An Analysis of Why Highly Similar Enzymes Evolve Differently
a Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030|, http://www.100md.com
b Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030|, http://www.100md.com
ABSTRACT|, http://www.100md.com
The TEM-1 and SHV-1 ß-lactamases are important contributors to resistance to ß-lactam antibiotics in gram-negative bacteria. These enzymes share 68% amino acid sequence identity and their atomic structures are nearly superimposable. Extended-spectrum cephalosporins were introduced to avoid the action of these ß-lactamases. The widespread use of antibiotics has led to the evolution of variant TEM and SHV enzymes that can hydrolyze extended-spectrum antibiotics. Despite being highly similar in structure, the TEM and SHV enzymes have evolved differently in response to the selective pressure of antibiotic therapy. Examples of this are at residues Arg164 and Asp179. Among TEM variants, substitutions are found only at position 164, while among SHV variants, substitutions are found only at position 179. To explain this observation, the effects of substitutions at position 164 in both TEM-1 and SHV-1 on antibiotic resistance and on enzyme catalytic efficiency were examined. Competition experiments were performed between mutants to understand why certain substitutions preferentially evolve in response to the selective pressure of antibiotic therapy. The data presented here indicate that substitutions at position Asp179 in SHV-1 and Arg164 in TEM-1 are more beneficial to bacteria because they provide increased fitness relative to either wild type or other mutants.
PENICILLIN and other ß-lactam antibiotics have been available since the 1940s to treat bacterial infections and are among the most often used antimicrobial agents (GHUYSEN 1991 ; NAVARRE and SCHNEEWIND 1999 ). The extensive use of these antibiotics has unfortunately led to the emergence of resistant strains of bacteria (ABRAHAM and CHAIN 1940 ). Among these strains, the production of ß-lactamase enzymes is the most common mechanism of resistance (ABRAHAM and CHAIN 1940 ; FRERE 1995 ). ß-Lactamases catalyze the hydrolysis of the amide bond present in the ß-lactam ring to create an ineffective antimicrobial agent. The enzymes can be divided into four classes (A, B, C, and D) based on primary sequence homology (AMBLER 1980 ). The class A, C, and D enzymes utilize an active site serine to hydrolyze ß-lactam antibiotics (MATAGNE et al. 1998 ). In contrast, class B enzymes, also known as the metallo-ß-lactamases, utilize zinc ions to catalyze the hydrolysis of ß-lactams (LIVERMORE and WOODFORD 2000 ). The rapid spread of ß-lactam resistance, both within and between species, is facilitated by the transmission of ß-lactamase-encoding genes via mobile genetic elements such as transposons and plasmids (GHUYSEN 1991(Fahd K. Majiduddin and Timothy Palzkill)
b Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030|, http://www.100md.com
ABSTRACT|, http://www.100md.com
The TEM-1 and SHV-1 ß-lactamases are important contributors to resistance to ß-lactam antibiotics in gram-negative bacteria. These enzymes share 68% amino acid sequence identity and their atomic structures are nearly superimposable. Extended-spectrum cephalosporins were introduced to avoid the action of these ß-lactamases. The widespread use of antibiotics has led to the evolution of variant TEM and SHV enzymes that can hydrolyze extended-spectrum antibiotics. Despite being highly similar in structure, the TEM and SHV enzymes have evolved differently in response to the selective pressure of antibiotic therapy. Examples of this are at residues Arg164 and Asp179. Among TEM variants, substitutions are found only at position 164, while among SHV variants, substitutions are found only at position 179. To explain this observation, the effects of substitutions at position 164 in both TEM-1 and SHV-1 on antibiotic resistance and on enzyme catalytic efficiency were examined. Competition experiments were performed between mutants to understand why certain substitutions preferentially evolve in response to the selective pressure of antibiotic therapy. The data presented here indicate that substitutions at position Asp179 in SHV-1 and Arg164 in TEM-1 are more beneficial to bacteria because they provide increased fitness relative to either wild type or other mutants.
PENICILLIN and other ß-lactam antibiotics have been available since the 1940s to treat bacterial infections and are among the most often used antimicrobial agents (GHUYSEN 1991 ; NAVARRE and SCHNEEWIND 1999 ). The extensive use of these antibiotics has unfortunately led to the emergence of resistant strains of bacteria (ABRAHAM and CHAIN 1940 ). Among these strains, the production of ß-lactamase enzymes is the most common mechanism of resistance (ABRAHAM and CHAIN 1940 ; FRERE 1995 ). ß-Lactamases catalyze the hydrolysis of the amide bond present in the ß-lactam ring to create an ineffective antimicrobial agent. The enzymes can be divided into four classes (A, B, C, and D) based on primary sequence homology (AMBLER 1980 ). The class A, C, and D enzymes utilize an active site serine to hydrolyze ß-lactam antibiotics (MATAGNE et al. 1998 ). In contrast, class B enzymes, also known as the metallo-ß-lactamases, utilize zinc ions to catalyze the hydrolysis of ß-lactams (LIVERMORE and WOODFORD 2000 ). The rapid spread of ß-lactam resistance, both within and between species, is facilitated by the transmission of ß-lactamase-encoding genes via mobile genetic elements such as transposons and plasmids (GHUYSEN 1991(Fahd K. Majiduddin and Timothy Palzkill)