Ch-Aa metabolism

• Amino acid metabolism involves L-α-amino acids, protein turnover, new protein synthesis, urea synthesis, and amphybolic pathways

  
• Oxidative degradation of amino acids provides 10-15% of metabolic energy and depends on the organism's metabolic state
• Factors affecting amino acid oxidation include protein breakdown, dietary intake, and metabolic conditions like starvation or uncontrolled diabetes mellitus
• Around 20 amino acids are present in the body
• Amino acids undergo transamination, deamination (except for lysine, threonine, proline, and hydroxyproline), and dehydratase processes
• The liver plays a crucial role in the degradation of branched-chain amino acids and the removal of amino groups through transamination
• Metabolic fates of amino groups involve recycling for biosynthetic pathways or excretion as urea or uric acid
• The urea cycle is a series of reactions converting ammonia to urea, essential for eliminating nitrogen waste, mainly occurring in the liver
• Glutamate and glutamine are key amino acids in nitrogen metabolism, acting as general collectors of amino groups
• Ammonia, a toxic substance, is converted to nontoxic forms like glutamine for transport and excretion
• Liver enzymes like glutaminase and glutamine synthetase prevent ammonia leakage and facilitate its conversion to urea for excretion
• The urea cycle is a series of biochemical reactions that converts ammonia to urea, taking place in the liver and essential for the body to eliminate nitrogen waste

  
• The first two steps of the urea cycle involve the formation of N-acetyl glutamate from acetyl-CoA and glutamate, and the formation of carbamoyl phosphate from bicarbonate and ATP

  
• The urea cycle involves four steps: transamination, oxidative deamination of glutamate, transport of ammonia, and the urea cycle itself
• Regulation of the urea cycle can occur through the availability of substrates, allosteric activation of CPS I with N-acetylglutamate (NAG), and enzyme induction/repression
• Enzyme defects in the urea cycle can lead to potentially fatal consequences, such as depleted ATP, loss of consciousness, hypothermia, mental retardation, and hyperammonemia
• Defects in enzymes like carbamoyl-P synthetase (CPSI) and ornithine transcarbamoylase can result in hiperammonemia, encephalopathy, respiratory alkalosis, and other severe symptoms
• Arginine degradation involves ornithine transaminase, leading to increased plasma and urine ornithine levels
• Tyrosine degradation includes the hydroxylation of C-4 of phenylalanine by phenylalanine hydroxylase in the liver
• Histidine metabolism can be affected by folic acid deficiency and glutamate-formimino transferase deficiency, leading to urine FIGLU in histidase deficiency
• Tryptophan metabolism involves enzymes dependent on vitamin B6, with its deficiency causing increased excretion of kynurenine and xanthurenic acid, leading to greenish-yellow urine
• Methionine metabolism includes homocystinuria and homocystinemia due to defects in enzymes like cystathionine β-synthase, vitamin B6 deficiency, vitamin B12 deficiency, folic acid deficiency, and cystathionase