Background

A cost-effective pretreatment and saccharification process is a necessary prerequisite for utilizing lignocellulosic biomass (LCB) in biofuel and biomaterials production. Utilizing a multifunctional enzyme with both pretreatment and saccharification functions in a single step for simultaneous biological pretreatment and saccharification process (SPS) will be a green method of low cost and high efficiency. Manganese peroxidase (MnP, EC 1.11.1.13), a well-known lignin-degrading peroxidase, is generally preferred for the biological pretreatment of biomass. However, exploring the role and performance of MnP in LCB conversion will promote the application of MnP for lignocellulose-based biorefineries.

Results

In this study, we explored the ability of an MnP from Moniliophthora roreri, MrMnP, in LCB degradation. With Mn2+ and H2O2, MrMnP decomposed 5.0 g/L carboxymethyl cellulose to 0.14 mM of reducing sugar with a conversion yield of 5.0 mg/g, including 40 μM cellobiose, 70 μM cellotriose, 20 μM cellotetraose, and 10 μM cellohexaose, and degraded 1.0 g/L mannohexaose to 0.33 μM mannose, 4.08 μM mannotriose, and 4.35 μM mannopentaose. Meanwhile, MrMnP decomposed 5.0 g/L lichenan to 0.85 mM of reducing sugar with a conversion yield of 30.6 mg/g, including 10 μM cellotriose, 20 μM cellotetraose, and 80 μM cellohexose independently of Mn2+ and H2O2. Moreover, the versatility of MrMnP in LCB deconstruction was further verified by decomposing locust bean gum and wheat bran into reducing sugars with a conversion yield of 54.4 mg/g and 29.5 mg/g, respectively, including oligosaccharides such as di- and tri-saccharides. The catalytic mechanism underlying MrMnP degraded lignocellulose was proposed as that with H2O2, MrMnP oxidizes Mn2+ to Mn3+. Subsequently, it forms a complex with malonate, facilitating the degradation of CMC and mannohexaose into reducing sugars. Without H2O2, MrMnP directly oxidizes malonate to hydroperoxyl acetic acid radical to form compound I, which then attacks the glucosidic bond of lichenan.

Conclusion

This study identified a new function of MrMnP in the hydrolysis of cellulose and hemicellulose, suggesting that MrMnP exhibits its versatility in the pretreatment and saccharification of LCB. The results will lead to an in-depth understanding of biocatalytic saccharification and contribute to forming new enzymatic systems for using lignocellulose resources to produce sustainable and economically viable products and the long-term development of biorefinery, thereby increasing the productivity of LCB as a green resource.