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Chapter 18 Amino Acid Catabolism
Amino Acid Oxidation and Urea Production
Dietary Protein Digestion and Absorption
Proteins are broken down into amino acids through the digestive process.
Amino acids are the building blocks used in various metabolic pathways.
Key Metabolites and Pathways
Amino acids dispersed into key metabolic intermediates:
Acetyl CoA: Central compound in metabolism, utilized in pathways like the Krebs cycle.
Oxaloacetate: Vital for gluconeogenesis and amino acid metabolism.
Urea Cycle: Responsible for converting toxic ammonia from amino acid breakdown into urea for excretion.
Amino Acid Groups
Glucogenic Amino Acids: Can be converted into glucose (e.g., Alanine, Aspartate).
Ketogenic Amino Acids: Can be converted into ketone bodies (e.g., Leucine, Lysine).
Some amino acids are both glucogenic and ketogenic (e.g., Isoleucine, Phenylalanine).
Amino Acid Catabolism
Overview of Toxicity and Energy Utilization
Amino acids contain an amino group (toxic) and a carbon skeleton (energy).
Main processes:
Transamination: Removal of amino group to form glutamate and a-keto acid.
Deamination: Further processing of glutamate to release ammonia into the urea cycle.
Outline of Amino Acid Catabolism Sections
Digestion of amino acids.
Toxicity management: Transamination and Deamination reactions.
Formation of Carbamoyl Phosphate.
Transition to the Urea Cycle.
Metabolism of carbon skeletons into metabolic intermediates (e.g., Pyruvate, Succinyl-CoA).
Urea Cycle and Ammonia Excretion
Function and Importance
The Urea Cycle excretes nitrogenous waste in the form of urea, which is less toxic than ammonia and conserves water.
Needs energy (ATP) and water for processing.
Steps of the Urea Cycle
Carbamoyl Phosphate formation from ammonia and bicarbonate.
Conversion of Carbamoyl Phosphate to Citrulline.
Conversion of Citrulline to Argininosuccinate.
Decomposition of Argininosuccinate to form Arginine and Fumarate, and then to Urea.
Regulation by the availability of substrates, particularly during high protein intake.
Regulation of Amino Acid Catabolism
Long-term Regulation
Increased protein intake prompts the synthesis of urea cycle enzymes.
Decreased protein intake results in reduced enzyme concentrations.
Short-term Regulation
Arginine can allosterically activate the enzyme for carbamoyl phosphate formation, accelerating catabolism in a high protein diet.
Genetic Disorders and Metabolism
Common Disorders Affecting Amino Acid Catabolism
Phenylketonuria (PKU): Caused by the deficiency in phenylalanine hydroxylase, leads to accumulation of phenylalanine.
Tyrosinemia: Related to defects in enzymes for tyrosine metabolism.
Symptoms range from mental retardation to failure to thrive in untreated individuals.
Cofactors in Amino Acid Metabolism
PLP (Pyridoxal Phosphate): Essential for transamination reactions.
Biotin, S-Adenosylmethionine, and Tetrahydrofolate: Key cofactors participating in various amino acid transformations.
Conclusion
Amino acid catabolism involves complex cycles for energy and waste management, tightly regulated by nutrient availability and enzymatic activity. Understanding each step and its regulation impacts both health and treatment of metabolic disorders.
