Summary
Highlights
Proteins don't directly produce ATP but are crucial in its production, especially when carbs and fats are scarce. They are broken down into amino acids, which enter the Kreb cycle. Enzymes, which are proteins, catalyze metabolic pathways like glycolysis, the Kreb cycle, and oxidative phosphorylation, generating ATP.
Protein catabolism occurs during fasting, intense exercise, or when other energy sources are limited. Proteolysis, the breakdown of proteins into smaller polypeptides and amino acids, starts in the stomach with pepsin and continues in the small intestine with pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase). Final breakdown into free amino acids occurs at the brush border by peptidases, which are then absorbed into the bloodstream.
After proteolysis, free amino acids are transported to the liver, the primary site for their metabolism. Deamination is the removal of the amino group from amino acids, catalyzed by deaminases. The removed amino group forms toxic ammonia, which the liver converts into less toxic urea through the urea cycle (ornithine cycle). Urea is then excreted by the kidneys. The remaining carbon skeletons contribute to energy production.
Post-deamination, amino acid carbon skeletons are converted into various intermediates that enter metabolic pathways for energy. Key intermediates include pyruvate (from alanine, serine, glycine), Acetyl CoA (from leucine, isoleucine, lysine), Alpha-ketoglutarate (from glutamate, glutamine, arginine), Succinyl CoA (from isoleucine, methionine, valine, threonine), Fumarate (from phenylalanine, tyrosine), and Oxaloacetate (from aspartate, asparagine). These intermediates feed into the Kreb cycle or gluconeogenesis.
The Kreb cycle, located in the mitochondria, is central to aerobic respiration. It oxidizes Acetyl CoA (derived from carbohydrates, fats, and proteins) into carbon dioxide, producing NADH and FADH2, which are then used in the electron transport chain to generate ATP. The cycle involves several steps where intermediates like Acetyl CoA, oxaloacetate, citrate, isocitrate, alpha-ketoglutarate, succinyl CoA, succinate, fumarate, and malate are transformed, regenerating oxaloacetate to continue the cycle. Each turn produces three NADH, one FADH2, and one GTP, crucial for ATP production.