# DBT JRF 2020 — Question 128

Exam: DBT JRF 2020 · Section: B · Topic: Plant Biology · Difficulty: Hard

## Question

The broad-spectrum herbicide glyphosate, the active ingredient of Roundup, inhibits this enzyme

## Options

- **A.** 3-deoxy-7-phosphoheptulonate synthase
- **B.** Chorismate synthase
- **C.** Shikimate dehydrogenase
- **D.** 5-enolpyruvylshikimate-3-phosphate synthase

## Answer

Option D

## Explanation

Glyphosate (N-phosphonomethylglycine) is the active ingredient of Roundup® herbicide (Monsanto/Bayer). Target: EPSPS (5-enolpyruvylshikimate-3-phosphate synthase), EC 2.5.1.19, the penultimate enzyme in the shikimate pathway. EPSPS catalyses: Shikimate-3-phosphate + PEP (phosphoenolpyruvate) → 5-EPSP + $\text{P}_i$ . Glyphosate is a competitive inhibitor with respect to PEP — it binds in the PEP-binding site of EPSPS in the presence of shikimate-3-phosphate, forming a stable ternary complex. The shikimate pathway is essential in plants (and bacteria, fungi) for synthesising chorismate → phenylalanine, tyrosine, tryptophan (aromatic amino acids) + folate. Animals lack the shikimate pathway — they obtain aromatic amino acids from diet — explaining glyphosate's low acute toxicity to mammals. Glyphosate-resistant crops carry CP4-EPSPS from Agrobacterium (has low glyphosate affinity).

Source: https://mypyqbuddy.com/questions/the-broad-spectrum-herbicide-glyphosate-the-active-ingredient-of-6a05a8a02adafec470293298