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predicted that Phe158 and Ile 164 form hydrophobic interactions with bulky substituents on the substrate, suggesting that the loss of these residues would only affect binding of substrates with large aromatic substituents [ 37 ] . In the modeling studies on CcrA [ 42 ] , Asn233 was predicted to interact with the β-lactam carbonyl on substrate, and mutagenesis studies have supported this prediction [ 43 ] . Although Asn233 is sequence conserved in L1 [ 35 ] , it is located 14Å away from the modeled position of the β-lactam carbonyl and was predicted not to play a role in substrate binding to L1 [ 37 ] . On the other hand, the substrate-binding model predicted that Tyr228 was in position to offer a hydrogen bond to the β-lactam carbonyl and participate in an oxyanion hole that was proposed to form as the substrate was hydrolyzed [ 37 ] . By using the crystal structure and modeling studies on L1, Ullah et al.
Efforts to solve the crystal structure of one of the metallo-β-lactamases with a bound substrate molecule have failed, most likely due to the high activity of the enzymes towards all β-lactam containing antibiotics [ 37 54 ] . Therefore, computational studies have been used extensively to study substrate binding, the role of the Zn(II) ions in catalysis, the protonation state of the active site, and inhibitor binding [ 37 42 55 56 57 58 59 ] . All of the substrate binding models have made assumptions before the substrate was docked into the active site [ 37 42 ] , and some of these assumptions have been shown to be invalid for certain substrates [ 43 ] . With L1, two key assumptions were made: (1) the bridging hydroxide functions as the nucleophile during catalysis and (2) Zn 1 coordinates the β-lactam carbonyl [ 37 ] . With these assumptions and after energy minimizations, Ser224 was predicted to hydrogen bond to the substrate carboxylate [ 37 ] , reminiscent of the role predicted for Lys224 in CcrA [ 42 ] . Ullah et al.
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