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.

Reaction :-NH4+CO2+ASPARTATE+3ATP = UREA+FUMARATE+2ADP+2PIi+AMP+Ppi

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