Hyaluronan (HA), the essential [-3-GlcNAc-1-β-4-GlcA-1-β-]n matrix polysaccharide in vertebrates and molecular camouflage coating in select pathogens, is polymerized by "HA synthase" (HAS) enzymes. The first HAS identified three decades ago opened the window for new insights and biotechnological tools. This review discusses current understanding of HA biosynthesis, its biotechnological utility, and addresses some misconceptions in the literature. HASs are fascinating enzymes that polymerize two different UDP-activated sugars via different glycosidic linkages. Therefore, these catalysts were the first examples to break the "one enzyme/one sugar transferred" dogma. Three distinct types of these bifunctional glycosyltransferases (GTs) with disparate architectures and reaction modes are known. Based on biochemical and structural work, we present an updated classification system. Class I membrane-integrated HASs employ a processive chain elongation mechanism and secrete HA across the plasma membrane. This complex operation is accomplished by functionally integrating a cytosolic catalytic domain with a channel-forming transmembrane region. Class I enzymes, containing a single GT family-2 (GT-2) module that adds both monosaccharide units to the nascent chain, are further subdivided into two groups that construct the polymer with opposite molecular directionalities: Class I-R and I-NR elongate the HA polysaccharide at either the reducing or the non-reducing end, respectively. In contrast, Class II HASs are membrane-associated peripheral synthases with a non-processive, non-reducing end elongation mechanism using two independent GT-2 modules (one for each type of monosaccharide) and require a separate secretion system for HA export. We discuss recent mechanistic insights into HA biosynthesis that promise biotechnological benefits and exciting engineering approaches.

中文翻译：

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乙酰透明质酸合酶；三种独特双功能糖基转移酶的机制、神话和奥秘。

透明质酸 (HA) 是脊椎动物中必需的 [-3-GlcNAc-1-β-4-GlcA-1-β-]n 基质多糖，也是特定病原体的分子伪装涂层，由“HA 合酶”(HAS) 聚合而成。三十年前发现的第一个 HAS 为新见解和生物技术工具打开了窗口。这篇综述讨论了目前对 HA 生物合成及其生物技术用途的理解，并解决了文献中的一些误解。HAS 是一种令人着迷的酶，可通过不同的糖苷键聚合两种不同的 UDP 激活糖。因此，这些催化剂是打破“一种酶/一种糖转移”教条的第一个例子。已知三种不同类型的双功能糖基转移酶 (GT) 具有不同的结构和反应模式。基于生化和结构工作，我们提出了一个更新的分类系统。I 类膜集成 HAS 采用持续链延伸机制并跨质膜分泌 HA。这种复杂的操作是通过将胞质催化结构域与通道形成跨膜区域功能性整合来完成的。I 类酶包含一个 GT 家族 2 (GT-2) 模块，可将两个单糖单元添加到新生链上，并进一步细分为两组，它们构建具有相反分子方向性的聚合物：IR 类和 I-NR 类延长了HA 多糖分别位于还原端或非还原端。相比之下，II 类 HAS 是膜相关外周合酶，具有非进行性、非还原末端延伸机制，使用两个独立的 GT-2 模块（每种类型的单糖一个），并且需要单独的分泌系统来输出 HA。我们讨论了最近对 HA 生物合成的机制见解，这些见解有望带来生物技术效益和令人兴奋的工程方法。