In the citric acid cycle, the step that allowed us to form the fadh two molecule is this step here. In this step, Succinate is oxidized into fumarate, and the fad molecule is reduced into fadh two. So these two H atoms, along with one electron each, are extracted. They bind onto fad to form the fadh two. And these two electrons left over here, one here and one here, create a double bond to form this fumarate molecule. And the enzyme that catalyzes this step of the citric acid cycle is known as Succinate dehydrogenase.