Simpson Peter J, Jamieson Stuart J, Abou-Hachem Maher, Karlsson Eva Nordberg, Gilbert Harry J, Holst Olle, Williamson Michael P
Krebs Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, UK.
Biochemistry. 2002 May 7;41(18):5712-9. doi: 10.1021/bi012093i.
The solution structure is presented for the second family 4 carbohydrate binding module (CBM4-2) of xylanase 10A from the thermophilic bacterium Rhodothermus marinus. CBM4-2, which binds xylan tightly, has a beta-sandwich structure formed by 11 strands, and contains a prominent cleft. From NMR titrations, it is shown that the cleft is the binding site for xylan, and that the main amino acids interacting with xylan are Asn31, Tyr69, Glu72, Phe110, Arg115, and His146. Key liganding residues are Tyr69 and Phe110, which form stacking interactions with the sugar. It is suggested that the loops on which the rings are displayed can alter their conformation on substrate binding, which may have functional importance. Comparison both with other family 4 cellulose binding modules and with the structurally similar family 22 xylan binding module shows that the key aromatic residues are in similar positions, and that the bottom of the cleft is much more hydrophobic in the cellulose binding modules than the xylan binding proteins. It is concluded that substrate specificity is determined by a combination of ring orientation and the nature of the residues lining the bottom of the binding cleft.
本文给出了嗜热细菌海栖热袍菌木聚糖酶10A的第二个第4家族碳水化合物结合模块（CBM4-2）的溶液结构。CBM4-2能紧密结合木聚糖，具有由11条链形成的β-折叠三明治结构，并含有一个明显的裂隙。通过核磁共振滴定表明，该裂隙是木聚糖的结合位点，与木聚糖相互作用的主要氨基酸是Asn31、Tyr69、Glu72、Phe110、Arg115和His146。关键的配体残基是Tyr69和Phe110，它们与糖类形成堆积相互作用。有人提出，展示环的环可以在底物结合时改变其构象，这可能具有功能重要性。与其他第4家族纤维素结合模块以及结构相似的第22家族木聚糖结合模块进行比较表明，关键的芳香族残基处于相似位置，并且在纤维素结合模块中裂隙底部比木聚糖结合蛋白更疏水。得出的结论是，底物特异性由环的取向和结合裂隙底部残基的性质共同决定。
