Characterization of the CDP-d-mannitol biosynthetic pathway in Streptococcus pneumoniae 35A
作者:Quan Wang、Yanli Xu、Andrei V Perepelov、Yuriy A Knirel、Peter R Reeves、Alexander S Shashkov、Peng Ding、Xi Guo、Lu Feng
DOI:10.1093/glycob/cws113
日期:2012.12
Streptococcus pneumoniae is a major human pathogen associated with diseases worldwide. The capsular polysaccharides (CPSs) are considered a major virulence factor and are targets for a vaccine. d-Mannitol was found to be present in the CPS of several S. pneumoniae serotypes. Two genes, mnp1 and mnp2, which are located in the CPS gene cluster, were proposed to be responsible for the synthesis of NDP-d-mannitol (the nucleotide activated form of d-mannitol). However, the pathway has never been identified by experimental methods and we aimed to characterize it in the present study. To achieve this, the two genes, mnp1 and mnp2, were cloned and the gene products were overexpressed, purified, and analyzed in vitro for their respective enzymatic activities. Products of reactions catalyzed by Mnp1 and Mnp2 were detected by capillary electrophoresis and validated using electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy. We show that Mnp1 is responsible for the transfer of CMP from CTP to d-fructose-6-phosphate (Fru-6-P) to form CDP-d-fructose, whereas Mnp2 catalyzed the conversion of CDP-d-fructose to CDP-d-mannitol. Therefore, Mnp1 (renamed as mnpA) was identified as Fru-6-P cytidylyltransferase-encoding gene, and mnp2 (renamed as mnpB) as a CDP-d-fructose reductase-encoding gene. The kinetics of Mnp1 for the substrate (Fru-6-P and CTP) and of Mnp2 for the substrate (CDP-d-fructose) and the cofactor NADH or NADPH fitted the Michaelis–Menten model. The effects of temperature, pH and cations on the two enzymes were analyzed. This is the first time that the biosynthetic pathway of CDP-d-mannitol has been identified biochemically.
肺炎链球菌是一种与全球疾病相关的主要人类病原体。荚膜多糖(CPSs)被认为是主要的致病因子,也是疫苗的靶标。位于 CPS 基因簇中的两个基因 mnp1 和 mnp2 被认为负责合成 NDP-d-甘露醇(d-甘露醇的核苷酸活化形式)。然而,该途径从未通过实验方法确定,我们的目的是在本研究中确定其特征。为此,我们克隆了 mnp1 和 mnp2 这两个基因,并对基因产物进行了过表达、纯化和体外酶活性分析。用毛细管电泳法检测了 Mnp1 和 Mnp2 催化反应的产物,并用电喷雾离子化质谱法和核磁共振光谱法进行了验证。我们发现,Mnp1 负责将 CMP 从 CTP 转移到 d-果糖-6-磷酸(Fru-6-P),形成 CDP-d-果糖,而 Mnp2 则催化 CDP-d-果糖转化为 CDP-d-甘露醇。因此,Mnp1(改名为 mnpA)被确定为 Fru-6-P 胞苷酸转移酶编码基因,而 mnp2(改名为 mnpB)被确定为 CDP-d-果糖还原酶编码基因。Mnp1 对底物(Fru-6-P 和 CTP)以及 Mnp2 对底物(CDP-d-果糖)和辅助因子 NADH 或 NADPH 的动力学符合 Michaelis-Menten 模型。分析了温度、pH 值和阳离子对这两种酶的影响。这是首次从生物化学角度确定 CDP-d-甘露糖醇的生物合成途径。