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| - Insights into the Molecular Basis for the Carbenicillinase Activity of PSE-4β-Lactamase from Crystallographic and Kinetic Studies
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| - PSE-4 is a class A β-lactamase produced by strains of Pseudomonasaeruginosa and is highlyactive for the penicillin derivative carbenicillin. The crystal structure of the wild-type PSE-4 carbenicillinasehas been determined to 1.95 Å resolution by molecular replacement and represents the first structure ofa carbenicillinase published to date. A superposition of the PSE-4 structure with that of TEM-1 shows arms deviation of 1.3 Å for 263 Cα atoms. Most carbenicillinases are unique among class A β-lactamasesin that residue 234 is an arginine (ABL standard numbering scheme), while in all other class A enzymesthis residue is a lysine. Kinetic characterization of a R234K PSE-4 mutant reveals a 50-fold reduction inkcat/Km and confirms the importance of Arg 234 for carbenicillinase activity. A comparison of the structureof the R234K mutant refined to 1.75 Å resolution with the wild-type structure shows that Arg 234 stabilizesan alternate conformation of the Ser 130 side chain, not seen in other class A β-lactamase structures. Ourmolecular modeling studies suggest that the position of a bound carbenicillin would be shifted relative tothat of a bound benzylpenicillin in order to avoid a steric clash between the carbenicillin α-carboxylategroup and the conserved side chain of Asn 170. The alternate conformation of the catalytic Ser 130 inwild-type PSE-4 may be involved in accommodating this shift in the bound substrate position.
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