Biorefining Research Institute, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada.
Biorefining Research Institute, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada; Universidade Tecnológica Federal do Paraná, Programa de Pós-Graduação em Engenharia Ambiental, Câmpus Londrina, CEP: 86036-370, Londrina, PR, Brazil.
Enzyme Microb Technol. 2021 Jun;147:109780. doi: 10.1016/j.enzmictec.2021.109780. Epub 2021 Mar 18.
Lignin is an abundant natural plant aromatic biopolymer containing various functional groups that can be exploited for activating lignin for potential commercial applications. Applications are hindered due to the presence of a high content of methyl/methoxyl groups that affects reactiveness. Various chemical and enzymatic approaches have been investigated to increase the functionality in transforming lignin. Among these is demethylation/demethoxylation, which increases the potential numbers of vicinal hydroxyl groups for applications as phenol-formaldehyde resins. Although the chemical route to lignin demethylation is well-studied, the biological route is still poorly explored. Bacteria and fungi have the ability to demethylate lignin and lignin-related compounds. Considering that appropriate microorganisms possess the biochemical machinery to demethylate lignin by cleaving O-methyl groups liberating methanol, and modify lignin by increasing the vicinal diol content that allows lignin to substitute for phenol in organic polymer syntheses. Certain bacteria through the actions of specific O-demethylases can modify various lignin-related compounds generating vicinal diols and liberating methanol or formaldehyde as end-products. The enzymes include: cytochrome P-aryl-O-demethylase, monooxygenase, veratrate 3-O-demethylase, DDVA O-demethylase (LigX; lignin-related biphenyl 5,5'-dehydrodivanillate (DDVA)), vanillate O-demethylase, syringate O-demethylase, and tetrahydrofolate-dependent-O-demethylase. Although, the fungal counterparts have not been investigated in depth as in bacteria, O-demethylases, nevertheless, have been reported in demethylating various lignin substrates providing evidence of a fungal enzyme system. Few fungi appear to have the ability to secrete O-demethylases. The fungi can mediate lignin demethylation enzymatically (laccase, lignin peroxidase, manganese peroxidase, O-demethylase), or non-enzymatically in brown-rot fungi through the Fenton reaction. This review discusses details on the aspects of microbial (bacterial and fungal) demethylation of lignins and lignin-model compounds and provides evidence of enzymes identified as specific O-demethylases involved in demethylation.
木质素是一种丰富的天然植物芳香性生物聚合物，含有各种官能团，可以被开发利用来激活木质素，以实现潜在的商业应用。然而，由于存在大量的甲基/甲氧基基团，这影响了其反应活性，因此其应用受到了限制。为了提高木质素的功能，人们已经研究了各种化学和酶法方法。其中包括脱甲基/脱甲氧基反应，这可以增加潜在的邻位羟基数量，从而将木质素应用于酚醛树脂。虽然木质素脱甲基的化学途径已经得到了很好的研究，但生物途径仍未得到充分探索。细菌和真菌具有脱甲基化木质素和木质素相关化合物的能力。考虑到合适的微生物具有通过切断 O-甲基基团释放甲醇的生物化学机制来脱甲基化木质素，并通过增加邻二醇含量来修饰木质素的能力，从而使木质素能够替代酚类在有机聚合物合成中的作用。某些细菌通过特定的 O-脱甲基酶的作用，可以修饰各种木质素相关化合物，生成邻二醇，并释放甲醇或甲醛作为终产物。这些酶包括：细胞色素 P-芳基-O-脱甲基酶、单加氧酶、藜芦醇 3-O-脱甲基酶、DDVA O-脱甲基酶（LigX；木质素相关联苯 5,5'-脱氢二香草酸（DDVA））、香草酸 O-脱甲基酶、丁香酸 O-脱甲基酶和四氢叶酸依赖性-O-脱甲基酶。尽管真菌对应物的研究不如细菌深入，但 O-脱甲基酶已被报道存在于各种木质素底物的脱甲基化中，这为真菌酶系统提供了证据。少数真菌似乎具有分泌 O-脱甲基酶的能力。真菌可以通过酶促（漆酶、木质素过氧化物酶、锰过氧化物酶、O-脱甲基酶）或通过 Fenton 反应在褐腐真菌中非酶促地介导木质素脱甲基化。本综述讨论了微生物（细菌和真菌）木质素和木质素模型化合物脱甲基的各个方面，并提供了被鉴定为参与脱甲基的特定 O-脱甲基酶的证据。
