Krebs Cycle

Several things occur in the Krebs cycle. It takes place in the mitochondrial matrix. The electron transport chain is located in the cristae of a mitochondria. The enzymes used during the Krebs cycle are found within the mitochondrial matrix excluding succinate dehydrogenase, which is bound to the inner mitochondrial membrane. Protein complexes located in the inner membrane perform the transfer and the gradual release of energy is used to pump protons into the intermembrane space of the mitochondria. Some components of the electron transport system are embedded into the inner membrane.

The inner membrane is permeable to oxygen, carbon dioxide, and water. Its structure is highly complex, including all of the complexes of the electron transport system, the ATP synthetase complex, and transport proteins. The folds found on the inner membrane are ordered into lamillae (which are layers), also known as cristae. The inner mitochondrial membrane of certain tissues contain a large amount of thermogenin (an uncoupling protein) which acts as an uncoupler by forming another pathway for the protons to flow down from the electron transport system back to matrix.

The Krebs cycle begins with the oxidation of pyruvate. This produces one CO2 and one acetyl-CoA. The Acetyl-coA reacts with oxaloacetate to form citrate. Citrate is converted back to oxaloacetate through a series of reactions. This produces 2 cO2 and uses 3 NAD+ and 3 H+. It consumes 3 H20 and one FAD, which produces FADH+. After the first turn, one ATP is produced as well as 3 NADH, 1 FADH2, and 2 CO2. The cycle turns again, and the outcome produces a total of 2 ATP, 6 NADH, 2 FADH2, and 4 CO2. So for each molecule of glucose, six NADH2+, two FADH2, and two ATP are produced. During the oxidation process, most electrons (e-) are accepted by NAD+ and NADH is formed. Electrons can be taken by FAD, forming FADH2.

A substrate-level phosphorylation in the Krebs cycle generates GTP that then goes through a transphosphorylation with ADP catalyzed by nucleoside diphosphate kinase. Two succinate-CoA ligases with different nucleotide specificities integrate and regulate the central metabolic pathways that involve the Krebs cycle. The products of substrate-level phosphorylation, ATP and GTP, may represent a pool of nucleotide that has a different phosphorylation potential than the ATP made by oxidative phosphorylation and may be channeled to meet specific needs within mitochondria