BACKGROUND Pectic substances are the major polysaccharide components of the middle lamella and primary cell wall of dicotyledonous plants. They consist of homogalacturonan 'smooth' regions and highly rhamnified 'hairy' regions of rhamnogalacturonan. The backbone in rhamnogalacturonan-l (RG-l), which is composed of alternating galacturonic acid and rhamnose residues, is the substrate for a new class of enzymes known as rhamnogalacturnoases (RGases). RGase A is a novel enzyme implicated in the enzymatic degradation of RG-l. RESULTS The structure of RGase A from Aspergillus aculeatus has been solved by the single isomorphous replacement method including anomalous scattering (SIRAS method) to 2.0 A resolution. The enzyme folds into a large right-handed parallel beta helix, with a core composed of 13 turns of beta strands. Four parallel beta sheets (PB1, PB1a, PB2 and PB3), formed by the consecutive turns, are typically separated by a residue in the conformation of a left-handed alpha helix. As a consequence of the consecutive turns, 32% of all residues have their sidechains aligned at the surface or in the interior of the parallel beta helix. The aligned residues at the surface are dominated by threonine, aspartic acid and asparagine, whereas valine, leucine and isoleucine are most frequently found in the interior. A very large hydrophobic cavity is found in the interior of the parallel beta helix. The potential active site is a groove, oriented almost perpendicular to the helical axis, containing a cluster of three aspartic acid residues and one glutamic acid residue. The enzyme is highly glycosylated; two N-linked and eighteen O-linked glycosylation sites have been found in the structure. CONCLUSIONS Rhamnogalacturonase A from A. aculeatus is the first three-dimensional structure of an enzyme hydrolyzing glycoside bonds within the backbone of RG-l. The large groove, which is the potential active site of RGase A, is also seen in the structures of pectate lyases. Two catalytic aspartic acid residues, which have been proposed to have a catalytic role, reside in this area of RGase A. The distance between the aspartic acid residues is consistent with the inverting mechanism of catalysis. The glycan groups bound to RGase A are important to the stability of the crystal, as the carbohydrate moiety is involved in most of the intermolecular hydrogen bonds.

中文翻译：

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刺曲霉鼠李糖半乳糖醛酸酶A的晶体结构：右旋平行β螺旋。

背景技术果胶物质是双子叶植物中层和初级细胞壁的主要多糖成分。它们由高半乳糖醛酸聚糖的“光滑”区域和高度鼠李糖醛酸半乳糖醛酸的“毛状”区域组成。由交替的半乳糖醛酸和鼠李糖残基组成的鼠李糖半乳糖醛酸聚糖-1（RG-1）中的骨架是被称为鼠李糖半乳糖醛酸酶（RGase）的一类新型酶的底物。RGase A是一种新型酶，涉及RG-1的酶促降解。结果通过包括异常散射（SIRAS方法）在内的单一同构置换方法（分辨率为2.0 A的分辨率）已解决了刺曲霉RGase A的结构。该酶折叠成一个大的右手平行β螺旋，其核心由13匝β链组成。由连续匝形成的四个平行的β折叠（PB1，PB1a，PB2和PB3）通常由左旋α螺旋构象中的残基分隔。连续转弯的结果是，所有残基中的32％的侧链在平行β螺旋的表面或内部对齐。表面排列的残基主要由苏氨酸，天冬氨酸和天冬酰胺占据，而缬氨酸，亮氨酸和异亮氨酸最常见于内部。在平行β螺旋的内部发现非常大的疏水腔。潜在的活性位点是几乎垂直于螺旋轴取向的凹槽，其中包含一簇三个天冬氨酸残基和一个谷氨酸残基。该酶高度糖基化；在该结构中发现了两个N-连接和十八个O-连接的糖基化位点。结论棘孢曲霉的鼠李糖半乳糖醛酸酶A是水解RG-1骨架内糖苷键的酶的第一个三维结构。在果胶酸裂合酶的结构中也可见到较大的凹槽，这是RGase A的潜在活性位点。已经提出具有催化作用的两个催化天冬氨酸残基位于RGase A的该区域中。天冬氨酸残基之间的距离与催化的转化机理一致。结合到RGase A上的聚糖基对于晶体的稳定性很重要，因为碳水化合物部分参与了大多数分子间氢键。刺突是水解RG-1主链内糖苷键的酶的第一个三维结构。在果胶酸裂合酶的结构中也可见到较大的凹槽，这是RGase A的潜在活性位点。已经提出具有催化作用的两个催化天冬氨酸残基位于RGase A的该区域中。天冬氨酸残基之间的距离与催化的转化机理一致。结合到RGase A上的聚糖基对于晶体的稳定性很重要，因为碳水化合物部分参与了大多数分子间氢键。刺突是水解RG-1主链内糖苷键的酶的第一个三维结构。在果胶酸裂合酶的结构中也可见到较大的凹槽，这是RGase A的潜在活性位点。已经提出具有催化作用的两个催化天冬氨酸残基位于RGase A的该区域中。天冬氨酸残基之间的距离与催化的转化机理一致。结合到RGase A上的聚糖基对于晶体的稳定性很重要，因为碳水化合物部分参与了大多数分子间氢键。已经提出具有催化作用的两个催化天冬氨酸残基位于RGase A的该区域中。天冬氨酸残基之间的距离与催化的转化机理一致。结合到RGase A上的聚糖基对于晶体的稳定性很重要，因为碳水化合物部分参与了大多数分子间氢键。已经提出具有催化作用的两个催化天冬氨酸残基位于RGase A的该区域中。天冬氨酸残基之间的距离与催化的转化机理一致。结合到RGase A上的聚糖基对于晶体的稳定性很重要，因为碳水化合物部分参与了大多数分子间氢键。