Cellular Biochemistry and Metabolism 2 (CLS 333 ) Dr. Samah Kotb Nasr Eldeen Lecturer of Biochemistry Urea Cycle

UREA CYCLE UREA CYCLE Chapter 9

Urea Cycle The ammonia resulting from the deamination of amino acids is converted into urea in the liver by a cyclic pathway known as urea cycle.

It was shown that urea formation from ammonia involves three specific compounds namely Ornithine, Citrulline and Arginine.

Arginine is a standard amino acid found in proteins. Ornithine and citrulline are non-essential amino acids. They do not occur building blocks of protein molecule but are synthesis inside cells.

Urea Cycle  The structures of the three compounds suggest that they might be related in a sequence with ornithine acting as a precursor of citrulline and citrulline as a precursor of arginine:-

It was also shown that arginine could be hydrolyzed by the enzyme Arginase into ornithine + urea. Thus:

Urea Cycle Urea Cycle A.A molecule of ornithine combines with one molecule of NH 3 and one of CO 2 to form citrulline.

B. A second molecule of NH 3 is added to citrulline to form arginine. C. Arginine is then hydrolyzed to give urea + ornithine to start another turn of the cycle.

Urea Cycle

Urea is a neutral, non-toxic, water-soluble substance. It passes into the blood and is filtered at the kidney nephrons and appears in urine as a waste product of N-metabolism.

Urea Cycle  Urea Cycle Several Complex Steps 1.The first ammonia to enter the urea cycle arises in the form of free ammonia by the oxidative deamination of glutamate inside the mitochondria catalyzed by the enzyme Glutamate Dehydrogenase.

The Urea Cycle  Urea Cycle Several Complex Steps This free ammonia is immediately combined with CO 2 formed in the mitochondria to form a molecule of carbamoyl-phosphate. The reaction is ATP-dependent and is catalyzed by the enzyme carbamoyl phosphate synthetase (CPS):-

Urea Cycle  Urea Cycle Several Complex Steps Now the carbamoyl phosphate donates its carbamoyl group to ornithine to form citrulline and release phosphate in a reaction catalyzed by the enzyme ornithine transcarbomylase (OTC):-

The citrulline formed now leaves the mitochondria and passes into the cytosol of liver cells.

Urea Cycle  Urea Cycle Several Complex Steps 2)The second molecule of ammonia is now introduce in the form of aspartate which is acquired from Glutamate by the enzyme Aspartate transaminase:-

Urea Cycle  Urea Cycle Several Complex Steps Now aspartate and citrulline undergo a condensation reaction in the presence of ATP to form the compound argininosuccinate. The reaction is catalyzed by the cytosolic enzyme argininosuccinate synthetase:-

In the next step argininosuccinate is cleaved by the enzyme argininosuccinate lyase to form arginine + Fumarate:-

Urea Cycle  Urea Cycle Several Complex Steps

Urea Cycle  Urea Cycle Several Complex Steps As seen in the last reaction of the urea cycle the liver enzyme arginase cleaves arginine to yield urea and ornithine. Ornithine can now enter the mitochondria again to start another turn of the cycle.

Urea Cycle  Overall Urea Synthesis  Note that for every molecule of urea formed a molecule of HCO 3 - (CO 2 ) is consumed. Thus the urea cycle disposes of 2 waste products; ammonia + CO 2. Thus it helps in the regulation of blood PH.

Clinical significance of urea

 1 ) Physiologic changes in urea concentration  The concentration of urea depends on diet. The individuals taking food with high protein content have a higher serum urea concentration than those taking food with low protein content. Urea is normally lower in children and women because of their lower muscular mass than in men.

A) Increased levels In healthy individuals, the blood urea concentration increases with dietary protein. The concentration of urea increases with age. In late pregnancy because of increased protein intake.

B) Decreased levels Blood urea concentration is lower in children during growth than in adults. In early normal pregnancy. In individuals on a low-protein, high-carbohydrate diet.

2) Pathologic changes in urea concentration A) Increased urea levels in: Excessive urea production - Intake of high-protein diet 12 hours before blood sampling. - Enhanced catabolism of body protein.

Incomplete urea clearance from the body in diseases: - Acute renal failure. - Glomerulonephritis. - Acquired hemolytic anemia (autoimmune). - Acute pancreatitis. - Acute post-streptococcal.

B) Decreased urea levels in: Acromegaly. Acute and subacute necrosis of the liver. After glucose infusion. After hemodialysis. Celiac disease. Cystic fibrosis. Eclampsia.