Crennell Susan J, Hreggvidsson Gudmundur O, Nordberg Karlsson Eva
Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
J Mol Biol. 2002 Jul 19;320(4):883-97. doi: 10.1016/s0022-2836(02)00446-1.
Cellulose is one of the most abundant polysaccharides in nature and microorganisms have developed a comprehensive system for enzymatic breakdown of this ubiquitous carbon source, a subject of much interest in the biotechnology industry. Rhodothermus marinus produces a hyperthermostable cellulase, with a temperature optimum of more than 90 degrees C, the structure of which is presented here to 1.8 A resolution. The enzyme has been classified into glycoside hydrolase family 12; this is the first structure of a thermophilic member of this family to have been solved. The beta-jelly roll fold observed has identical topology to those of the two mesophilic members of the family whose structures have been elucidated previously. A Hepes buffer molecule bound in the active site may have triggered a conformational change to an active configuration as the two catalytic residues Glu124 and Glu207, together with dependent residues, are observed in a conformation similar to that seen in the structure of Streptomyces lividans CelB2 complexed with an inhibitor. The structural similarity between this cellulase and the mesophilic enzymes serves to highlight features that may be responsible for its thermostability, chiefly an increase in ion pair number and the considerable stabilisation of a mobile region seen in S. lividans CelB2. Additional aromatic residues in the active site region may also contribute to the difference in thermophilicity.
纤维素是自然界中最丰富的多糖之一，微生物已经开发出一套完整的酶促系统来分解这种无处不在的碳源，这是生物技术产业中备受关注的一个课题。海栖热袍菌产生一种超嗜热纤维素酶，其最适温度超过90摄氏度，本文给出了该酶分辨率为1.8埃的结构。该酶已被归类为糖苷水解酶家族12；这是该家族嗜热成员的首个已解析结构。观察到的β-果冻卷折叠与该家族先前已阐明结构的两个嗜温成员具有相同的拓扑结构。结合在活性位点的一个Hepes缓冲分子可能触发了向活性构象的构象变化，因为观察到两个催化残基Glu124和Glu207以及相关残基处于与与抑制剂复合的变铅青链霉菌CelB2结构中所见类似的构象。这种纤维素酶与嗜温酶之间的结构相似性有助于突出可能导致其热稳定性的特征，主要是离子对数目的增加以及在变铅青链霉菌CelB2中看到的一个可移动区域的显著稳定。活性位点区域中额外的芳香族残基也可能导致嗜热性的差异。
