Biochemistry

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Module
Review Articles
Video Lectures
Practice Questions
Flashcards
Metabolism
23
22
64
148
Nutrition
6
5
17
30
Genetics
11
11
33
66
Cellular
6
5
18
29
Molecular
6
5
18
35

Module	Review Articles	Video Lectures	Practice Questions	Flashcards
Metabolism	23	22	64	148
Nutrition	6	5	17	30
Genetics	11	11	33	66
Cellular	6	5	18	29
Molecular	6	5	18	35

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Sample Summary page

Urea Cycle

Writer: Yazan AlHabil, M.D.

Keywords: Biochemistry; urea cycle

Overview

Ammonia, a toxic byproduct of protein metabolism, must be safely transported and excreted to prevent toxicity. This is achieved primarily through the glutamine and alanine cycles, which convert free ammonia into non-toxic carriers. In the liver, ammonia enters the urea cycle, a critical detoxification pathway that converts nitrogen into urea for renal excretion. The urea cycle involves a series of mitochondrial and cytosolic reactions, with carbamoyl phosphate synthetase 1 (CPS1) catalyzing the rate-limiting step. This process is regulated by N-acetylglutamate and requires inputs like aspartate, which provides an amino group. Efficient functioning of this cycle is vital for maintaining nitrogen balance and preventing hyperammonemia.

Origin of Ammonia

Ammonia is a toxic byproduct of protein metabolism, and to avoid toxicity, it is transported in a non-toxic form. There are two primary mechanisms for ammonia transport:

1. Glutamine Cycle (Most Common):

o Ammonia is transported as an amine group attached to glutamine.

o In the liver, glutamine synthetase adds free ammonia to glutamate to form glutamine.

o The ammonia is then cleaved from glutamine when it reaches the liver.

2. Alanine Cycle:

o In this cycle, ammonia is transported as an amine group on alanine, which is synthesized in skeletal muscle.

o Alanine is created by alanine transaminase (ALT) from pyruvate in muscle tissue. In the liver, alanine is converted back to pyruvate. (Figure 1).

Figure 1. The Alanine Cycle

Urea Cycle Reactions

The urea cycle consists of a series of reactions that occur in both the mitochondria and the cytosol. Table 1 and Figure 2 provide an overview of this process.

Figure 2. The Urea Cycle

Table 1: The Urea Cycle’s Key Reactions

Reaction	Substrate	Enzyme (+ Site of Reaction)	Product(s)	Special Features
1. Creation of Carbamoyl Phosphate	HCO₃⁻, NH₃	Carbamoyl Phosphate Synthetase 1 (CPS1) (Mitochondria)	Carbamoyl Phosphate	Rate-limiting step. Requires 2 ATP. Activated by N-acetylglutamate, which is stimulated by excess amino acid catabolism.
2. Creation of Citrulline	Carbamoyl Phosphate, Ornithine	Ornithine Carbamoyltransferase (Mitochondria)	Citrulline	Citrulline is transported into the cytosol in exchange for ornithine.
3. Creation of Argininosuccinate	Citrulline, Aspartate	Argininosuccinate Synthetase (Cytosol)	Argininosuccinate	ATP is consumed (ATP → AMP + 2 Pi). Aspartate donates its amino group.
4. Hydrolysis of Argininosuccinate	Argininosuccinate	Argininosuccinase (Cytosol)	Arginine, Fumarate	Fumarate is a product, and arginine is formed.
5. Hydrolysis of Arginine	Arginine, H₂O	Arginase (Cytosol)	Urea, Ornithine	Urea is produced and enters the bloodstream for renal excretion. Ornithine is transported back to mitochondria.