Mølgaard A, Kauppinen S, Larsen S
Centre for Crystallographic Studies, University of Copenhagen, Copenhagen, DK-2100, Denmark.
Structure. 2000 Apr 15;8(4):373-83. doi: 10.1016/s0969-2126(00)00118-0.
The complex polysaccharide rhamnogalacturonan constitutes a major part of the hairy region of pectin. It can have different types of carbohydrate sidechains attached to the rhamnose residues in the backbone of alternating rhamnose and galacturonic acid residues; the galacturonic acid residues can be methylated or acetylated. Aspergillus aculeatus produces enzymes that are able to perform a synergistic degradation of rhamnogalacturonan. The deacetylation of the backbone by rhamnogalacturonan acetylesterase (RGAE) is an essential prerequisite for the subsequent action of the enzymes that cleave the glycosidic bonds.
The structure of RGAE has been determined at 1.55 A resolution. RGAE folds into an alpha/beta/alpha structure. The active site of RGAE is an open cleft containing a serine-histidine-aspartic acid catalytic triad. The position of the three residues relative to the central parallel beta sheet and the lack of the nucleophilic elbow motif found in structures possessing the alpha/beta hydrolase fold show that RGAE does not belong to the alpha/beta hydrolase family.
Structural and sequence comparisons have revealed that, despite very low sequence similarities, RGAE is related to seven other proteins. They are all members of a new hydrolase family, the SGNH-hydrolase family, which includes the carbohydrate esterase family 12 as a distinct subfamily. The SGNH-hydrolase family is characterised by having four conserved blocks of residues, each with one completely conserved residue; serine, glycine, asparagine and histidine, respectively. Each of the four residues plays a role in the catalytic function.
复合多糖鼠李半乳糖醛酸聚糖是果胶毛状区域的主要组成部分。它可以有不同类型的碳水化合物侧链连接到由鼠李糖和半乳糖醛酸残基交替组成的主链中的鼠李糖残基上；半乳糖醛酸残基可以被甲基化或乙酰化。棘孢曲霉产生能够协同降解鼠李半乳糖醛酸聚糖的酶。鼠李半乳糖醛酸聚糖乙酰酯酶（RGAE）对主链进行脱乙酰化是后续切割糖苷键的酶发挥作用的必要前提。
已确定RGAE的结构，分辨率为1.55埃。RGAE折叠成α/β/α结构。RGAE的活性位点是一个开放裂缝，包含丝氨酸 - 组氨酸 - 天冬氨酸催化三联体。这三个残基相对于中央平行β折叠片层的位置以及在具有α/β水解酶折叠结构中未发现亲核肘部基序，表明RGAE不属于α/β水解酶家族。
结构和序列比较表明，尽管序列相似性非常低，但RGAE与其他七种蛋白质相关。它们都是一个新的水解酶家族——SGNH水解酶家族的成员，该家族包括碳水化合物酯酶家族12作为一个独特的亚家族。SGNH水解酶家族的特征是有四个保守的残基块，每个块分别有一个完全保守的残基；分别是丝氨酸、甘氨酸、天冬酰胺和组氨酸。这四个残基中的每一个在催化功能中都发挥作用。
