Crystal structure of metagenomic &bgr;-xylosidase/ &agr;-L-arabinofuranosidase activated by calcium

T. Matsuzawa, S. Kaneko, N. Kishine, Z. Fujimoto, K. Yaoi
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引用次数: 13

Abstract

The crystal structure of metagenomic β-xylosidase/α-l-arabinofuranosidase CoXyl43, activated by calcium ions, was determined in its apo and complexed forms with xylotriose or l-arabinose in the presence and absence of calcium. The presence of calcium ions dramatically increases the kcat of CoXyl43 for p-nitrophenyl β-d-xylopyranoside and reduces the Michaelis constant for p-nitrophenyl α-l-arabinofuranoside. CoXyl43 consists of a single catalytic domain comprised of a five-bladed β-propeller. In the presence of calcium, a single calcium ion was observed at the centre of this catalytic domain, behind the catalytic pocket. In the absence of calcium, the calcium ion was replaced with one sodium ion and one water molecule, and the positions of these cations were shifted by 1.3 Å. The histidine-319 side chain, which coordinates to the 2-hydroxyl oxygen atom of the bound xylose molecule in the catalytic pocket, also coordinates to the calcium ion, but not to the sodium ion. The calcium-dependent increase in activity appears to be caused by the structural change in the catalytic pocket induced by the tightly bound calcium ion and coordinating water molecules, and by the protonation state of glutamic acid-268, the catalytic acid of the enzyme. Our findings further elucidate the complex relationship between metal ions and glycosidases.
钙活化的宏基因组&bgr;-木糖苷酶/ &agr;- l -阿拉伯糖醛酸苷酶的晶体结构
在钙离子激活下,测定了宏基因组β-木糖苷酶/α-l-阿拉伯糖糠糖苷酶CoXyl43的载子和与木糖或l-阿拉伯糖在有钙和无钙情况下的络合形式。钙离子的存在显著提高了CoXyl43对对硝基苯β-d-木吡喃苷的kcat,降低了对硝基苯α-l-阿拉伯糖脲苷的Michaelis常数。CoXyl43由一个由五叶β-螺旋桨组成的单一催化结构域组成。在钙存在的情况下,在催化结构域的中心,在催化口袋后面,观察到一个单一的钙离子。在没有钙的情况下,钙离子被一个钠离子和一个水分子取代,这些阳离子的位置移动了1.3 Å。组氨酸-319侧链与催化袋中结合木糖分子的2-羟基氧原子配位,也与钙离子配位,但不与钠离子配位。钙依赖性的活性增加似乎是由紧密结合的钙离子和配位的水分子引起的催化口袋结构变化以及酶的催化酸谷氨酸-268的质子化状态引起的。我们的发现进一步阐明了金属离子与糖苷酶之间的复杂关系。
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