Glyphosate inhibits EPSP synthetase, a key enzyme in the shikimate pathway that produces aromatic amino acids essential for plants. The correct answer is (2) aromatic amino acids.

Question Breakdown

EPSP synthetase (5-enolpyruvylshikimate-3-phosphate synthase) catalyzes the reaction between phosphoenolpyruvate (PEP) and shikimate-3-phosphate (S3P) to form EPSP, a precursor in the shikimate pathway. Glyphosate structurally mimics PEP, binding tightly to the enzyme’s active site and blocking this step, which halts synthesis of phenylalanine, tyrosine, and tryptophan.

Option Analysis

• (1) Aliphatic amino acids: Incorrect. These non-aromatic amino acids (e.g., glycine, alanine, leucine) are synthesized via different pathways, not involving EPSP synthetase or the shikimate route.​

• (2) Aromatic amino acids: Correct. EPSP synthetase is specific to the shikimate pathway, producing phenylalanine, tyrosine, and tryptophan—aromatic amino acids vital for protein synthesis and plant secondary metabolites.

• (3) Basic amino acids: Incorrect. Basic amino acids (e.g., lysine, arginine, histidine) derive from aspartate or glutamate pathways, unaffected by glyphosate.​

• (4) Acidic amino acids: Incorrect. Acidic amino acids (e.g., aspartic acid, glutamic acid) come from separate biosynthetic routes, not the shikimate pathway.​

Plants lack an alternative pathway for these aromatics, leading to glyphosate’s herbicidal action, while animals do not have this pathway.​

Glyphosate EPSP synthetase aromatic amino acids synthesis is a critical topic in plant physiology and herbicide mechanisms, especially for competitive exams like GATE Life Sciences. This herbicide targets the shikimate pathway, where EPSP synthetase plays a pivotal role.

Mechanism of Action

Glyphosate structurally mimics phosphoenol pyruvate (PEP), competing for the enzyme’s active site. It binds more tightly to the EPSP synthase-S3P complex, preventing formation of 5-enolpyruvylshikimate-3-phosphate (EPSP). This disruption starves plants of essential aromatic amino acids: phenylalanine, tyrosine, and tryptophan.

Why Aromatic Amino Acids?

The shikimate pathway, absent in animals, exclusively produces these three aromatic amino acids in plants, bacteria, and fungi. Inhibition leads to protein synthesis failure and secondary metabolite shortages, causing plant death.

Exam Relevance

In MCQs, options like aliphatic, basic, or acidic amino acids test pathway specificity. Aromatic amino acids are the precise target, making option (2) correct for glyphosate-related questions.​

This knowledge aids in understanding herbicide resistance (e.g., mutated EPSPS) and biotech applications.​