Aromatic pathway entry step: DAHP synthetase catalyzes the condensation of which substrates to form 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) in the shikimate pathway?

Introduction / Context:The shikimate pathway is central to the biosynthesis of aromatic amino acids (phenylalanine, tyrosine, tryptophan) in bacteria, fungi, and plants. The first committed step is catalyzed by DAHP synthetase, making it a classic enzyme target in antimicrobial and herbicidal strategies.

Given Data / Assumptions:

• The product is DAHP (3-deoxy-D-arabino-heptulosonate-7-phosphate).
• We must identify the two substrates condensed by DAHP synthetase.
• Phenylalanine is an end product/feedback effector, not a substrate.

Concept / Approach:

DAHP synthetase condenses phosphoenolpyruvate (PEP) with erythrose-4-phosphate (E4P). This C3 + C4 condensation yields the seven-carbon DAHP, committing carbon flow into the shikimate pathway toward chorismate and downstream aromatic compounds.

Step-by-Step Solution:

Verification / Alternative check:

Pathway schematics consistently show PEP + E4P → DAHP; later steps proceed via shikimate, EPSP, and chorismate to the aromatic amino acids.

Why Other Options Are Wrong:

E4P only or PEP only: incomplete; both are required.

Phenylalanine: an end-product and regulator, not a substrate at this step.

Common Pitfalls:

Confusing substrates with allosteric regulators; mixing up shikimate pathway entry (DAHP) with downstream branch points.

Final Answer:

both (a) and (b)