Transamination, Deamination, Urea cycle and its Disorders : USMLE Step 1

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Transamination:
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Transamination is a process that involves the transfer of an amino group (NH2) from one amino acid to a keto acid.
This transfer is catalyzed by enzymes known as transaminases or aminotransferases.
The amino group from one amino acid is transferred to a keto acid, resulting in the formation of a new amino acid and a different keto acid.
The enzyme alanine aminotransferase (ALT) catalyzes the transamination of alanine and α-ketoglutarate to produce pyruvate and glutamate.
Another example is the transamination of aspartate and α-ketoglutarate to form oxaloacetate and glutamate.
Transamination reactions play a crucial role in amino acid interconversion and the generation of various metabolic intermediates.

Deamination:
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Deamination is the process of removing an amino group (NH2) from an amino acid.
This process typically leads to the formation of an α-keto acid and ammonia (NH3).
Deamination reactions are essential for the elimination of excess nitrogen from the body.
Ammonia generated during deamination is typically converted into urea in the liver through the urea cycle.
For example, the deamination of glutamine by the enzyme glutaminase results in the formation of glutamate and ammonia.


Urea Cycle Disorders (UCDs):
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Ornithine Transcarbamylase Deficiency (OTC Deficiency):

OTC deficiency is the most common UCD and occurs due to a deficiency in the enzyme ornithine transcarbamylase.
This enzyme is responsible for the conversion of carbamoyl phosphate and ornithine into citrulline in the mitochondria.
With OTC deficiency, there is a buildup of ammonia in the bloodstream because carbamoyl phosphate cannot be effectively incorporated into the urea cycle.
Symptoms may include lethargy, vomiting, seizures, and coma. This disorder primarily affects males.
Citrullinemia:

Citrullinemia is caused by a deficiency in either argininosuccinate synthetase or argininosuccinate lyase.
Argininosuccinate synthetase deficiency results in the accumulation of citrulline and ammonia.
Argininosuccinate lyase deficiency leads to elevated levels of argininosuccinate, citrulline, and ammonia.
Symptoms may include irritability, poor feeding, and neurological problems.
Arginase Deficiency:

Arginase deficiency is a rare UCD resulting from a deficiency in the enzyme arginase.
This enzyme is responsible for converting arginine into urea and ornithine.
In the absence of arginase, arginine accumulates in the bloodstream.
Symptoms may include developmental delays, spasticity, and intellectual disabilities.
CPS I Deficiency:

Carbamoyl phosphate synthetase I (CPS I) deficiency is a severe UCD caused by a deficiency in CPS I, the enzyme responsible for generating carbamoyl phosphate in the mitochondria.
With CPS I deficiency, there is a failure to incorporate ammonia into the urea cycle.
Symptoms appear shortly after birth and may include seizures, coma, and respiratory distress.
ASL Deficiency:

Argininosuccinate lyase (ASL) deficiency results from a deficiency in the ASL enzyme.
This enzyme is involved in the conversion of argininosuccinate to arginine and fumarate.
ASL deficiency leads to elevated levels of argininosuccinate, citrulline, and ammonia.
Symptoms can range from mild to severe and may include developmental delays and intellectual disabilities.

NAGS Deficiency:

N-acetylglutamate synthase (NAGS) deficiency is a rare UCD that affects the enzyme NAGS, which plays a crucial role in the regulation of CPS I.
A deficiency in NAGS results in reduced activation of CPS I, leading to impaired urea cycle function.
Symptoms may include hyperammonemia and neurological issues.

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