This image is a comparison of penicillin and D-alanyl-D-alanine, the group which the carboxypeptidase binds to in the transpeptidation cross-linking process. Penicillin inhibits carboxypeptidase's cross-linking capabilites by acylating the enzyme and preventing the formation of terminal D-alanine cross-linking. Penicillin has nine atoms which are similar to the D-alanyl-D-alanine molecule. Penicillin binds to the transpeptidase and individual strands of N-acetylmuramic acid (NAM) are not able to link together to form a rigid structure. The complementary atoms are indicated by the same colored background.