Glyphosate inhibits which enzyme?
Glyphosate N-phosphonomethyl glycine inhibits key enzyme of aromatic amino acid pathway. 5-enolpyruvylshikimate-3-phosphate synthase normally condenses shikimate-3-phosphate and phosphoenolpyruvate to EPSP in chloroplast stroma, penultimate step of shikimate pathway. Glyphosate mimics PEP, occupying PEP binding site in EPSPS-S3P complex forming stable dead-end complex preventing enolpyruvyl transfer. Plants cannot synthesize chorismate, precursor of phenylalanine, tyrosine, tryptophan, causing accumulation of shikimate, depletion of aromatic amino acids, cessation of protein synthesis, and death within days. EPSPS encoded by nuclear gene but targeted to plastid via transit peptide. Class I EPSPS sensitive to glyphosate, Class II from CP4 strain tolerant due to alanine to glycine substitution reducing glyphosate affinity while maintaining catalytic efficiency. Identification of EPSPS as glyphosate target explained herbicidal mode of action and enabled engineering of tolerant crops via overexpression of resistant enzyme. Structural biology shows glyphosate occupies extended conformation in active site, interacting with conserved Lys22, Arg124, Asp313 residues. Binding prevents closure of domain needed for catalysis. Overexpression of sensitive EPSPS also confers tolerance via gene amplification observed in glyphosate resistant Amaranthus palmeri. Inhibition leads to starvation of phenylalanine needed for protein synthesis and secondary metabolite production. This detailed mechanism of EPSP synthase inhibition explains broad-spectrum herbicidal activity and basis
Ref: Steinrücken Biochem Biophys Res Commun 1980 glyphosate EPSPS; Funke PNAS 2006 Roundup Ready molecular basis; NCBI NBK22387; https://www.pnas.org/doi/10.1073/pnas.0603638103