Most microorganisms do not produce sialic acid (sialate), and those that do appear to use a biosynthetic mechanism distinct from mammals. Genetic hybrids of nonpathogenic, sialate-negative laboratory Escherichia coli K-12 strains designed for the de novo synthesis of the polysialic acid capsule from E. coli K1 proved useful in elucidating the genetics and biochemistry of capsule biosynthesis. In this article we propose a dynamic model of sialometabolism to investigate the effects of biosynthetic neu (N-acetylneuraminic acid) and catabolic nan (N-acylneuraminate) mutations on the flux of intermediates through the sialate synthetic pathway. Intracellular sialate concentrations were determined by high pH anion exchange chromatography with pulsed amperometric detection. The results indicated that a strain carrying a null defect in the gene encoding polysialyltransferase (neuS) accumulated > 50 times more CMP–sialic acid than the wild type when strains were grown in a minimal medium supplemented with glucose and casamino acids. Metabolic accumulation of CMP–sialic acid depended on a functional sialic acid synthase (neuB), as shown by the inability of a strain lacking this enzyme to accumulate a detectable endogenous sialate pool. The neuB mutant concentrated trace sialate from the medium, indicating its potential value for quantitative analysis of free sialic acids in complex biological samples. The function of the sialate aldolase (encoded by nanA) in limiting intermediate flux through the synthetic pathway was determined by analyzing free sialate accumulation in neuA (CMP–sialic acid synthetase) nanA double mutants. The combined results demonstrate how E. coli avoids a futile cycle in which biosynthetic sialate induces the system for its own degradation and indicate the feasibility of generating sialooligosaccharide precursors through targeted manipulation of sialate metabolism.
大多数微
生物不产生
硅铝酸盐(sialic acid),而那些产生
硅铝酸盐的微
生物似乎使用了一种与哺乳动物不同的
生物合成机制。事实证明,非致病性、对ialate 阴性的实验室大肠杆菌 K-12 株系的
基因杂交设计用于从大肠杆菌 K1 中重新合成多聚ialic 酸胶囊,这对阐明胶囊
生物合成的遗传学和
生物化学非常有用。在这篇文章中,我们提出了一个
硅铝酸盐代谢动态模型,以研究
生物合成neu(
N-乙酰神经氨酸)和分解nan(
N-乙酰神经氨酸)突变对
硅铝酸盐合成途径中间产物通量的影响。通过高 pH 阴离子交换色谱法和脉冲安培检测法测定了细胞内
硅铝酸盐的浓度。结果表明,当菌株在补充了
葡萄糖和 casamino 酸的最小
培养基中生长时,携带多聚糖基转移酶(neuS)
基因缺失的菌株积累的
CMP-sialic酸是野生型的 50 倍以上。
CMP-sialic酸的代谢积累依赖于功能性的sialic酸合成酶(neuB),缺乏该酶的菌株无法积累可检测到的内源性sialate池。neuB 突变体从
培养基中浓缩了微量的
硅铝酸盐,这表明它在定量分析复杂
生物样本中游离的
硅铝酸盐方面具有潜在价值。通过分析 neuA(
CMP-sialic acid synthetase)nanA 双突变体中游离
硅铝酸盐的积累,确定了
硅铝酸盐
醛缩酶(由 nanA 编码)在限制合成途径中间通量方面的功能。综合结果表明了大肠杆菌如何避免
生物合成的
硅铝酸盐诱导系统自身降解的徒劳循环,并表明了通过有针对性地操纵
硅铝酸盐代谢生成
硅铝
寡糖前体的可行性。