Question

Steps in the metabolism of Tyrosine.

A. Phenylalanine \(\xrightarrow{\text{Phenylalanine hydroxylase}}\) Tyrosine

B. Homogentisate \(\rightarrow\) Fumarate

C. Tyrosine \(\xrightarrow{\text{Tyrosine Transaminase}}\) hydroxyphenyl pyruvate

D. Phenylalanine \(\xrightarrow{\text{Phenylalanine dehydrogenase}}\) Homogentisate

Choose the correct answer from the options given below:

• A. A and B only \
• B. A, B and D only \
• C. C and D only \
• D. A and C only

Hint:

Phenylalanine Hydroxylase adds an $-\text{OH}$ group to Phenylalanine to create Tyrosine, which is then targeted by Tyrosine Transaminase.

Answer 1

The Correct Option is D

Step 1: Concept Phenylalanine and tyrosine are aromatic amino acids whose catabolic breakdown pathways are closely linked.

Step 2: Meaning The pathway begins with the conversion of phenylalanine to tyrosine, followed by a series of downstream steps that convert tyrosine into glucogenic and ketogenic intermediates.

Step 3: Analysis * Step A: Phenylalanine is converted to tyrosine by the action of the enzyme phenylalanine hydroxylase. This is a correct forward reaction step. * Step B: Homogentisate is converted to 4-maleylacetoacetate by homogentisate 1,2-dioxygenase, which eventually breaks down into fumarate and acetoacetate. It does not convert to fumarate in a single direct step. * Step C: Tyrosine undergoes transamination by tyrosine transaminase to form $p$-hydroxyphenylpyruvate. This is a correct forward reaction step. * Step D: Phenylalanine is not converted to homogentisate by a dehydrogenase enzyme.

Step 4: Conclusion Statements A and C accurately describe direct reactions in the standard aromatic amino acid metabolic pathway. This matches option D. Final Answer: (D)