During each Krebs cycle, eight reactions should occur. As the reactions happen, the energy currency of the cell, adenosine triphosphate, known as ATP, is made.
Electrons are also given off during the Krebs cycle, which fuel the process of oxidative phosphorylation. Oxidative phosphorylation is a major source of ATP and energy.
The first reaction in the cycle is the addition of an acetyl group to oxaloacetate by citrate synthase. Citrate is then converted to isocitrate by aconitase. In the third reaction, isocitrate dehydrogenase removes a carbon atom from isocitrate to form carbon dioxide and 2-oxoglutarate. During this process, electrons are transferred to the carrier molecule NADH. In the fourth step, the 2-oxoglutarate dehydrogenase complex removes another carbon atom to form carbon dioxide, transfers electrons to NADH and transfers the remaining molecule to coenzyme A to form succinyl-CoA. In the fifth step, succinyl-CoA synthase makes GTP and succinate from succinyl-CoA. GTP is then converted to ATP. In the sixth reaction, the succinyl dehydrogenase complex extracts a hydrogen atom from succinate to form fumarate. In the seventh reaction, fumarase adds water to fumarate to form malate. In the final reaction, malate dehydrogenase produces oxaloacetate from malate, producing electrons in the process, which are transferred to NADH.