This image is an enlargement of the active site for Penicillinase. The substrate molecule, (((N-Benzyloxycarbonyl) amino) methyl) phosphonate, is an analog of the intermediate state in Beta-lactam hydrolysis. By binding a intermediate analog, an enzyme-substrate complex tends to remain in this form rather than producing a product. Enzymes bind substrate-analogs with greater affinity than either the substrate or product. As a result, crystallization of these structures can be elucidated. All of the amino acids surrounding the Penicillin analog are polar residues with the acception of Isoleucine 235. Polar residues facilitate the binding of the polar Penicillin-analog. Successful binding of the Penicilllin analog inactivates the drug. As with the Carboxypeptidase-Cefotaxime complex, the residues are colored from the N-terminus = more blue to the C-terminus = more green.