About Penicillin G Natural -lactamase sensitive pencillin, A narrow spectrum antibiotic. Mechanism of Action of Penicillin G It is a bactericidal drug. Penicillin is a Beta lactum antibiotic. It binds to the penicillin binding protein in the cell wall of bacteria and inhibits the enzyme transpeptidase which is responsible for the transpeptidation reaction in peptidoglycan synthesis. It also activates the endogenous enzyme which degrades the peptidoglycan.These results in inhibition of protein synthesis and there by cell wall synthesis. Penicillin is more active against gram positive organism than gram negative organism.