Xiang Dao Feng, Narindoshvili Tamari, Raushel Frank M
Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States.
Biochemistry. 2025 Feb 4;64(3):591-599. doi: 10.1021/acs.biochem.4c00704. Epub 2025 Jan 24.
The exterior surface of the human pathogen is coated with a capsular polysaccharide (CPS) that consists of a repeating sequence of 2-5 different sugars that can be modified with various molecular decorations. In the HS:2 serotype from strain NCTC 11168, the repeating unit within the CPS is composed of d-ribose, -acetyl-d-galactosamine, and a d-glucuronic acid that is further amidated with either serinol or ethanolamine. The d-glucuronic acid moiety is also decorated with d-glycero-l-gluco-heptose. Here, we show that two different GT2 glycosyltransferases catalyze the transfer of -acetyl-d-galactosamine from UDP-NAc-d-galactosamine furanoside to the C4-hydroxyl group of the d-glucuronamide moiety at the growing end of the capsular polysaccharide chain. Catalytic activity was not observed with glycosides of d-glucuronic acid, and thus, the C6-carboxylate of the d-glucuronic acid moiety must be amidated prior to chain elongation. One of these enzymes comprises the N-terminal domain of Cj1438 (residues 1-325) and the other is from the N-terminal domain of Cj1434 (residues 1-327). These two glycosyltransferases are ∼87% identical in sequence, but it is not clear why there are two glycosyltransferases from the same gene cluster that apparently catalyze the same reaction. This discovery represents the second polymerizing glycosyltransferase that has been isolated and functionally characterized for the biosynthesis of the capsular polysaccharide in the HS:2 serotype of .
人类病原体的外表面覆盖有荚膜多糖（CPS），其由2 - 5种不同糖类的重复序列组成，这些糖类可通过各种分子修饰进行改性。在菌株NCTC 11168的HS:2血清型中，CPS内的重复单元由d - 核糖、N - 乙酰 - d - 半乳糖胺和一种d - 葡萄糖醛酸组成，该d - 葡萄糖醛酸进一步与丝氨醇或乙醇胺酰胺化。d - 葡萄糖醛酸部分还装饰有d - 甘油 - l - 葡萄糖 - 庚糖。在这里，我们表明两种不同的GT2糖基转移酶催化N - 乙酰 - d - 半乳糖胺从UDP - NAc - d - 半乳糖胺呋喃糖苷转移到荚膜多糖链生长末端的d - 葡萄糖醛酰胺部分的C4 - 羟基上。d - 葡萄糖醛酸的糖苷未观察到催化活性，因此，d - 葡萄糖醛酸部分的C6 - 羧酸盐必须在链延长之前酰胺化。其中一种酶包含Cj1438的N端结构域（第1 - 325位氨基酸残基），另一种来自Cj1434的N端结构域（第1 - 327位氨基酸残基）。这两种糖基转移酶的序列相似度约为87%，但尚不清楚为什么来自同一基因簇的两种糖基转移酶显然催化相同的反应。这一发现代表了第二种已被分离并对HS:2血清型的荚膜多糖生物合成进行功能表征的聚合糖基转移酶。
