Pannala Venkat R, Dash Ranjan K
Biotechnology and Bioengineering Center and Department of Physiology, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA.
Biotechnology and Bioengineering Center and Department of Physiology, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA.
Free Radic Biol Med. 2015 Jan;78:42-55. doi: 10.1016/j.freeradbiomed.2014.10.508. Epub 2014 Oct 29.
The thioredoxin system, which consists of a family of proteins, including thioredoxin (Trx), peroxiredoxin (Prx), and thioredoxin reductase (TrxR), plays a critical role in the defense against oxidative stress by removing harmful hydrogen peroxide (H2O2). Specifically, Trx donates electrons to Prx to remove H2O2 and then TrxR maintains the reduced Trx concentration with NADPH as the cofactor. Despite a great deal of kinetic information gathered on the removal of H2O2 by the Trx system from various sources/species, a mechanistic understanding of the associated enzymes is still not available. We address this issue by developing a thermodynamically consistent mathematical model of the Trx system which entails mechanistic details and provides quantitative insights into the kinetics of the TrxR and Prx enzymes. Consistent with experimental studies, the model analyses of the available data show that both enzymes operate by a ping-pong mechanism. The proposed mechanism for TrxR, which incorporates substrate inhibition by NADPH and intermediate protonation states, well describes the available data and accurately predicts the bell-shaped behavior of the effect of pH on the TrxR activity. Most importantly, the model also predicts the inhibitory effects of the reaction products (NADP(+) and Trx(SH)2) on the TrxR activity for which suitable experimental data are not available. The model analyses of the available data on the kinetics of Prx from mammalian sources reveal that Prx operates at very low H2O2 concentrations compared to their human parasite counterparts. Furthermore, the model is able to predict the dynamic overoxidation of Prx at high H2O2 concentrations, consistent with the available data. The integrated Prx-TrxR model simulations well describe the NADPH and H2O2 degradation dynamics and also show that the coupling of TrxR- and Prx-dependent reduction of H2O2 allowed ultrasensitive changes in the Trx concentration in response to changes in the TrxR concentration at high Prx concentrations. Thus, the model of this sort is very useful for integration into computational H2O2 degradation models to identify its role in physiological and pathophysiological functions.
硫氧还蛋白系统由一组蛋白质组成，包括硫氧还蛋白（Trx）、过氧化物还原酶（Prx）和硫氧还蛋白还原酶（TrxR），通过清除有害的过氧化氢（H2O2）在抵御氧化应激中起关键作用。具体而言，Trx将电子传递给Prx以清除H2O2，然后TrxR以NADPH作为辅助因子维持还原型Trx的浓度。尽管已经从各种来源/物种收集了大量关于硫氧还蛋白系统清除H2O2的动力学信息，但对相关酶的机制理解仍然缺乏。我们通过开发一个热力学一致的硫氧还蛋白系统数学模型来解决这个问题，该模型包含机制细节，并提供对TrxR和Prx酶动力学的定量见解。与实验研究一致，对现有数据的模型分析表明，这两种酶均通过乒乓机制起作用。所提出的TrxR机制，包括NADPH的底物抑制和中间质子化状态，很好地描述了现有数据，并准确预测了pH对TrxR活性影响的钟形行为。最重要的是，该模型还预测了反应产物（NADP(+)和Trx(SH)2）对TrxR活性的抑制作用，而目前尚无合适的实验数据。对来自哺乳动物来源的Prx动力学现有数据的模型分析表明，与人类寄生虫对应物相比，Prx在非常低的H2O2浓度下起作用。此外，该模型能够预测在高H2O2浓度下Prx的动态过氧化，与现有数据一致。整合的Prx-TrxR模型模拟很好地描述了NADPH和H2O2的降解动力学，并且还表明，在高Prx浓度下，TrxR和Prx依赖性H2O2还原的耦合允许Trx浓度响应TrxR浓度的变化而发生超敏感变化。因此，这种类型的模型对于整合到计算H2O2降解模型中以确定其在生理和病理生理功能中的作用非常有用。
