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Antimicrobial Agents and Chemotherapy, August 2005, p. 3421-3427, Vol. 49, No. 8
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.8.3421-3427.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Amino Acid Residues That Contribute to Substrate Specificity of Class A ß-Lactamase SME-1

Verna and Marrs McLean Department of Biochemistry and Molecular Biology,¹ Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030²

Received 3 March 2005/ Returned for modification 5 May 2005/ Accepted 26 May 2005

Carbapenem antibiotics are used as antibiotics of last resort because they possess a broad spectrum of antimicrobial activity and are not easily hydrolyzed by ß-lactamases. Recently, class A enzymes, such as the SME-1, NMC-A, and IMI-1 ß-lactamases, have been identified with the capacity to hydrolyze carbapenem antibiotics. Traditional class A ß-lactamases, such as TEM-1 and SHV-1, are unable to hydrolyze carbapenem antibiotics and exhibit some differences in sequence from those that are able to hydrolyze carbapenem antibiotics. The positions that differ may contribute to the unique substrate specificity of the class A carbapenemase SME-1. Codons in the SME-1 gene representing residues 104, 105, 132, 167, 237, and 241 were randomized by site-directed mutagenesis, and functional mutants were selected for the ability to hydrolyze imipenem, ampicillin, or cefotaxime. Although several positions are important for hydrolysis of ß-lactam antibiotics, no single position was found to uniquely contribute to carbapenem hydrolysis. The results of this study support a model whereby the carbapenemase activity of SME-1 is due to a highly distributed set of interactions that subtly alter the structure of the active-site pocket.

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