Urea Cycle

By Dr Musaddika Shaikh Dentowesome @drmusaddikashaikh

Urea cycle :-

  • It is end product of protein metabolism.
  • It is also known as urea Cycle or ornithine cycle or Krebs hensleit cycle.
  • Nitrogen of amino acids get converted to ammonia which is toxic to body.
  • Ammonia gets converted into urea and detoxified.
  • Urea is mainly synthesized in liver and transported to kidney for excreation.
  • Urea synthesis in 5steps cyclic process with 5 distinct enzymes.
  • First two enzymes – mitochondria and Rest – cytosol

Formation of Carbamoyl Phosphate :- Carbamoyl Phosphate synthase 1st present in mitochondria catalyses condensation of NH ions with CO2 to form carbamoyl phosphate. This step consumes 2ATP and is irreversible.

Formation of citrulline :- It is synthesized from carbamoyl phosphate synthetase 1st and Ornithine by transcarbomyle. Ornithine is regenerated and used in urea cycle.

Formation of Arginosuccinate :- It condenses citrulline with aspartate to produce Arginosuccinate. 2amino group of urea is involved in reaction. This step requires ATP which is cleaved to AMP.

Formation of Argentine :- Arginosuccinate breakes to form argenine and fumarate.

Formation of Urea :- Arginase is final enzyme which cleaves argenine to yeild urea and orthinine. Orthinine enters mitochondria for its reuse in urea cycle.


Reference :- Biochemistry book U. Satyanarayan, U Chakrapani. Google website

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