Emiola Akintunde, George John, Andrews Steven S
School of Health, Sports and Bioscience, University of East London, London, United Kingdom.
Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America.
PLoS One. 2015 Apr 28;10(4):e0121216. doi: 10.1371/journal.pone.0121216. eCollection 2014.
Lipid A is a highly conserved component of lipopolysaccharide (LPS), itself a major component of the outer membrane of Gram-negative bacteria. Lipid A is essential to cells and elicits a strong immune response from humans and other animals. We developed a quantitative model of the nine enzyme-catalyzed steps of Escherichia coli lipid A biosynthesis, drawing parameters from the experimental literature. This model accounts for biosynthesis regulation, which occurs through regulated degradation of the LpxC and WaaA (also called KdtA) enzymes. The LpxC degradation signal appears to arise from the lipid A disaccharide concentration, which we deduced from prior results, model results, and new LpxK overexpression results. The model agrees reasonably well with many experimental findings, including the lipid A production rate, the behaviors of mutants with defective LpxA enzymes, correlations between LpxC half-lives and cell generation times, and the effects of LpxK overexpression on LpxC concentrations. Its predictions also differ from some experimental results, which suggest modifications to the current understanding of the lipid A pathway, such as the possibility that LpxD can replace LpxA and that there may be metabolic channeling between LpxH and LpxB. The model shows that WaaA regulation may serve to regulate the lipid A production rate when the 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) concentration is low and/or to control the number of KDO residues that get attached to lipid A. Computation of flux control coefficients showed that LpxC is the rate-limiting enzyme if pathway regulation is ignored, but that LpxK is the rate-limiting enzyme if pathway regulation is present, as it is in real cells. Control also shifts to other enzymes if the pathway substrate concentrations are not in excess. Based on these results, we suggest that LpxK may be a much better drug target than LpxC, which has been pursued most often.
脂多糖（LPS）是革兰氏阴性菌外膜的主要成分，而脂质A是脂多糖中高度保守的组分。脂质A对细胞至关重要，并能引发人类和其他动物强烈的免疫反应。我们利用实验文献中的参数，建立了大肠杆菌脂质A生物合成九个酶催化步骤的定量模型。该模型考虑了生物合成调控，这种调控通过LpxC和WaaA（也称为KdtA）酶的调控降解来实现。LpxC降解信号似乎源于脂质A二糖浓度，这是我们根据先前结果、模型结果和新的LpxK过表达结果推导出来的。该模型与许多实验结果相当吻合，包括脂质A的产生速率、LpxA酶缺陷型突变体的行为、LpxC半衰期与细胞世代时间的相关性以及LpxK过表达对LpxC浓度的影响。其预测结果也与一些实验结果不同，这表明需要对目前对脂质A途径的理解进行修正，例如LpxD可能替代LpxA的可能性，以及LpxH和LpxB之间可能存在代谢通道化。该模型表明，当2-酮-3-脱氧-D-甘露糖辛酸（KDO）浓度较低时，WaaA调控可能用于调节脂质A的产生速率，和/或控制连接到脂质A上的KDO残基数量。通量控制系数的计算表明，如果忽略途径调控，LpxC是限速酶，但如果存在途径调控（实际细胞中就是如此），LpxK是限速酶。如果途径底物浓度不过量，控制也会转移到其他酶上。基于这些结果，我们认为LpxK可能是比LpxC更好的药物靶点，而LpxC一直是研究最多的靶点。
