Summary
Highlights
Considering that one glucose molecule produces two pyruvate molecules, and thus two acetyl-CoA molecules, the Krebs cycle yields are doubled for each glucose. The cycle produces 4 carbon dioxide molecules (2 per acetyl-CoA), 6 NADH molecules (3 per acetyl-CoA), 2 FADH2 molecules (1 per acetyl-CoA), and 2 ATP molecules (1 per acetyl-CoA) directly from the Krebs cycle loop.
The Krebs cycle is a central metabolic pathway where amino acids can feed in and out at various points, such as through alpha-ketoglutarate or succinate. Fatty acids can also enter the cycle as acetyl-CoA. This flexibility allows the body to utilize different macronutrients for energy or synthesize them as needed.
In situations of glucose starvation (e.g., low-carb diets), oxaloacetate can be pulled out of the Krebs cycle to be converted back into glucose. If oxaloacetate is depleted, incoming acetyl-CoA (from fatty acid breakdown) has nothing to bind to, leading to its accumulation. Accumulated acetyl-CoA then combines to form ketones, which can be released into the bloodstream and used by the brain for energy, as the brain typically has sufficient oxaloacetate.
Succinyl-CoA releases its CoA to become succinate, a four-carbon molecule. This release of energy allows for the direct production of ATP (or GTP, which can then transfer its phosphate to ADP to make ATP). The enzyme responsible is succinyl-CoA synthetase. Succinate then converts to fumarate, reducing FAD to FADH2, catalyzed by succinate dehydrogenase. Fumarate becomes malate via fumarase, adding water, and finally, malate converts back to oxaloacetate, reducing NAD+ to NADH, through malate dehydrogenase.
The video begins with a quick recap of glycolysis, where glucose is broken down into two pyruvate molecules, producing NADH and ATP. Pyruvate, a three-carbon molecule, needs to enter the mitochondria for the Krebs cycle but must first be transformed into acetyl-CoA, a two-carbon molecule. This conversion involves losing a carbon as carbon dioxide, adding coenzyme A (CoA), and reducing NAD+ to NADH.
The process of NAD+ becoming NADH is crucial. NAD+ steals two hydrogen atoms from carbon molecules. One hydrogen atom is taken as a whole (proton and electron), while the other is taken as just an electron, leaving a free proton (H+) in the solution. This means NADH carries both hydrogens and electrons, which are vital for the electron transport chain.
The conversion of pyruvate to acetyl-CoA relies on several B vitamin derivatives. Thiamine pyrophosphate (TPP, from vitamin B1) is needed to remove carbon dioxide. Pantothenic acid (from vitamin B5) is required for coenzyme A, and niacin (from vitamin B3) is part of NAD+. This highlights why B vitamin complexes are essential for cellular respiration. The enzyme facilitating this step is pyruvate dehydrogenase.
Acetyl-CoA, a two-carbon molecule with a CoA, enters the mitochondria. It binds with oxaloacetate, a four-carbon molecule, to form citrate, a six-carbon molecule. The CoA detaches during this process, facilitated by the enzyme citrate synthase. Citrate then rearranges into isocitrate, a six-carbon molecule, with the help of aconitase, involving the removal and addition of water.
Isocitrate converts to alpha-ketoglutarate, a five-carbon molecule, by losing a carbon as carbon dioxide. This step is catalyzed by isocitrate dehydrogenase and also involves the reduction of NAD+ to NADH. Alpha-ketoglutarate then becomes succinyl-CoA, a four-carbon molecule, by losing another carbon as carbon dioxide, gaining a CoA, and producing another NADH via alpha-ketoglutarate dehydrogenase.