Tyrosine is an aromatic amino acid synthesized in plants, bacteria, and fungi through the shikimate pathway—a complex metabolic route not present in animals. This pathway converts simple carbohydrate precursors into aromatic amino acids like tyrosine, phenylalanine, and tryptophan.

Understanding the biosynthesis of tyrosine involves identifying key intermediates and cofactors at various steps. This article clarifies the roles of compounds labeled A, B, C, and D in the tyrosine biosynthesis pathway.

Overview of Tyrosine Biosynthesis via the Shikimate Pathway

The biosynthesis starts with shikimic acid, which undergoes phosphorylation and further enzymatic transformations to produce chorismic acid—a central branch point metabolite. Chorismic acid is converted into prephenic acid, which then leads to the formation of tyrosine through subsequent reactions including transamination.

The pathway steps relevant here are:

• Shikimic acid → A → shikimic acid-5-phosphate → B → C → chorismic acid → prephenic acid → D → transaminase → tyrosine.

Identifying A, B, C, and D

Step A: Phosphorylation of Shikimic Acid

• This step involves the ATP-dependent phosphorylation of shikimic acid to form shikimate-5-phosphate.

• The enzyme shikimate kinase catalyzes this reaction.

• Therefore, A is ATP, the phosphate donor.

Step B: Formation of 5-Enolpyruvylshikimate-3-phosphate (EPSP)

• Shikimate-5-phosphate condenses with phosphoenolpyruvate (PEP) to form 3-enolpyruvylshikimate-5-phosphate (EPSP).

• This reaction is catalyzed by EPSP synthase.

• Hence, B is phosphoenolpyruvate (PEP).

Step C: Conversion to Chorismic Acid

• EPSP is converted to chorismic acid by chorismate synthase.

• This is a key branch point leading to aromatic amino acid biosynthesis.

• Thus, C is 3-enolpyruvylshikimate-5-phosphate, the immediate precursor to chorismic acid.

Step D: Formation of p-Hydroxyphenylpyruvic Acid

• Prephenic acid undergoes rearrangement and oxidation to form p-hydroxyphenylpyruvic acid.

• This intermediate is then transaminated to produce tyrosine.

• Therefore, D is p-hydroxyphenylpyruvic acid.

Summary of Identifications

Correct Option Based on Identifications

The correct set matching these identifications is:

(1) ATP, phosphoenolpyruvic acid, 3-enolpyruvyl shikimic acid-5-phosphate, p-hydroxyphenylpyruvic acid