Chapter 20-Amino Acid Metabolism

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Transport of nitrogen: → Apoptosis- programmed cell death- and protein turnover (majority of proteins turn over every few days) lead to an excess of NH 3 as proteins are degraded. Free NH 3 is toxic . Most NH 3 is transported in blood as Gln or Ala (from the alanine cycle) to the liver. Urea Cycle Urea is produced in a 5 step process in the liver. The urea cycle was proposed by Krebs. Nitrogens in urea come from NH 3 and Asp. See figure 21.12 in book for the urea cycle. The urea cycle requires 4 ATP/cycle. →Step 1: Carbamoyl phosphate is formed from ammonia, bicarbonate and ATP.
→Step 2: citrulline is formed from ornithine and carbamoyl phosphate by the enzyme ornithine transcarbamoylase. →Step 3: :arginosuccinate is formed from citrulline and Aspartic acid via a citrullyl-AMP intermediate by the enzyme argininosuccinate synthetase. (This step requires 2 ATP) →Step 4: Argininosuccinate is converted to fumarate (TCA intermediate) and arginine by the enzyme argininosuccinate lyase. →Step 5: Arginine is hydrolyzed to form ornithine and urea by the enzyme arginase. The ornithine can then combine with carbmoyl phosphate to begin the cycle again. → Note the "linking" of the Urea Cycle and the TCA cycle (the so-called Krebbs "bicycle"). →Steps 1 and 2 occur in the mitochondria, and steps 3-5 occur in the cytosol. There is a citruline/ornithine exchange transpoter in the inner mitochondrial membrane. →The urea then travels to the kidneys for filtration and excretion. →The main regulation is controlled by need and diet. A high protein diet and starvation activate the cycle. N-acetylglutamate activates carbamoyl phosphate synthase. Metabolic Disorders: NH 3 is toxic because excess NH 3 converts a-ketoglutarate to Glu. This results in a break in the TCA cycle, reduced ATP, and leads to coma and death. →The therapy to treat any metabolic disorders that result in a decrease in the urea cycle are (1) limit protein intake, (2) remove excess NH3 with a compound the will bind the NH3 and be excreted, and (3) replace TCA cycle intermediates. Tyrosine Biosynthesis
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Chapter 20-Amino Acid Metabolism

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