Question:
A patient has high phenylalanine levels despite normal phenylalanine hydroxylase. Which cofactor deficiency is most likely responsible?
A patient has high phenylalanine levels despite normal phenylalanine hydroxylase. Which cofactor deficiency is most likely responsible?
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Phenylalanine hydroxylase requires BH4. BH4 deficiency can mimic phenylketonuria.
Updated On: May 14, 2026
- Tetrahydrofolate
- Tetrahydrobiopterin (BH4)
- Thiamine
- Pyridoxine
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The Correct Option is B
Solution and Explanation
Concept:
Phenylalanine hydroxylase converts phenylalanine to tyrosine. This enzyme requires tetrahydrobiopterin, also called BH4, as a cofactor.
Step 1: Recall the reaction.
\[ \text{Phenylalanine} \rightarrow \text{Tyrosine} \] This reaction is catalyzed by phenylalanine hydroxylase.
Step 2: Identify the required cofactor.
The required cofactor is: \[ BH_4 \]
Step 3: Interpret the condition.
If phenylalanine hydroxylase is normal but phenylalanine is high, the problem is likely BH4 deficiency.
Step 1: Recall the reaction.
\[ \text{Phenylalanine} \rightarrow \text{Tyrosine} \] This reaction is catalyzed by phenylalanine hydroxylase.
Step 2: Identify the required cofactor.
The required cofactor is: \[ BH_4 \]
Step 3: Interpret the condition.
If phenylalanine hydroxylase is normal but phenylalanine is high, the problem is likely BH4 deficiency.
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