Tormo J, Lamed R, Chirino A J, Morag E, Bayer E A, Shoham Y, Steitz T A
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT, USA.
EMBO J. 1996 Nov 1;15(21):5739-51.
The crystal structure of a family-III cellulose-binding domain (CBD) from the cellulosomal scaffoldin subunit of Clostridium thermocellum has been determined at 1.75 A resolution. The protein forms a nine-stranded beta sandwich with a jelly roll topology and binds a calcium ion. conserved, surface-exposed residues map into two defined surfaces located on opposite sides of the molecule. One of these faces is dominated by a planar linear strip of aromatic and polar residues which are proposed to interact with crystalline cellulose. The other conserved residues are contained in a shallow groove, the function of which is currently unknown, and which has not been observed previously in other families of CBDs. On the basis of modeling studies combined with comparisons of recently determined NMR structures for other CBDs, a general model for the binding of CBDs to cellulose is presented. Although the proposed binding of the CBD to cellulose is essentially a surface interaction, specific types and combinations of amino acids appear to interact selectively with glucose moieties positioned on three adjacent chains of the cellulose surface. The major interaction is characterized by the planar strip of aromatic residues, which align along one of the chains. In addition, polar amino acid residues are proposed to anchor the CBD molecule to two other adjacent chains of crystalline cellulose.
嗜热栖热放线菌（Clostridium thermocellum）纤维小体支架蛋白亚基中III型纤维素结合结构域（CBD）的晶体结构已在1.75埃分辨率下测定。该蛋白形成具有果冻卷拓扑结构的九股β折叠三明治结构，并结合一个钙离子。保守的表面暴露残基映射到分子相对两侧的两个特定表面。其中一个面由一条芳香族和极性残基的平面线性条带主导，推测该条带与结晶纤维素相互作用。其他保守残基包含在一个浅沟中，其功能目前未知，且在其他CBD家族中未曾观察到。基于建模研究并结合最近测定的其他CBD的NMR结构比较，提出了CBD与纤维素结合的通用模型。尽管所提出的CBD与纤维素的结合本质上是一种表面相互作用，但特定类型和组合的氨基酸似乎选择性地与位于纤维素表面三条相邻链上的葡萄糖部分相互作用。主要相互作用的特征是芳香族残基的平面条带，它沿着其中一条链排列。此外，极性氨基酸残基被认为将CBD分子锚定到结晶纤维素的另外两条相邻链上。
