Biochemistry | Protein Metabolism | Urea Cycle | Made Easy by Dr. Apeksha Niraula | BPKIHS | Part 4

A video by:
Dr. Bharat Kc (MBBS 2014, BPKIHS)

Nitrogen Excretion and the Urea Cycle
The excess NH4+ is converted into urea and then excreted. Such organisms are referred to as ureotelic

Urea Cycle: Hans Krebs and Kurt Henseleit in 1932
Site: Mitochondria + Cytosol
One nitrogen of the urea molecule is supplied by free ammonia, and the other nitrogen by aspartate

The carbon and oxygen of urea are derived from – HCO3

Urea is produced by the liver, and then is transported in the blood to the kidneys for excretion in the urine

Reactions of the cycle

The first two reactions leading to the synthesis of urea occur in the mitochondria, whereas the remaining cycle enzymes are located in the cytosol.
Carbamoyl Phosphate Synthase I Initiates Urea Biosynthesis
Condensation of CO2, ammonia, and ATP to form carbamoyl phosphate is catalyzed by mitochondrial carbamoyl phosphate synthase I

Carbamoyl phosphate synthetase catalyzes the required three steps

Carbamoyl phosphate synthetase – Rate limiting enzyme is active only in the presence of its allosteric activator N-acetylglutamate

The urea cycle is linked to gluconeogenesis

Pyrophosphate is rapidly hydrolyzed, and so the equivalent of four molecules of ATP are consumed

Synthesis of fumarate by the urea cycle is important because it is a precursor for glucose synthesis

Fumarate is hydrated to malate, which is in turn oxidized to oxaloacetate
Metabolic Disorders of Urea Synthesis

Potentially fatal and cause coma when ammonia concentrations become high

Characterized by hyperammonemia, encephalopathy, and respiratory alkalosis

Loss of consciousness may be a consequence of ATP depletion

A high concentration of ammonia sequesters a α-­ ketoglutarate to form glutamate, thus depleting the TCA cycle of important intermediates and reducing ATP production

Clinical symptoms common to all urea cycle disorders include vomiting, avoidance of high-protein foods, intermittent ataxia, irritability, lethargy and severe mental retardation

Music credit:
Aakash Gandhi - lifting dreams

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