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Urea Cycle - Introduction - The urea cycle is linked to the oxidation of amino acids as an energy source. - It does not constitute a protein degradation pathway. Protein degradation comes from proteasomes or lysosomes. - Energy Sources - To obtain energy from molecules, they must be degraded into monomers and subunits. - Examples: - Proteins must be degraded into amino acids (sourced by proteasomes or lysosomes) - Glycogen must be degraded into glucose monomers. - DNA is degraded into nucleotides, which can serve as an energy source (ATP, TTP, CTP, GTP). - Triglycerides must be degraded into free fatty acids to be oxidized. - Role of the Urea Cycle - The urea cycle functions as a "waste collector". - It eliminates toxic amino groups, allowing the use of other metabolic pathways (glycolysis and the citric acid cycle) for oxidation. - The urea cycle does not generate ATP production. - Types of Amino Acids - Glucogenic amino acids - Their destiny is complete oxidation via: 1. Glycolysis

Pyruvate

Acetyl-CoA

Citric Acid Cycle

Respiratory Chain (for complete oxidation). - Used for glucose production (gluconeogenesis) in the liver or glycolysis in muscles. - Ketogenic amino acids - Their destiny is acetyl-CoA production. - Acetyl-CoA can enter the citric acid cycle for oxidation or be converted into ketone bodies in case of accumulation. - Cannot enter the glycolysis cycle or be used for gluconeogenesis. - Physiological Conditions for Protein Degradation - In a fasting state, proteins are degraded into amino acids for energy production. - In case of excess dietary protein, amino acids are degraded to obtain energy via lysosomal pathways. - Protein Degradation - Lysosomal degradation - Selection of specific proteins via the KFERQ motif. - Chaperones: proteins that identify the KFERQ sequence to bring proteins to the lysosome, where they are cut and degraded, allowing the activation of the urea cycle. - Proteasomal degradation - Always active, aiming to eliminate old or damaged proteins through ubiquitination. - A process that requires several interactions with ubiquitin enzymes. - Urea Cycle - The urea cycle primarily occurs in the mitochondria and the cytosol. The preparation process takes place in the mitochondria, while termination and urea production happen in the cytosol. - Key steps of the cycle 1. OAA (Oxaloacetate) is transformed by aminotransferase into aspartate (transfer of an amino group from one amino acid to another) 2. Carbamoyl phosphate (produced from bicarbonate) is transformed into citrulline. 3. OAA and $\alpha$-ketoglutarate consumption in the urea cycle; the cycle uses amino acids to produce urea which will be excreted. - Ammonia Intoxication - Ammonia is toxic: it causes the formation of free radicals. For this reason, it is converted into urea to be safely eliminated from the blood. - Interconnections with Other Pathways - Products of the urea cycle can feed into other metabolic pathways, such as glycolysis or the citric acid cycle, but this is not their primary function. ## Lipolysis and Lipases - Lipases - Hormone-sensitive lipase, pancreatic lipases, and lipoprotein lipases. - Hormone-sensitive lipase: activated by hormones, especially in situations of energy need when glucose is low. - Pancreatic lipase: active in the intestinal lumen for the digestion of specific fats, primarily acting during food consumption. - Lipoprotein lipases: activated by apolipoproteins, permanently available in the blood, while recycling fatty acids from lipoproteins. - Lipase Activations - Hormone-sensitive lipase: Activated by glucagon. - Lipoprotein lipase: Activated by the presence of apolipoproteins, especially Apo C-II. - Pancreatic lipase: Active during food consumption, often associated with insulin release. - Regulation - Insulin inhibits access to fatty acids in the presence of high glucose, while epinephrine (adrenaline) increases the need for fatty acid release. - Calculation of Energy Yields - For a $10$-carbon fatty acid: - How many rounds of $\beta$-oxidation? Answer: $4$. - Production for each round of $\beta$-oxidation: $1$ Acetyl-CoA, $1$ NADH, $1$ FADH$. - <strong>NADH produced</strong>:$2.5$ATP per NADH. - <strong>FADH₂ produced</strong>:$1.5$ATP per FADH₂. - <strong>Total</strong>: Approximately$64$$ ATP produced after deducting activation costs. - Conclusions - The urea cycle is essential for eliminating nitrogenous waste while supporting related metabolic pathways. - Lipases are involved in lipid metabolism, crucial for managing energy reserves, connecting digestion to cellular utilization. - Other considerations - Importance of metabolic regulation based on substrate availability and the need to maintain energy homeostasis (utilization of amino acids, carbohydrates, and fats under various circumstances). - Clinical relevance - Understanding these cycles is essential for managing pathologies such as obesity, diabetes, or metabolic diseases, where the management of body nutrients and energy is crucial.