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How does penicillin inhibit enzymes that bacteria use to build cell walls?

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A multi-panel diagram illustrates how a substrate binds to an enzyme's active site through induced fit, leading to product formation, and shows how non-substrates and allosteric regulators can alter enzyme activity by binding to different sites on the enzyme. — Enzyme In the induced-fit theory of enzyme-substrate binding, a substrate approaches the surface of an enzyme (step 1 in box A, B, C) and causes a change in the enzyme shape that results in the correct alignment of the catalytic groups (triangles A and B; circles C and D represent substrate-binding groups on the enzyme that are essential for catalytic activity). The catalytic groups react with the substrate to form products (step 2). The products then separate from the enzyme, freeing it to repeat the sequence (step 3). Boxes D and E represent examples of molecules that are too large or too small for proper catalytic alignment. Boxes F and G demonstrate binding of an inhibitor molecule (I and I′) to an allosteric site, thereby preventing interaction of the enzyme with the substrate. Box H illustrates binding of an allosteric activator (X), a nonsubstrate molecule capable of reacting with the enzyme.

Penicillin functions by inhibiting the enzymes responsible for cell wall synthesis in actively replicating bacteria. Specifically, these antibiotics—classified as β-lactam antibiotics—interfere with the final stages of cell wall construction, which involves the cross-linking of strands of wall material.12

By inhibiting these essential enzymes, penicillin prevents the bacteria from maintaining the structural integrity of their protective cell walls. Furthermore, the drug activates other enzymes that actively break down the existing wall structure. Because human cells lack cell walls, penicillin is selectively toxic to bacteria and does not harm human cells. This process is only effective against microorganisms that are actively replicating and producing new cell wall material.1