Feedback control in amino acid biosynthesis: Anthranilate synthase, the first committed enzyme after the branch point in tryptophan biosynthesis, shows feedback inhibition and repression in response to which effector?

Introduction / Context:
Amino acid biosynthetic pathways are tightly regulated to conserve resources. Tryptophan biosynthesis provides a classic example where the end product controls both enzyme activity (feedback inhibition) and gene expression (repression), matching supply to cellular demand.

Given Data / Assumptions:

• Anthranilate synthase catalyzes the first committed step toward tryptophan.
• Feedback regulation typically uses the pathway end product.
• We must identify the effector that inhibits the enzyme and represses gene expression.

Concept / Approach:
L-tryptophan acts as the key effector: it allosterically inhibits anthranilate synthase, lowering pathway flux when tryptophan is abundant. In many bacteria, tryptophan also participates in repression via a repressor protein or attenuation mechanisms, decreasing transcription of trp operon genes when intracellular tryptophan is high.

Step-by-Step Solution:
Identify the end product of the pathway (L-tryptophan). Recall that end products commonly mediate feedback and repression. Confirm that L-tryptophan inhibits anthranilate synthase and represses trp genes. Select L-tryptophan.

Verification / Alternative check:
Genetic and biochemical studies show that high tryptophan lowers anthranilate synthase activity and reduces expression of trp operon genes through repressor binding/attenuation.

Why Other Options Are Wrong:
Hydantoins and pyruvate are not pathway end products here; L-serine participates in other biosyntheses but is not the primary feedback effector for anthranilate synthase.

Common Pitfalls:
Confusing feedback inhibition (rapid, post-translational) with repression (slower, transcriptional); both are triggered by the same end product in this case.

Final Answer:
L-tryptophan.