Watch this one video and never forget what Ornithine cycle is

Watch this one video and never forget what Ornithine cycle is. NEET Zoology XI Excretion and Osmoregulation

Urea formation:
Proteins → amino acids → keto acids (by Deamination) → enter Krebs Cycle → energy. CO_2 is formed during decarboxylation.
Step 1: Formation of Carbamoyl phosphate: Condensation of CO_2, NH_3 and ATP to form Carbamoyl phosphate occurs in the liver mitochondria in the presence of the enzyme Carbamoyl phosphate
synthase I.

This reaction is the rate limiting reaction of the Urea cycle and is allosterically activated by the presence of N-acetylglutamate, which enhances the affinity of the CPS-I enzyme for the A TP molecule.

Step 2: Synthesis of L-Citrulline: This reaction is catalyzed in the mitochondria by L-Ornithine ‘transcarbamoylase enzyme (OTC). While this reaction occurs in the mitochondrial matrix, the formation of ornithine and the subsequent reaction occur in the liver cell cytosol.


Step 3: Conversion of L-Citrulline to Argino-succinate: L-Citrulline combines with L-Aspartate (The second ammonia aroUf) donor) in presence of Mg^(2+) - ATP to form Arginosuccinate. This reaction is catalyzed by Arginosuccinic acid synthase enzyme. This reaction requires ATP which is converted later to AMP + Mg^2 - PPi.

Step 4: Cleavage of Arginosuccinate: This reaction is catalyzed by Arginosuccinase enzyme which deserves into the formation of L-Arginine and Fumarate. L-Arginine then continues ahead. Arginosuccinate

Step 5: Cleavage of Arginine releasing urea and reforming Ornithine: L-Arginine is broken down by addition of water to produce L-Ornithine and Urea. The L-Ornithine so formed then re-enters mitochondria to run another Urea Cycle. This reaction is catalyzed by Arginase enzyme.

And hence the cycle is complete. This cycle is hence referred to as Krebs Hensleit Cycle (after the name of its discoverers). Most of the urea is produced in the liver.
The process is divided into 2 parts :
(i) Chemical part - in liver.
(ii) Mechanical part - in kidney.

Fig. Ornithine cycle
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