Urea cycle
**1. Ammonia Toxicity and Nitrogen Elimination:**
- Nitrogen is a component of amino acids, which are the building blocks of proteins.
- During protein metabolism, amino acids are broken down, and excess nitrogen is released in the form of ammonia (NH3), a highly toxic compound.
**2. Urea Cycle Steps:**
1. **Step 1: Formation of Carbamoyl Phosphate**
- The cycle begins with the conversion of ammonia and bicarbonate into carbamoyl phosphate, catalyzed by the enzyme carbamoyl phosphate synthetase I (CPS I).
2. **Step 2: Citrulline Formation**
- Carbamoyl phosphate combines with ornithine to form citrulline, a reaction catalyzed by the enzyme ornithine transcarbamylase (OTC).
3. **Step 3: Formation of Argininosuccinate**
- Citrulline reacts with aspartate to form argininosuccinate, catalyzed by the enzyme argininosuccinate synthetase.
4. **Step 4: Cleavage of Argininosuccinate**
- Argininosuccinate is cleaved into arginine and fumarate by the enzyme argininosuccinase.
5. **Step 5: Formation of Urea**
- Arginine is hydrolyzed to form urea and ornithine by the enzyme arginase.
- Ornithine produced in the final step re-enters the cycle to react with carbamoyl phosphate, completing the cycle.
**3. Fate of Urea:**
- Urea is transported from the liver to the kidneys through the bloodstream.
- In the kidneys, urea is excreted in the urine, contributing to the elimination of excess nitrogen from the body.
**4. Regulation of the Urea Cycle:**
- The urea cycle is tightly regulated to ensure efficient nitrogen disposal and prevent ammonia toxicity.
- Key regulatory points include the control of CPS I activity by N-acetylglutamate, which is synthesized from glutamate.
**5. Disorders and Clinical Significance:**
- Deficiencies in the enzymes involved in the urea cycle can lead to urea cycle disorders, resulting in the accumulation of ammonia and its toxic effects on the central nervous system.
- Urea cycle disorders are often diagnosed in newborns through newborn screening tests and require specialized medical management and dietary interventions.
The urea cycle is a complex and vital metabolic pathway that helps maintain nitrogen homeostasis in the body and prevent the harmful effects of ammonia buildup.
Urea Cycle Quiz
1. The primary site where the urea cycle takes place is:
a) Kidneysb) Liver
c) Heart
d) Lungs
2. The urea cycle helps eliminate excess:
a) Oxygenb) Carbon dioxide
c) Nitrogen
d) Hydrogen
3. Which enzyme catalyzes the conversion of ammonia and bicarbonate into carbamoyl phosphate?
a) Arginaseb) Carbamoyl phosphate synthetase I
c) Argininosuccinase
d) Ornithine transcarbamylase
4. The urea cycle involves the conversion of citrulline and aspartate into:
a) Arginineb) Urea
c) Fumarate
d) Ornithine
5. The primary function of the urea cycle is to:
a) Produce glucoseb) Synthesize proteins
c) Eliminate excess nitrogen
d) Break down lipids
6. The enzyme that catalyzes the hydrolysis of arginine to form urea and ornithine is:
a) Carbamoyl phosphate synthetase Ib) Argininosuccinase
c) Arginase
d) Ornithine transcarbamylase
7. The urea cycle is primarily responsible for the elimination of:
a) Carbon dioxideb) Water
c) Ammonia
d) Glucose
8. The urea cycle takes place primarily in which organ of the body?
a) Heartb) Kidneys
c) Liver
d) Lungs
9. Urea is transported from the liver to the kidneys through:
a) Bloodstreamb) Lymphatic system
c) Nervous system
d) Endocrine system
10. Which enzyme is responsible for the conversion of citrulline and aspartate into argininosuccinate?
a) Arginaseb) Carbamoyl phosphate synthetase I
c) Argininosuccinase
d) Ornithine transcarbamylase
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