Department of Chemistry, University of Eastern Finland, 80130 Joensuu, Finland.
Department of Pharmacy, University of Helsinki, 00790 Helsinki, Finland.
Molecules. 2022 Apr 20;27(9):2655. doi: 10.3390/molecules27092655.
Acetylated glucuronoxylan is one of the most common types of hemicellulose in nature. The structure is formed by a β-(1→4)-linked D-xylopyranosyl (Xyl) backbone that can be substituted with an acetyl group at -2 and 3 positions, and α-(1→2)-linked 4--methylglucopyranosyluronic acid (MeGlcA). Acetyl xylan esterases (AcXE) that target mono- or doubly acetylated Xyl are well characterized; however, the previously studied AcXE from (AcXE) was the first to remove the acetyl group from 2--MeGlcA-3--acetyl-substituted Xyl units, yet structural characteristics of these enzymes remain unspecified. Here, six homologs of AcXE were produced and three crystal structures of the enzymes were solved. Two of them are complex structures, one with bound MeGlcA and another with acetate. All homologs were confirmed to release acetate from 2--MeGlcA-3--acetyl-substituted xylan, and the crystal structures point to key structural elements that might serve as defining features of this unclassified carbohydrate esterase family. Enzymes comprised two domains: N-terminal CBM domain and a C-terminal SGNH domain. In AcXE and all studied homologs, the sequence motif around the catalytic serine is Gly-Asn-Ser-Ile (GNSI), which differs from other SGNH hydrolases. Binding by the MeGlcA-Xyl ligand is directed by positively charged and highly conserved residues at the interface of the CBM and SGNH domains of the enzyme.
乙酰化葡糖醛酸木聚糖是自然界中最常见的半纤维素类型之一。其结构由β-(1→4)-连接的 D-吡喃木糖（Xyl）主链组成，该主链可以在-2 和 3 位被乙酰基取代，在 4-位被α-(1→2)-连接的 4-O-甲基吡喃葡萄糖醛酸（MeGlcA）取代。靶向单乙酰化或双乙酰化 Xyl 的乙酰木聚糖酯酶（AcXE）已有很好的研究；然而，先前研究的 AcXE 是第一个从 2--MeGlcA-3--乙酰取代的 Xyl 单元上移除乙酰基的酶，但这些酶的结构特征仍未明确。本文产生了六个 AcXE 的同源物，并解析了三种酶的晶体结构。其中两种为复合物结构，一种与 MeGlcA 结合，另一种与醋酸盐结合。所有同源物均被证实能从 2--MeGlcA-3--乙酰取代的木聚糖中释放醋酸盐，晶体结构指出了可能作为这个未分类的碳水化合物酯酶家族特征的关键结构元素。酶由两个结构域组成：N 端 CBM 结构域和 C 端 SGNH 结构域。在 AcXE 和所有研究的同源物中，催化丝氨酸周围的序列基序为 Gly-Asn-Ser-Ile（GNSI），与其他 SGNH 水解酶不同。MeGlcA-Xyl 配体的结合由酶的 CBM 和 SGNH 结构域界面处带正电荷和高度保守的残基所引导。
