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Example sentences for: ccra
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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.
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.
All crystallographically characterized metallo-β-lactamases have a flexible amino acid chain that extends over the active site [ 37 42 44 45 46 47 48 49 ] . Previous NMR studies on CcrA have shown that this loop "clamps down" on substrate or inhibitor upon binding, and there is speculation that the distortion of substrate upon clamping down of the loop may drive catalysis [ 50 ] . The crystal structure of L1 showed that there is a large loop that extends over the active site, and modeling studies have predicted that two residues, Ile164 and Phe158, make significant contacts with large, hydrophobic substituents at the 2' or 6' positions on penicillins, cephalosporins, or carbapenems [ 37 ] . To test this prediction, Ile 164 and Phe158 were changed from large, hydrophobic residues to alanines to afford the I164A and F158A mutants.
There are over 300 distinct β-lactamases known, and these enzymes have been grouped by a number of classification schemes [ 8 9 10 11 12 13 14 15 ] . For example, Bush has developed a scheme, based on the enzymes' molecular properties, that has four distinct β-lactamase groups [ 10 15 ] . One of the more alarming groups are the Bush group 3 enzymes, which are Zn(II) dependent enzymes that hydrolyze nearly all known β-lactam containing antibiotics and for which there are no or very few known clinical inhibitors [ 9 14 16 17 18 19 ] . The metallo-β-lactamases have been further divided by Bush into subgroups based on amino acid sequence identity: the Ba enzymes share a >23% sequence identity, require 2 Zn(II) ions for full activity, prefer penicillins and cephalosporins as substrates, and are represented by metallo-β-lactamase CcrA from Bacteroides fragilis, the Bb enzymes share a 11% sequence identity with the Ba enzymes, require only 1 Zn(II) ion for full activity, prefer carbapenems as substrates, and are represented by the metallo-β-lactamase imiS from Aeromonas sobria, and the Bc enzymes have only 9 conserved residues with the other metallo-β-lactamases, require 2 Zn(II) ions for activity, contain a different metal binding motif than the other metallo-β-lactamases, prefer penicillins as substrates, and are represented by the metallo-β-lactamase L1 from Stenotrophomonas maltophilia [ 9 ] . A similar grouping scheme (B1, B2, and B3) based on structural properties of the metallo-β-lactamases has recently been offered [ 41 ] . The diversity of the group 3 β-lactamases is best exemplified by the enzymes' vastly differing efficacies towards non-clinical inhibitors; these differences predict that one inhibitor may not inhibit all metallo-β-lactamases [ 18 20 21 22 23 24 25 26 27 28 29 ] . To combat this problem, we are characterizing a metallo-β-lactamase from each of the subgroups in an effort to identify a common structural or mechanistic aspect of the enzymes that can be targeted for the generation of an inhibitor.
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