Krebs/ Citric Acid/ Tricarboxylic Cycle
Known by many names, Krebs cycle or citric acid cycle (CAC) takes place in the matrix of mitochondria. This is where the mitochondrial DNA is found and where fatty acid breakdown takes place. The matrix has a much higher protein concentration than in the cytosol, which is the surrounding cellular fluid all around the mitochondria.
The Krebs cycle contributes indirectly to the cellular ATP supplies. The cycle involves 8 chemical reactions that use Acetyl CoA and Oxaloacetic acid to produce 6 NADH, 2 FADH, and 4 CO2 and 2 ATP (per 2 pyruvate).
The CAC cycle includes 8 major steps. In the first step of the cycle, a 2-carbon molecule (Acetyl-CoA) and a 4-carbon molecule (Oxaloacetate) are combined to form a 6-carbon molecule known as, “Citrate.” The Citrate molecule will undergo multiple chemical changes and at the end it will go back to its original 4-Carbon structure. Every time a carbon molecule loses one carbon, one CO2 is produced. Thus, 2 CO2 molecules are generated during the conversion of citrate molecule to oxaloacetate molecule.
Electron Transport Chain and Oxidative Phosphorylation:
Now, you may have noticed that the preceding reaction processes don’t actually produce the enormous amounts of ATP that middle school science classes said they would. Though some amazing science happens in the past few reactions, they were all in preparation for the real ATP maker: the electron transport chain and oxidative phosphorylation process. The previous reactions produced electron carrier molecules, ie. NADH and FADH2, which are now used in this process of 4 different reactions to power various protein complexes (I, II, III, IV). The electrons from the electron carriers are transferred to the complexes to power their actions, which primarily is to pump H+ protons from the mitochondrial matrix to the intermembrane space. Electrons are also shared through the different complexes in a cascading effect through inner membrane mobile electron carriers ubiquinone (coenzyme Q) and cytochrome C. The cascading electrons finish their journey through the entire aerobic metabolic cycle with an O2 molecule, producing H2O.
All of the protons which are concentrated on the intermembrane space are then utilized for powering the main powerhouse, the ATP synthase. The ATP synthase is an embedded protein, similar to a motor, which passes protons through its passage through to the other side. The passing protons power a rotary motor, literally spinning the protein, and inducing conformational changes on the surface of the protein. These conformational changes open up particular active sites on the synthase protein which allows for ADP and Pi to bind and phosphorylate to produce ATP. Unlike many of the other mentioned processes, this reaction is powered not by potential energy, but by kinetic energy of the rotating ATP synthase.