The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that processes acetyl CoA to produce energy. In each turn of the cycle, one mole of acetyl CoA is converted into two moles of carbon dioxide (CO2), generating one mole of FADH2, three moles of NADH, and one mole of GTP, which can be converted to ATP. It's important to note that while citrate is produced during the cycle, it is not considered a final product since it is subsequently utilized in the cycle.
One significant reaction in the citric acid cycle is the conversion of alpha-ketoglutarate to succinyl CoA, which is catalyzed by alpha-ketoglutarate dehydrogenase. This reaction shares similarities with the pyruvate dehydrogenase reaction, as both involve the generation of NADH and the use of cofactors such as TPP, lipoate, and CoA. However, the specific substrates and cofactors differ between these two dehydrogenase reactions.
Another key reaction is the conversion of succinyl CoA to succinate, which produces GTP through substrate-level phosphorylation. This process involves breaking a thioester bond, releasing energy that facilitates the conversion of GDP and inorganic phosphate into GTP. Although GTP can be converted to ATP, it is not always utilized for this purpose.
In terms of molecular structure, citrate is considered prochiral, meaning it is symmetric but behaves as if it were chiral during interactions with enzymes like aconitase. In contrast, succinate, while also symmetric, does not exhibit prochirality as it can be randomized in its orientation during reactions with succinate dehydrogenase.
When considering the overall conversion of pyruvate to carbon dioxide, the process yields four moles of NADH (one from pyruvate oxidation and three from the citric acid cycle), one mole of FADH2, and one mole of ATP, highlighting the cycle's efficiency in energy production.
Additionally, the glyoxylate cycle, which is not present in humans but occurs in plants, fungi, and bacteria, allows these organisms to utilize acetyl CoA for energy and the synthesis of biosynthetic precursors. This cycle enables the conversion of acetyl CoA back into carbohydrates, a process that is not possible in human metabolism.
