Cellobiohydrolase hydrolyzes crystalline cellulose on hydrophobic faces.
Cellobiohydrolase hydrolyzes crystalline cellulose on hydrophobic faces.
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DOI：
10.1074/jbc.m110.216556
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发表时间：
2011-04-01
期刊：
影响因子：
--
通讯作者：
Ding SY
中科院分区：
文献类型：
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作者：
Liu YS;Baker JO;Zeng Y;Himmel ME;Haas T;Ding SY
植物生物质的生物降解在自然界中是一个缓慢的过程，而纤维素的水解也被广泛认为是拟议的将木质纤维材料转化为生物燃料的工业过程中的一个限速步骤。众所周知，一组酶，包括内切和外切纤维素酶以及纤维二糖酶，需要协同作用才能将纤维素水解成葡萄糖。这些酶的详细分子机制还没有令人信服地阐明。本文利用原子力显微镜(AFM)对里氏木霉胞外纤维素酶I(CBH I)作用下瓦隆尼亚纤维素晶体的结构变化进行了实时成像。在原子力显微镜下，可以观察到单一的酶分子只与纤维素晶体的一面结合，显然是疏水性的一面。加入CbHI后，纤维素的表面粗糙度开始增加，在11h内，纤维素晶体的总尺寸变小。有趣的是，这种尺寸减小显然只发生在晶体的宽度，而高度保持相对恒定。此外，测得的纤维素晶体的横截面形状由不对称变为近对称。这些观察到的CbH I作用所带来的变化可能构成了第一次直接的可视化，支持了胞外纤维素酶选择性地水解纤维素的疏水表面的观点。天然纤维素中疏水表面的有限可及性可能是导致纤维素水解速度限制缓慢的重要原因。
Biodegradation of plant biomass is a slow process in nature, and hydrolysis of cellulose is also widely considered to be a rate-limiting step in the proposed industrial process of converting lignocellulosic materials to biofuels. It is generally known that a team of enzymes including endo- and exocellulases as well as cellobiases are required to act synergistically to hydrolyze cellulose to glucose. The detailed molecular mechanisms of these enzymes have yet to be convincingly elucidated. In this report, atomic force microscopy (AFM) is used to image in real-time the structural changes in Valonia cellulose crystals acted upon by the exocellulase cellobiohydrolase I (CBH I) from Trichoderma reesei. Under AFM, single enzyme molecules could be observed binding only to one face of the cellulose crystal, apparently the hydrophobic face. The surface roughness of cellulose began increasing after adding CBH I, and the overall size of cellulose crystals decreased during an 11-h period. Interestingly, this size reduction apparently occurred only in the width of the crystal, whereas the height remained relatively constant. In addition, the measured cross-section shape of cellulose crystal changed from asymmetric to nearly symmetric. These observed changes brought about by CBH I action may constitute the first direct visualization supporting the idea that the exocellulase selectively hydrolyzes the hydrophobic faces of cellulose. The limited accessibility of the hydrophobic faces in native cellulose may contribute significantly to the rate-limiting slowness of cellulose hydrolysis.