Integrated Computational Modeling of the Kinetics and Regulation of NADPH Oxidase 2 Assembly, Activation, Electron Flow, and Superoxide Production
Authors: Sadri Shima , Tomar Namrata , Cowley Allen W. , Audi Said H. , Dash Ranjan K. 
Published: April 2020

IntroductionReactive oxygen species (ROS) play a crucial role in key physiological processes, including cell signaling. However, ROS overproduction leads to oxidative stress, which plays a critical role in cell injury/death and pathogenesis of many diseases, including cardiovascular disease. Members of the NADPH oxidase (NOX) family that comprise of membrane and cytosolic components are known to be the major non‐mitochondrial sources of ROS in most mammalian cells. NOX2 is the most complex, widely expressed, and well‐studied of the NOX isoforms. NOX2 is activated upon assembly of its membrane‐bound subunits gp91phox and p22phox with the cytosolic subunits p40phox, p47phox, p67phox, and Rac facilitating electron flow and ROS production. Specifically, upon NOX2 assembly and activation, electrons are transferred from substrate NADPH to molecular O2 through different redox centers of NOX2 complex (further regulated by pH) resulting in oxidized NADP+ and reduced O2−. (superoxide). Yet, there is a lack of mechanistic and quantitative understanding of the kinetics and regulation of NOX2 assembly and the relative contributions of NOX2 subunits towards NOX2 activation, electron flow, and ROS production.MethodWe developed an integrated computational model to quantitatively describe the kinetics and regulation of NOX2 assembly, activation, electron flow, and ROS production. The model incorporates our hypothesized “random rapid equilibrium binding mechanism” for NOX2 assembly and activation, regulations by guanine nucleotides (GTP, GDP), and mutual binding enhancements between individual cytosolic subunits (p40phox, p47phox, p67phox, Rac). The model also considers the thermodynamics of electron transfer through different redox centers of NOX2 complex and biphasic inhibition by pH. The model uses diverse published experimental data to modularly estimate the unknown model parameters using the built‐in “fmincon” optimization algorithm in MATLAB.ResultsThe modeling and analysis show that NOX2 is differentially activated and regulated by p40phox, p47phox, p67phox, and Rac subunits. In addition, the assembly of NOX2 subunits is regulated by GTP, GDP, and other cytosolic subunits. The model can quantify enhancements in the binding affinities of p47phox, p67phox and Rac subunits by GTP, reductions in the binding affinities of the same cytosolic subunits by GDP, mutual binding enhancement between p47phox, p67phox and Rac subunits, and enhancement in the binding affinity of p47phox by p40phox. The model can also quantify the binding affinities of NADPH and O2 for NOX2 complex and the maximal NOX2 activities as functions of pH.ConclusionThe proposed model provides a quantitative and integrated understanding of the kinetics and regulation of NOX2 assembly, activation, and the importance of these processes in facilitating electron transfer and ROS production under different conditions. The model also serves as a mechanistic and quantitative framework for investigating the critical role of NOX2 complex‐mediated ROS production in regulating diverse cellular mechanisms under physiological and pathophysiological conditions.Support or Funding InformationNIH P01‐GM066730, P01‐HL116264, U01‐HL122199