Mitochondrial Complex III, (Cyt. bc1 complex; CoQ-Cyt. c reductase), is responsible for energy production through oxidative phosphorylation. This enzyme complex in the inner mitochondrial membrane plays a central role in the transfer of electrons from ubiquinol to Cyt. c, ultimately contributing to the generation of ATP. The important Complex III components are: Cytochrome b (with two hemes, bL and bH), Cyt. c1 and a high-potential iron-sulfur cluster protein. The complex contains two distinct ubiquinone binding sites: the cytoplasmic Qo site and the matrix side Qi site, each involved in different steps of the electron transfer process. Complex III accepts electrons from Complex I and Complex II in the form of ubiquinol. Complex III operates the “Q-cycle” which includes: - Oxidation of ubiquinol at the Qo site, releasing two H+ into the intermembrane space; - Sequential transfer of electrons: one to Cyt. c via the iron-sulfur cluster and Cyt. c1, and the other through Cyt. bL and bH to reduce another ubiquinone at the Qi site; - Recycling of ubiquinone and pumping of additional protons, contributing to the proton gradient across the inner mitochondrial membrane, necessary for ATP synthesis at Complex V. Ubiquinone can pass electrons to O2 forming superoxide (an important ROS, which can leak into the mitochondrial matrix and the intermembrane space and act as a signaling molecule or induce oxidative stress. AkrivisBio’s Mitochondrial Complex III Activity Assay is a simple, sensitive means of following Complex III activity in isolated mitochondria, with a sensitivity of about 1 mU of Complex III activity per well.
