木质纤维素生物预处理的现状潜力进展与挑战外文翻译Word格式文档下载.docx

1、尽管由于许多固有的局限性没有引起多少注意，生物预处理还是由于其自身的许多优势而存在很大潜力，包括更环保、耗能更少、反应产生抑制剂更少、副产物更少等。在白蚁和白腐菌方面不断取得的科技成果为实现这些利益，发展新一代生物预处理技术提供了理论依据。本文综述了以木质素降解酶为主的酶系统，描述了当前对微生物降解植物细胞壁的理解，对比了生物与化学的预处理过程。还对生物制浆的成果进行了总结，提供了一个未来生物预处理过程的发展方向。简介 获得可再生燃料和化学制剂的唯一方式是通过利用绿色植物吸收太阳能，再以有机碳源的形式存储起来。大自然还开发了各种途径以额外的最小输入能量来利用和回收这些植物材料。这样做，大自然能

2、够一直保持一个可持续发展的平衡的生态系统数百万年。如何利用木质纤维素的生物分解来进行生物燃料和生化生产是这些天然生物过程需要解决的主要障碍，他们往往最节能并且对环境产生的影响不大。随着化石燃料资源的衰退和对气候变化的担忧，发展生物质燃料和化学制剂显得愈发紧迫。例如， 到2022年，每年生产的360亿加仑可再生燃料中，生物燃料必须占到210亿加仑。未来生化和生物燃料发展的基础是生物质原料的供应。所有的类型中，木质纤维素、木质生物、作物残留物、草和藻类的生物质能含量是最丰富的。木质生物质是地球上最丰富的可再生生物资源，在地球上,每年可生产109 200吨，其中只有3%用于诸如造纸工业的非食品领域。

3、目前纤维素的消费量与谷物消费持平，是钢铁消费的3倍。为了既能将这些材料用于生产生物燃料又不与人类的粮食供应构成冲突，未来的生物炼制将以木质生物质原料为主。植物细胞壁（PCW）是存储能源和有机碳的主要材料。PCW的组成和结构决定了以它为原料来设计下游加工流程生产各种目标分子。植物由有序排列的有壁细胞组成。细胞壁中含有不同比例的混合纤维素（Ca.40%）、半纤维素（Ca.20 - 30%）和（Ca.20 - 30%）。纤维素是一种葡萄糖单元由-1,4-糖苷键联系在一起的线性聚合物。半纤维素是许多糖（木糖、甘露糖、半乳糖、阿拉伯糖、鼠李糖）单体的杂聚合物形成的随机非晶态结构。另一方面，木质素是由一个

4、包含三种DPH（对香豆醇、松柏醇、芥子醇）的大分子单体交叉链接组成。木质纤维素是一个紧凑的复杂结构。其中一部分含有复杂晶体，和多糖紧密连接成的层状超细纤维形成的稳定剂来防止它们被水解酶和其他外部因素分解 。以木材为例来解释其结构：通常，支持细胞死亡后的管腔可以作为水分运输的通道，其内层是一个成分未知的异构混合部分，内层外面的次生壁可以进一步分为S3, S2 ,S1三个子层，三个子层都是由纤维素超细纤维嵌入在不同的半纤维素和木质素的一个非晶体混合物中组成的。纤维素的浓度最高的是S2子层,并且向中间层依次减少。富含半纤维素的S3层是最靠近导管的。中层的木质素浓度最高，然而,由于次生壁要厚得多，所以

5、它包含了大部分的木质素（60 - 80%）。木质化是在初生壁发起的，邻近S1层中靠近形成层处进行纤维素沉积的细胞，然后形成于细胞间层和初生壁。在初生壁、结晶和无定形纤维素核心围绕着半纤维素聚合物。草类植物中，细胞间层、初生壁中含有更多的碳-碳键耦合形成的DPH，使之变成了一个有很多支链的聚合物。相比之下，-O-4耦合的DPH会在二级细胞壁中形成一个相对线性更强的聚合物。初生壁中高度支链化的木质素比次生壁中的线性木质素更能抑制细胞壁的降解。大多数生物过程集中于用糖作为能源和碳源通过发酵获得不同的产品。主要的糖单元是葡萄糖，在植物细胞壁结构中呈有界的共价纤维素聚合物。从物理化学加工中获得糖是一个重

6、大的生物精炼的瓶颈。锁在纤维素和半纤维素聚合物中的糖单位是用于发酵生产生物燃料和生物化学制剂的唯一能源和碳源。纤维素聚合物的生化分解一般是由称为木纤维质酵素的纤维素酶完成的。由极其复杂而且种类繁多的纤维素复合而成的木质纤维素，有专门用来抵御攻击的结构。木质素和半纤维素的复杂的结构和疏水性的细胞壁可以防止酶与纤维素聚合物接触。因此，木质素和半纤维素的结构需要被消减或修改为可以允许纤维素酶随意移动的自由空间。这通常是通过一个预处理的过程来解决的。 我们越来越多的需要新的预处理方法。我们正在进入一个工业生物技术、合成生物学、代谢工程和系统生物学的新时代，这些新兴技术和学科提供新的工具微生物用以生产诸

7、如碳氢化合物的先进生物燃料。在使用这些工具进行生物燃料生产方面的真正进步将是有限的，如果给这些微生物供应的糖仍然是一个主要障碍。考虑到自然已经通过进化创造了复合酶作为生物催化剂，它有能力通过选择性地断裂化学键之间的基本单位解开木质素分子的复杂结构，我们应该寻求利用类似的物理化学加工过程，利用木质素和半纤维素降解酶实现在预处理中整合和糖化的终极目标。这些酶在传统的预处理之前或之后使用以实现减少，并最终取代热化学处理，从而减少整个预处理在大分子水平和简化工艺的严重影响。 在可持续发展和能源效率的前提下，生物工艺由于其可以在自然环境下发生且环保的特点优于理化过程。基于此的生物预处理未来将吸引更多的关

8、注。本文将在这个话题上提供一个全面的调查。本文将在目前生物预处理工艺的理论基础上，概述先进的知识，指出信息和技术的差距。最后,本文还提供了一些猜测，提出一些未来研究和发展方向。需要指出的是,理想的预处理过程需要木质素和半纤维素的解构。本文将，主要侧重于木质素降解。本文首先从木质素分解酶系统入手，其次是不同的微生物如何分解木质素。对生物预处理和热化学预处理作出比较，然后提供生物制浆的应用实例，得出结论并展望未来。外文文献原文：Status of Biological Pretreatment of Lignocellulosics: Potential, Progress and Challen

9、gesShulin Chen, Xiaoyu Zhang, Deepak Singh, Hongbo Yu, Xuewei Yang Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164.School of Life Science & Technology, Huazhong University of Science & Technology,Wuhan, Hubei, P.R.China, 430074.Abstract The feasibility of

10、 producing biofuels and biochemicals from lignocellulosic biomass via the biochemical platform depends largely on advancing technologies obtaining sugars from the cellulose and hemicelluloses of the plant cell walls. This paper provides an overview on the merit and challenges related to developing b

11、iological pretreatment processes as a new alternative to break the barriers of the plant cell wall structure for subsequent enzymatic extraction of sugars from cellulose polymer. Although having attracted little attention due to many inherent limitations, biological pretreatment has great potential

12、because of the multiple benefits, including being more environmentally benign, less energy intensive, less inhibitor production, and co-product possibilities. Increasing knowledge on termite and white-rot fungi provides insights in developing a new generation of pretreatment technologies to realize

13、these benefits. This review summarizes the enzyme system primarily lignin degrading enzymes, describes current understanding of biodegradation of plant cell walls by microorganisms, compares biological versus thermochemical pretreatment processes. The review also summarizes the progress in biopulpin

14、g, suggests a future perspective for developing the biological pretreatment processes. Introduction The only way to obtain renewable transportation fuels and chemicals is through the use of plant biomass that stores the intercepted solar energy via photosynthesis in the forms of organic carbon. Natu

15、re has also developed various pathways for utilizing and recycling these plant materials with minimum input of additional energy. In doing so, nature has been able to maintain a sustainable, yet balanced ecosystem for millions of years. These naturally occurring biological processes should be adopte

16、d in addressing major barriers of biorefinering - utilization of lignocellulosics for biofuels and biochemical production as they are often the most energy efficient yet creates little impact to the environment. There has been increasing urgency for developing biomass based fuels and chemicals as th

17、e concerns over climate change increases and fossil fuel resources decline. For example, the 36 billion gallons of renewable fuels production per year by 2022 of which 21 billion gallons must be advanced biofuels 1. The base for future biofuel and biochemical development is the supply of biomass fee

18、dstock. Among all types of biomass, lignocellulosics, the cell walls of trees, crop residues, grasses 2 and algae 3 is most abundant. Lignocellulosic biomass is the most abundant renewable biological resource on earth, with a yearly production of 200109tons 4, 5, only 3% of which is used in non-food

19、 areas, such as the paper and pulp industries 6. Current cellulose consumption is threefold higher than steel consumption and equals cereal consumption 7. As the use of these materials for the production of biofuel does not constitute a conflict with the producing food for human consumption, lignoce

20、llulosic biomass will be the major feedstock for the future biorefineries. The plant cell walls （PCW） are the primary materials where energy and organic carbon are stored. The composition and structure of PCW determines the design of the down stream processes using PCW as raw materials to produce va

21、rious target molecules. Plant consists of an orderly arrangement of cells with walls composed of varying amounts of a mixture of cellulose （ca. 40%）, hemicellulose （ca. 20-30%） and lignin （ca. 20-30%） 8. Cellulose is a linear polymer of D-glucose units linked by -1, 4-glycosidic bonds. Hemicellulose

22、 is heteropolymer containing many sugar monomers （xylose, mannose, galactose, arabinose, and rhamnose） forming random amorphous structures. Lignin on the other hand, is a cross linked macromolecule consisting of primarily three monolignol monomers （p-coumerayl alcohol, coniferyl alcohol, and sinapyl

23、 alcohol） that are methoxylated to various degree. Lignincellulose is a compact, in part crystalline complex, and polysaccharide components which form microfibers are densely packed in layers of lignin, protecting them against the activity of hydrolytic enzymes and other external factors, serving as

24、 a stabilizer of the complex structure 9. Such a structure can be illustrated using wood as an example. Generally, the supporting cell has a lumen after the cell is dead, which can be more or less empty or filled with water.The inner layer is a heterogeneous mixture of components of unknown composit

25、ion.The secondary wall outside the inner layer can be further divided into three sublayers （S3, S2, S1） all consisting of cellulose microfibrils embedded in an amorphous mixture of different hemicelluloses and lignin 10. The concentration of the cellulose is highest in the S2 sublayer of the seconda

26、ry wall and decrease towards the middle lamella. The S3 layer, which is near the lumen is rich in hemicelluloses. Lignin has the highest concentration in middle lamella. However, since the secondary wall is much thicker, it contains most of the lignin （60-80%）. Lignification is initiated in the prim

27、ary walls adjacent to the corners of the cell undergoing cellulose deposition in the S1 layers near the cambium and then proceeds in the intercellular layers and primary walls 11. In the primary cell wall, crystalline and amorphous cellulose core is surrounded by hemicellulose polymer. In grass, the

28、 middle lamella and primary wall contains more C-C coupling of monolignols into a highly branched polymer. In contrast, the -O-4 coupling of monolignols leads to a relatively linear polymer in the secondary cell wall 12. The highly branched lignins in the primary cell walls are more inhibitory to ce

29、ll wall degradation than the linear polymer in the secondary walls 13. The essence of converting lignocellulosics to fuels and chemicals is to obtain the desirable form of organic carbon molecules from the PCW to be used either as precursor molecules or energy sources for the targeted fuel products.

30、 There are typically two major platforms for biomass conversion. The first one is biochemical platform in which various sugar molecules are first obtained from the biomass. The sugars are then used by microorganisms to be converted subsequently to target fuel molecules such as ethanol. Whereas the o

31、ther is thermochemical platform in which the lignocellulosics is either first broken into a mixture of molecules or simple molecules. Some of the molecules, upon separation from the mixture, can be used directly as fuels; whereas the simple molecules, upon further processing, can be synthesized as f

32、uel molecules. For the purpose of this paper, our discussions will be limited to the biochemical platform. Most biological processes focus on the use of the sugars as the energy and carbon source through fermentation to derive different products. The predominant sugar unit is glucoses that are bounded covalently in the form of cellulose polymer within the plant cell wall structure. Obtaining the sugars from the PCW is one