Mechanistic Studies on Enzymatic Reactions by Electrospray Ionization MS Using a Capillary Mixer with Adjustable Reaction Chamber Volume for Time-Resolved Measurements
作者:Derek J. Wilson、Lars Konermann
DOI:10.1021/ac0355348
日期:2004.5.1
Mass spectrometry (MS)-based techniques have enormous potential for kinetic studies on enzyme-catalyzed processes. In particular, the use of electrospray ionization (ESI) MS for steady-state measurements is well established. However, there are very few reports of MS-based studies in the pre-steady-state regime, because it is difficult to achieve the time resolution required for this type of experiment. We have recently developed a capillary mixer with adjustable reaction chamber volume for kinetic studies by ESI-MS with millisecond time resolution (Wilson, D. J.; Konermann, L. Anal. Chem. 2003, 75, 6408−6414). Data can be acquired in kinetic mode, where the concentrations of selected reactive species are monitored as a function of time, or in spectral mode, where entire mass spectra are obtained for selected reaction times. Here, we describe the application of this technique to study the kinetics of enzyme reactions. The hydrolysis of p-nitrophenyl acetate by chymotrypsin was chosen as a simple chromophoric model system. On-line addition of a “makeup solvent” immediately prior to ionization allowed the pre-steady-state accumulation of acetylated chymotrypsin to be monitored. The rate constant for acetylation, as well as the dissociation constant of the enzyme−substrate complex obtained from these data, is in excellent agreement with results obtained by conventional stopped-flow methods. Bradykinin was chosen to illustrate the performance of the ESI-MS-based method with a nonchromophoric substrate. In this case, the unfavorable rate constant ratio for acylation and deacylation of the enzyme precluded measurements in the pre-steady-state regime. Steady-state experiments were carried out to determine the turnover number and the Michaelis constant for bradykinin. The methodologies used in this work open a wide range of possibilities for future ESI-MS-based kinetic assays in enzymology.
基于质谱(MS)的技术在酶催化过程的动力学研究方面潜力巨大。特别是,利用电喷雾离子化 (ESI) 质谱进行稳态测量已得到广泛认可。然而,基于 MS 的前稳态研究报告却很少,因为很难达到这类实验所需的时间分辨率。我们最近开发了一种可调节反应室容积的毛细管混合器,用于利用具有毫秒级时间分辨率的 ESI-MS 进行动力学研究(Wilson, D. J.; Konermann, L. Anal. Chem. 2003, 75, 6408-6414)。数据可以在动力学模式下获取,在该模式下,选定反应物的浓度随时间变化而受到监控;也可以在光谱模式下获取,在该模式下,选定反应时间的整个质谱图都会被获取。在此,我们将介绍如何应用该技术研究酶反应的动力学。糜蛋白酶水解对硝基苯乙酸酯的过程被选为一个简单的发色模型系统。在电离前立即在线添加 "补充溶剂",可监测乙酰化糜蛋白酶的前稳态积累。从这些数据中得到的乙酰化速率常数以及酶-底物复合物的解离常数与传统的停流方法得到的结果非常吻合。选择缓激肽来说明基于 ESI-MS 的方法在处理非色素底物时的性能。在这种情况下,酶的酰化和脱酰化的不利速率常数比排除了在前稳态状态下进行测量的可能性。为了确定缓激肽的周转次数和迈克尔常数,我们进行了稳态实验。这项工作中使用的方法为未来基于 ESI-MS 的酶学动力学测定提供了广泛的可能性。