Abstract
Succinate dehydrogenase (SDH) is an inner mitochondrial membrane protein complex that links the Krebs cycle to the electron transport system. It can produce significant amounts of superoxide ([Formula: see text]) and hydrogen peroxide (H2O2); however, the precise mechanisms are unknown. This fact hinders the development of next-generation antioxidant therapies targeting mitochondria. To help address this problem, we developed a computational model to analyze and identify the kinetic mechanism of [Formula: see text] and H2O2 production by SDH. Our model includes the major redox centers in the complex, namely FAD, three iron-sulfur clusters, and a transiently bound semiquinone. Oxidation state transitions involve a one- or two-electron redox reaction, each being thermodynamically constrained. Model parameters were simultaneously fit to many data sets using a variety of succinate oxidation and free radical production data. In the absence of respiratory chain inhibitors, model analysis revealed the 3Fe-4S iron-sulfur cluster as the primary [Formula: see text] source. However, when the quinone reductase site is inhibited or the quinone pool is highly reduced, [Formula: see text] is generated primarily by the FAD. In addition, H2O2 production is only significant when the enzyme is fully reduced, and fumarate is absent. Our simulations also reveal that the redox state of the quinone pool is the primary determinant of free radical production by SDH. In this study, we showed the importance of analyzing enzyme kinetics and associated side reactions in a consistent, quantitative, and biophysically detailed manner using a diverse set of experimental data to interpret and explain experimental observations from a unified perspective.
Highlights
Succinate dehydrogenase (SDH) is a heterotetrametric protein attached to the mitochondrial inner membrane of eukaryotes and many bacteria
This manner of reduction can be inhibited by atpenins, potent competitive inhibitors of quinone binding at the Qp site
Using Saccharomyces cerevisiae as an experimental model, several groups determined that the Q site is a strong contender for most reactive oxygen species (ROS) production [38, 53]
Summary
Succinate dehydrogenase (SDH) is a heterotetrametric protein attached to the mitochondrial inner membrane of eukaryotes and many bacteria. Mammalian SDH kinetics and free radical production needed to sustain the reverse reaction of complex II [21]. In this setting, succinate replaces oxygen as the final electron acceptor. The combination of a highly reduced Q pool and a hyperpolarized membrane potential drives complex I to enter the so-called reverse electron transport (RET) state. Mutations that alter ROS production by SDH have been implicated in some cancers [31,32,33] These new lines of evidence inevitably raise the question that SDH plays a more deterministic role in health and disease than previously thought. They suggest that the enzyme is an overlooked target in developing therapies that target mitochondrial oxidative stress
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