Computational Analysis of Reactive Oxygen Species Generation by Mitochondria Resulting from Substrate Manipulation

Abstract

Mitochondria tightly couple the transport of electrons and proton pumping with oxidative phosphorylation to synthesize ATP. However, such energetic processes can “leak” electrons to oxygen forming reactive oxygen species (ROS). Increased ROS associated with oxidative stress has been implicated in numerous pathological conditions. To better understand the complex system regulating ROS, we present a mechanistic mitochondrial model that includes the electron transport chain, tricarboxylic acid cycle kinetics, mitochondrial calcium, sodium and proton handling, membrane ion transport processes that is coupled to oxidative phosphorylation, generation of ROS by complexes I and III, and matrix and extramitochondrial antioxidant defenses. Consistent with earlier study we found that in heart mitochondria respiring in presence of complex-II linked substrate succinate there is an increased ROS production with increasing membrane potential. Studies from experiments suggest that this relationship between ROS production and ΔѰm due to substrate modification is attributed to the process of Reverse Electron Transport. However, the model suggests that ROS generation with increasing membrane potential in presence of succinate is not because of reverse electron flow, but occurs because of the fact that as the mitochondrial membrane potential increases, it becomes harder for the protons to be pumped out to the p-side, thereby decreasing proton pumpingflux, and the most favorable pathway for the electrons is to move forward and cause the single electron reduction of oxygen to produce more ROS. Furthermore the mechanism of malonate induced reduction in ROS production is not because of decrease in reverse electron flow due to decrease in succinate, but occurs owing to the fact that since succinate oxidation decreases, there is tendency of electrons to flow across the respiratory chain to maintain the diminishing ΔѰm rather than leaking them to produce ROS.