Muraleedharan Madhu Nair, Zouraris Dimitrios, Karantonis Antonis, Topakas Evangelos, Sandgren Mats, Rova Ulrika, Christakopoulos Paul, Karnaouri Anthi
1Biochemical Process Engineering, Chemical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, Luleå, Sweden.
2Laboratory of Physical Chemistry and Applied Electrochemistry, School of Chemical Engineering, National Technical University of Athens, Athens, Greece.
Biotechnol Biofuels. 2018 Oct 28;11:296. doi: 10.1186/s13068-018-1294-6. eCollection 2018.
Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that oxidatively cleave recalcitrant lignocellulose in the presence of oxygen or hydrogen peroxide as co-substrate and a reducing agent as electron donor. One of the possible systems that provide electrons to the LPMOs active site and promote the polysaccharide degradation involves the mediation of phenolic agents, such as lignin, low-molecular-weight lignin-derived compounds and other plant phenols. In the present work, the interaction of the bulk insoluble lignin fraction extracted from pretreated biomass with LPMOs and the ability to provide electrons to the active site of the enzymes is studied.
The catalytic efficiency of three LPMOs, namely LPMO9 with C1/C4 regioselectivity, LPMO9D which is a C1 active LPMO and LPMO9C which is a C4 LPMO, was evaluated in the presence of different lignins. It was correlated with the physicochemical and structural properties of lignins, such as the molecular weight and the composition of aromatic and aliphatic hydroxyl groups. Moreover, the redox potential of lignins was determined with the use of large amplitude Fourier Transform alternating current cyclic voltammetry method and compared to the formal potential of the Cu (II) center in the active site of the LPMOs, providing more information about the lignin-LPMO interaction. The results demonstrated the existence of low-molecular weight lignin-derived compounds that are diffused in the reaction medium, which are able to reduce the enzyme active site and subsequently utilize additional electrons from the insoluble lignin fraction to promote the LPMO oxidative activity. Regarding the bulk lignin fractions, those isolated from the organosolv pretreated materials served as the best candidates in supplying electrons to the soluble compounds and, finally, to the enzymes. This difference, based on biomass pretreatment, was also demonstrated by the activity of LPMOs on natural substrates in the presence and absence of ascorbic acid as additional reducing agent.
Lignins can support the action of LPMOs and serve indirectly as electron donors through low-molecular-weight soluble compounds. This ability depends on their physicochemical and structural properties and is related to the biomass source and pretreatment method.
裂解多糖单加氧酶（LPMOs）是一类依赖铜的酶，在氧气或过氧化氢作为共底物以及还原剂作为电子供体的存在下，可氧化裂解难降解的木质纤维素。一种可能为LPMOs活性位点提供电子并促进多糖降解的系统涉及酚类试剂的介导作用，如木质素、低分子量木质素衍生化合物及其他植物酚类。在本研究中，对从预处理生物质中提取的大量不溶性木质素部分与LPMOs的相互作用以及向酶活性位点提供电子的能力进行了研究。
在不同木质素存在的情况下，评估了三种LPMOs的催化效率，即具有C1/C4区域选择性的LPMO9、C1活性LPMO9D和C4 LPMO9C。其与木质素的物理化学和结构性质相关，如分子量以及芳香族和脂肪族羟基的组成。此外，利用大振幅傅里叶变换交流循环伏安法测定了木质素的氧化还原电位，并与LPMOs活性位点中铜（II）中心的形式电位进行比较，从而提供了更多关于木质素-LPMO相互作用的信息。结果表明，反应介质中存在扩散的低分子量木质素衍生化合物，这些化合物能够还原酶活性位点，并随后利用来自不溶性木质素部分的额外电子来促进LPMO的氧化活性。对于大量木质素部分，从有机溶剂预处理材料中分离出的那些木质素是向可溶性化合物以及最终向酶提供电子的最佳候选者。基于生物质预处理的这种差异，在有无抗坏血酸作为额外还原剂的情况下，LPMOs对天然底物的活性也得到了证明。
木质素可支持LPMOs的作用，并通过低分子量可溶性化合物间接作为电子供体。这种能力取决于它们的物理化学和结构性质，并且与生物质来源和预处理方法有关。
