Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States.
Department of Biochemistry & Biophysics, Texas A&M University, College Station, Texas 77843, United States.
Biochemistry. 2023 Jan 3;62(1):134-144. doi: 10.1021/acs.biochem.2c00633. Epub 2022 Dec 19.
is a human pathogen and the leading cause of food poisoning in the United States and Europe. Surrounding the exterior surface of this bacterium is a capsular polysaccharide (CPS) that consists of a repeating sequence of common and unusual carbohydrate segments. At least 10 different heptose sugars have thus far been identified in the various strains of . The accepted biosynthetic pathway for the construction of the 6-deoxy-heptoses begins with the 4,6-dehydration of GDP-d--d--heptose by a dehydratase, followed by an epimerase that racemizes C3 and/or C5 of the product GDP-6-deoxy-4-keto-d--heptose. In the final step, a C4-reductase catalyzes the NADPH reduction of the resulting 4-keto product. However, in some strains and serotypes of , there are two separate C4-reductases with different product specificities in the gene cluster for CPS formation. Five pairs of these tandem C4-reductases were isolated, and the catalytic properties were ascertained. In four out of five cases, one of the two C4-reductases is able to catalyze the isomerization of C3 and C5 of GDP-6-deoxy-4-keto-d--heptose, in addition to the catalysis of the reduction of C4, thus bypassing the requirement for a separate C3/C5-isomerase. In each case, the 3'-end of the gene for the first C4-reductase contains a poly-G tract of 8-10 guanine residues that may be used to control the expression and/or catalytic activity of either C4-reductase. The three-dimensional structure of the C4-reductase from serotype HS:15, which only does a reduction of C4, was determined to 1.45 Å resolution in the presence of NADPH and GDP.
是一种人类病原体，也是美国和欧洲食源性疾病的主要致病原。这种细菌的外表面被一层荚膜多糖（CPS）所包裹，它由重复的常见和不常见的碳水化合物片段组成。迄今为止，在不同的 菌株中已经鉴定出至少 10 种不同的庚糖。公认的 6-脱氧-庚糖生物合成途径始于脱水酶对 GDP-d--d--庚糖的 4,6-脱水作用，然后是将产物 GDP-6-脱氧-4-酮-d--庚糖的 C3 和/或 C5 外消旋化的差向异构酶。在最后一步，C4-还原酶催化生成的 4-酮产物的 NADPH 还原。然而，在某些 菌株和血清型中，CPS 形成基因簇中存在两种具有不同产物特异性的独立 C4-还原酶。从该基因簇中分离出了 5 对串联 C4-还原酶，并确定了它们的催化特性。在五种情况中的四种中，两种 C4-还原酶中的一种除了能够催化 C4 的还原之外，还能够催化 GDP-6-脱氧-4-酮-d--庚糖的 C3 和 C5 的异构化，从而绕过了对单独的 C3/C5-异构酶的需求。在每种情况下，第一个 C4-还原酶基因的 3' 端都含有一个 8-10 个鸟嘌呤残基的多 G 序列，该序列可能用于控制任一 C4-还原酶的表达和/或催化活性。HS:15 血清型的 C4-还原酶的三维结构在 NADPH 和 GDP 的存在下被确定为 1.45 Å 的分辨率。
