Final answer:
Penicillin G acts as a inhibitor by forming an irreversible covalent bond with the transpeptidase enzyme, halting bacterial cell wall synthesis.
Step-by-step explanation:
Penicillin G forms a covalent bond with a serine residue at the active site of transpeptidase, which classifies it as a inhibitor. This type of inhibitor resembles the normal substrate of an enzyme but reacts irreversibly with it, resulting in a permanent inactivation of the enzyme. Penicillin's effectiveness comes from the significant strain in its ß-lactam ring, making this reaction irreversible and thus inhibiting the enzyme transpeptidase that catalyzes cell wall synthesis in bacteria.