Question:
Which amino acid needs to be supplemented through diet in a patient with cystathionine beta synthase deficiency?
Which amino acid needs to be supplemented through diet in a patient with cystathionine beta synthase deficiency?
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Think about which product of the transsulfuration pathway cannot be formed when CBS is absent.
Updated On: Jun 23, 2026
- Cysteine
- Methionine
- Serine
- Tryptophan
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The Correct Option is A
Solution and Explanation
Step 1: Understand the enzyme involved.
Cystathionine beta synthase (CBS) catalyzes the condensation of homocysteine and serine to form cystathionine, using PLP as a cofactor.
Step 2: Trace the downstream pathway.
Cystathionine is then cleaved by cystathionase into cysteine and homoserine. This step is also PLP-dependent.
Step 3: Identify the consequence of CBS deficiency.
When CBS is non-functional, homocysteine accumulates and cystathionine cannot be formed. Since cystathionine is absent, cysteine cannot be synthesized via this transsulfuration route.
Step 4: Determine dietary requirement.
Cysteine is normally a conditionally essential amino acid synthesized from methionine. In CBS deficiency, this synthetic route is blocked, making cysteine an essential amino acid that must be supplemented in the diet.
Answer: Cysteine
Cystathionine beta synthase (CBS) catalyzes the condensation of homocysteine and serine to form cystathionine, using PLP as a cofactor.
Step 2: Trace the downstream pathway.
Cystathionine is then cleaved by cystathionase into cysteine and homoserine. This step is also PLP-dependent.
Step 3: Identify the consequence of CBS deficiency.
When CBS is non-functional, homocysteine accumulates and cystathionine cannot be formed. Since cystathionine is absent, cysteine cannot be synthesized via this transsulfuration route.
Step 4: Determine dietary requirement.
Cysteine is normally a conditionally essential amino acid synthesized from methionine. In CBS deficiency, this synthetic route is blocked, making cysteine an essential amino acid that must be supplemented in the diet.
Answer: Cysteine
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