A bacterial infection that cannot be treated with one sort of penicillin can be treated with a different sort. Use your knowledge of enzyme action to explain why the different sort of penicillin is effective in treating the infection.

Penicillin inhibits an enzyme that forms cross links in bacterial cell walls halting bacterial reproduction. Different penicillins have different shapes or structures. The penicillin that is not effective in treating the infection no longer fits or binds to the active site of the enzyme that form the cross links in the cell wall. Therefore, an enzyme-substrate complex is not formed and the pencillin does not inhibit bacterial cell wall production and growth. 
(whereas, the pencillin that is effective in treating the infection can bind to the active site of the enzyme, form an E-S complex and inhibit bacterial cell wall production and growth. )