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首页> 外文期刊>Biochemistry >CRYSTALLOGRAPHIC COMPLEXES OF GLUCOAMYLASE WITH MALTOOLIGOSACCHARIDE ANALOGS - RELATIONSHIP OF STEREOCHEMICAL DISTORTIONS AT THE NONREDUCING END TO THE CATALYTIC MECHANISM
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CRYSTALLOGRAPHIC COMPLEXES OF GLUCOAMYLASE WITH MALTOOLIGOSACCHARIDE ANALOGS - RELATIONSHIP OF STEREOCHEMICAL DISTORTIONS AT THE NONREDUCING END TO THE CATALYTIC MECHANISM

机译：葡糖淀粉酶与麦芽寡糖类似物的结晶学络合物-在非还原端的立体畸变与催化机理的关系

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摘要

Crystal structures at pH 4 of complexes of glucoamylase from Aspergillus awamori var. X100 with the pseudotetrasaccharides D-gluco-dihydroacarbose and acarbose have been refined to R-factors of 0.147 and 0.131 against data to 1.7 and 2.0-Angstrom resolution, respectively, The two inhibitors bind in nearly identical manners, each exhibiting a dual binding mode with respect to the location of the last wgar residues. The reduced affinity of D-gluco-dihydroacarbose (K-i = 10(-8) M) relative to acarbose (K-i = 10(-12) M) may stem in part from the weakening of hydrogen bonds of the catalytic water (Wat 500) to the enzyme. Steric contacts between the nonreducing end of D-gluco-dihydroacarbose and the catalytic water perturb Wat 500 from its site of optimal hydrogen bonding to the active site. Interactions within the active site displace the 6-hydroxymethyl group of the nonreducing end of both acarbose and D-gluco-dihydroacarbose toward a more axial position. In the case of D-gluco-dihydroacarbose the shift in the position of the 6-hydroxymethyl group occurs with a 12 degrees change in two dihedral angles of the glucopyranose ring toward a half-chair conformation The observed conformational distortion of the first residue of D-gluco-dihydroacarbose is consistent with the generation of a glucopyranosyl cation in the transition state. Comparable distortions of stereochemistry in model compounds require approximately 2 kcal/mol, not more than 25% of the energy necessary to form the half-chair conformation in glucose, The magnitude of stereochemical distortion observed in the active site of glucoamylase suggests that favorable electrostatic interactions between the putative glucopyranosyl cation intermediate and the active site must be more important in stabilizing the transition state than mechanical distortion of the substrate.
机译：泡盛曲霉的葡糖淀粉酶复合物在pH 4时的晶体结构。具有假四糖D-葡萄糖-二氢阿卡波糖和阿卡波糖的X100已分别针对数据分别精制至0.147和0.131的R因子，分辨率分别为1.7和2.0埃。两种抑制剂以几乎相同的方式结合，每种均表现出与关于最后的wgar残留物的位置。 D-葡萄糖-二氢阿卡波糖（Ki = 10（-8）M）相对于阿卡波糖（Ki = 10（-12）M）降低的亲和力可能部分源于催化水（Wat 500）氢键的减弱对酶。 D-葡萄糖-二氢阿卡波糖的非还原性末端与催化水之间的立体接触从其最佳氢键合位置到活性位点干扰了Wat 500。活性位点内的相互作用将阿卡波糖和D-葡萄糖-二氢阿卡波糖的非还原端的6-羟甲基移向更轴向的位置。在D-葡萄糖-二氢阿卡波糖的情况下，6-吡喃葡萄糖环的两个二面角朝半椅构象变化12度时会发生6-羟甲基位置的移位。观察到的D的第一个残基的构象变形-葡萄糖-二氢阿卡波糖与过渡态中的吡喃葡萄糖基阳离子的产生是一致的。模型化合物中立体化学的可比畸变需要大约2 kcal / mol，不超过在葡萄糖中形成半椅构象所需的能量的25％。在葡糖淀粉酶活性位点观察到的立体化学畸变的程度表明，良好的静电相互作用假定的吡喃葡萄糖基阳离子中间体和活性位点之间的稳定比底物的机械变形对稳定过渡态更重要。

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