BCH2022 Study Guide - Final Guide: Carbamoyl Phosphate, Citric Acid Cycle, Urea Cycle
Urea Cycle
NH3 + CO2 + aspartate + 3 ATP + 2H2O → urea (CH4N2O) + 2 ADP + 2 Pi + AMP +
PPi
Function
• To eliminate excess nitrogen
Location
• In the liver
• Mitochondrial matrix and cytosol
Starting Point
• Urea is produced from ammonia
Synthesis of carbamoyl phosphate (CP) (enzyme)
• Enzyme: carbamoyl phosphate (CP)
1. Ornithine + CP → Citrulline
o CP requires ATP → occurs in mitochondrial matrix
2. Citrulline + Aspartate → Argininosuccinate
o Occurs: mitochondria
3. Argininosuccinate → Arginine + Fumarate
o Occurs: cytosol
4. Arginine → Urea + Ornithine
o Ornithine: recycled back to mitochondria matrix to enable urea cycle
o Occurs in: cytosol
Know ATP is required
Connection with Citric Acid Cycle
• Fumarate is an intermediate in citric acid cycle (step 7): oxidised to form
Malate
Regulation of Urea Cycle
• Activity of urea cycle is regulated at the level of enzyme synthesis
• Carbomoyl phosphate synthestase I is allosterically activated by N-
acetylglutamate
• Balanced diet
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