For the field of virology, perhaps one of the most paradigm‐shifting events so far in the 21st century was the identification of the giant double‐stranded DNA virus that infects amoebae. Remarkably, this virus, known as Mimivirus, has a genome that encodes for nearly 1,000 proteins, some of which are involved in the biosynthesis of unusual sugars. Indeed, the virus is coated by a layer of glycosylated fibers that contain d‐glucose, N‐acetyl‐d‐glucosamine, l‐rhamnose, and 4‐amino‐4,6‐dideoxy‐d‐glucose. Here we describe a combined structural and enzymological investigation of the protein encoded by the open‐reading frame L780, which corresponds to an l‐rhamnose synthase. The structure of the L780/NADP⁺/UDP‐l‐rhamnose ternary complex was determined to 1.45 Å resolution and refined to an overall R‐factor of 19.9%. Each subunit of the dimeric protein adopts a bilobal‐shaped appearance with the N‐terminal domain harboring the dinucleotide‐binding site and the C‐terminal domain positioning the UDP‐sugar into the active site. The overall molecular architecture of L780 places it into the short‐chain dehydrogenase/reductase superfamily. Kinetic analyses indicate that the enzyme can function on either UDP‐ and dTDP‐sugars but displays a higher catalytic efficiency with the UDP‐linked substrate. Site‐directed mutagenesis experiments suggest that both Cys 108 and Lys 175 play key roles in catalysis. This structure represents the first model of a viral UDP‐l‐rhamnose synthase and provides new details into these fascinating enzymes.

中文翻译：

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来自多食棘阿米巴拟虫病毒的 UDP-l-鼠李糖合酶的高分辨率结构。

对于病毒学领域，21 世纪迄今为止最具范式转变的事件之一可能是鉴定了感染变形虫的巨型双链 DNA 病毒。值得注意的是，这种被称为Mimivirus 的病毒的基因组可编码近 1,000 种蛋白质，其中一些与异常糖类的生物合成有关。实际上，病毒是由含有糖基化的纤维的层涂覆d葡萄糖，Ñ乙酰基d -葡糖胺，升鼠李糖，和4-氨基-4,6-二脱氧d葡萄糖。在这里，我们描述了由开放阅读框 L780 编码的蛋白质的组合结构和酶学研究，它对应于一个l-鼠李糖合酶。L780/NADP ⁺ /UDP- l-鼠李糖三元复合物的结构确定为 1.45 Å 分辨率并精制为整体R- 19.9% 的因素。二聚体蛋白的每个亚基都采用双叶形外观，N 端结构域包含二核苷酸结合位点，C 端结构域将 UDP 糖定位到活性位点。L780 的整体分子结构将其置于短链脱氢酶/还原酶超家族中。动力学分析表明，该酶可以作用于 UDP 和 dTDP 糖，但对连接 UDP 的底物显示出更高的催化效率。定点诱变实验表明 Cys 108 和 Lys 175 在催化中起关键作用。这种结构代表了病毒 UDP- l-鼠李糖合酶的第一个模型，并为这些迷人的酶提供了新的细节。