Kumar Rajeev, Wyman Charles E
Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California, Riverside, 1084 Columbia Avenue, Riverside, California, 92507; BioEnergy Science Center (BESC), Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831-6422.
Biotechnol Bioeng. 2014 Jul;111(7):1341-53. doi: 10.1002/bit.25218. Epub 2014 May 1.
Cellulase enzymes contribute a major fraction of the total cost for biological conversion of lignocellulosic biomass to fuels and chemicals. Although a several fold reduction in cellulase production costs and enhancement of cellulase activity and stability have been reported in recent years, sugar yields are still lower at low enzyme doses than desired commercially. We recently reported that hemicellulose xylan and its oligomers strongly inhibit cellulase and that supplementation of cellulase with xylanase and β-xylosidase would significantly reduce such inhibition. In this study, mannan polysaccharides and their enzymatically prepared hydrolyzates were discovered to be strongly inhibitory to fungal cellulase in cellulose conversion (>50% drop in % relative conversion), even at a small concentration of 0.1 g/L, and inhibition was much greater than experienced by other known inhibitors such as cellobiose, xylooligomers, and furfural. Furthermore, cellulase inhibition dramatically increased with heteromannan loading and mannan substitution with galactose side units. In general, enzymatically prepared hydrolyzates were less inhibitory than their respective mannan polysaccharides except highly substituted ones. Supplementation of cellulase with commercial accessory enzymes such as xylanase, pectinase, and β-glucosidase was effective in greatly relieving inhibition but only for less substituted heteromannans. However, cellulase supplementation with purified heteromannan specific enzymes relieved inhibition by these more substituted heteromannans as well, suggesting that commercial preparations need to have higher amounts of such activities to realize high sugar yields at the low enzyme protein loadings needed for low cost fuels production.
纤维素酶的成本在将木质纤维素生物质生物转化为燃料和化学品的总成本中占很大比例。尽管近年来已报道纤维素酶生产成本降低了几倍，且纤维素酶活性和稳定性有所提高，但在低酶剂量下，糖产量仍低于商业预期。我们最近报道，半纤维素木聚糖及其低聚物会强烈抑制纤维素酶，用木聚糖酶和β-木糖苷酶补充纤维素酶可显著降低这种抑制作用。在本研究中，发现甘露聚糖多糖及其酶解产物在纤维素转化过程中对真菌纤维素酶具有强烈抑制作用（相对转化率下降>50%），即使在低至0.1 g/L的浓度下也是如此，且抑制作用远大于其他已知抑制剂，如纤维二糖、木寡糖和糠醛。此外，随着杂甘露聚糖负载量的增加以及甘露聚糖被半乳糖侧链取代，纤维素酶抑制作用显著增强。一般来说，酶解产物的抑制作用比其相应的甘露聚糖多糖弱，除了高度取代的情况。用木聚糖酶、果胶酶和β-葡萄糖苷酶等商业辅助酶补充纤维素酶可有效缓解抑制作用，但仅适用于取代度较低的杂甘露聚糖。然而，用纯化的杂甘露聚糖特异性酶补充纤维素酶也能缓解这些取代度较高的杂甘露聚糖的抑制作用，这表明商业制剂需要含有更高量的此类活性，才能在低成本燃料生产所需的低酶蛋白负载量下实现高糖产量。
