converted to biofuels and bio-based chemicals. Recent studies have shown that lytic polysaccharide monooxygenase (LPMOs) with auxiliary activity family 9 (AA9) are capable of efficiently depolymerizing the crystalline cellulose via regioselective oxidation reaction. Intriguingly, the catalysis by AA9 LPMOs requires reductant to provide electrons, and lignin and its phenolic derivatives can be oxidized, releasing reductant to activate the reaction. The activity of AA9 LPMOs can be enhanced by in-situ generation of H₂O₂ in the presence of O₂. Although scientific understanding of these enzymes remains somewhat unknown or controversial, structure modifications on AA9 LPMOs through protein engineering have emerged in recent years, which are prerequisite for their extensive applications in the development of cellulase-mediated lignocellulosic biorefinery processes. In this review, we critically comment on advances in studies for AA9 LPMOs, i.e., characteristic of AA9 LPMOs catalysis, external electron donors to AA9 LPMOs, especially the role of the oxidization of lignin and its derivatives, and AA9 LPMOs protein engineering as well as their extensive applications in the bioprocessing of lignocellulosic biomass. Perspectives are also highlighted for addressing the challenges.

处理顽固的木质纤维素生物质的一个关键步骤是将天然纤维素快速水解为可发酵的糖，然后将其转化为生物燃料和生物基化学品。最近的研究表明，具有辅助活性家族 9 (AA9) 的裂解多糖单加氧酶 (LPMOs) 能够通过区域选择性氧化反应有效地解聚结晶纤维素。有趣的是，AA9 LPMOs的催化需要还原剂提供电子，木质素及其酚类衍生物可以被氧化，释放还原剂来激活反应。AA9 LPMO 的活性可以通过在 O 2 存在下原位生成 H ₂ O ₂来_增强. 尽管对这些酶的科学认识仍然有些未知或存在争议，但近年来出现了通过蛋白质工程对 AA9 LPMO 进行的结构修饰，这是其在纤维素酶介导的木质纤维素生物精炼工艺开发中广泛应用的先决条件。在这篇综述中，我们批判性地评论了 AA9 LPMOs 的研究进展，即 AA9 LPMOs 催化的特性、AA9 LPMOs 的外部电子供体，特别是木质素及其衍生物的氧化作用、AA9 LPMOs 蛋白质工程以及它们在木质纤维素生物质的生物加工中的广泛应用。还强调了应对挑战的观点。