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Amino Acid Substitutions at Ambler Position Gly238 in the SHV-1 ß-Lactamase: Exploring Sequence Requirements for Resistance to Penicillins and Cephalosporins

Research Service,¹ Infectious Diseases Service, Louis Stokes Cleveland Department of Veterans Affairs Medical Center,⁴ Division of Infectious Diseases, University Hospitals of Cleveland,² Department of Biochemistry, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106³

Received 14 November 2001/ Returned for modification 16 March 2002/ Accepted 26 July 2002

Site saturation mutagenesis of the 238 position in the SHV ß-lactamase was performed to identify the complete sequence requirements needed for the extended spectrum ß-lactamase (ESBL) phenotype. MICs (in micrograms per milliliter) in an isogenic background, Escherichia coli DH10B, demonstrated that the Gly238Ala mutation conferred the most resistance to penicillins and cephalosporins. The absolute increase in resistance was greatest against cefotaxime for the Gly238Ala mutant (0.06 to 8 µg/ml). Except for the strain possessing the Gly238Pro ß-lactamase, ceftazidime MICs were also elevated. None of the mutant SHV ß-lactamases were expressed in as great an amount as the wild-type ß-lactamase. Kinetic analysis of the Gly238Ala mutant revealed that penicillin and cephalosporin substrates have a lower K_m for the enzyme because of this mutation. Ampicillin and piperacillin MICs were inversely proportional to the side chain volume of the amino acid in cases larger than Ser, suggesting that steric considerations may be a primary requirement for penicillin resistance. Secondary structural effects explain increased resistance to oxyiminocephalosporins. Based upon this study, we anticipate that additional mutations of Gly238 in the SHV ß-lactamase will continue to be discovered with an ESBL (ceftazidime or cefotaxime resistant) phenotype.

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