The conserved Class A ␤-lactamase active site residue Tyr-105 was substituted by saturation mutagenesis in TEM-1 ␤-lactamase from Escherichia coli in order to clarify its role in enzyme activity and in substrate stabilization and discrimination. Minimum inhibitory concentrations were calculated for E. coli cells harboring each Y105X mutant in the presence of various penicillin and cephalosporin antibiotics. We found that only aromatic residues as well as asparagine replacements conferred high in vivo survival rates for all substrates tested. At position 105, the small residues alanine and glycine provide weak substrate discrimination as evidenced by the difference in benzylpenicillin hydrolysis relative to cephalothin, two typical penicillin and cephalosporin antibiotics. Kinetic analyses of mutants of interest revealed that the Y105X replacements have a greater effect on K m than k cat , highlighting the importance of Tyr-105 in substrate recognition. Finally, by performing a short molecular dynamics study on a restricted set of Y105X mutants of TEM-1, we found that the strong aromatic bias observed at position 105 in Class A ␤-lactamases is primarily defined by a structural requirement, selecting planar residues that form a stabilizing wall to the active site. The adopted conformation of residue 105 prevents detrimental steric interactions with the substrate molecule in the active site cavity and provides a rationalization for the strong aromatic bias found in nature at this position among Class A ␤-lactamases.During the past decades, ␤-lactamase production (EC 3.5.2.6) has become a significant problem in bacterial strain resistance to widely used clinical antibiotics. Among these enzymes, the prevalent type has always been the Class A active site serine hydrolase ␤-lactamases, which have become model enzymes extensively studied by protein engineering with respect to site-directed or combinatorial mutagenesis (1-5), structure determination (6 -10), and molecular simulations (11-13). Over the years Escherichia coli TEM-1 ␤-lactamase has become an impressive example of the rapid evolution rate of proteins occurring within natural bacterial isolates subjected to selective pressure. Ever since the clinical introduction of ␤-lactam compounds and the discovery of TEM-1 ␤-lactamase, both in the 1940s (14), natural mutations have generated a large number of single and multiple mutants of this enzyme (for an extensive list see www.lahey.org/Studies/temtable.asp).The high rate of occurrence of mutated enzymes capable of hydrolyzing higher generation cephalosporins has stimulated research of ␤-lactamase adaptation to these new substrates in order to understand the molecular basis of this evolutionary chain of events. Consequently, a number of studies have successfully predicted the in vitro appearance of new mutations conferring resistance before their appearance in natural isolates (for an overall view, see Ref 1). To provide more information regarding these mutations in enzyme catalysis and/or substrate stabilization, ...

The diversity in substrate recognition spectra exhibited by various b-lactamases can result from one or a few mutations in the active-site area. Using Escherichia coli TEM-1 b-lactamase as a template that efficiently hydrolyses penicillins, we performed site-saturation mutagenesis simultaneously on two opposite faces of the active-site cavity. Residues 104 and 105 as well as 238, 240, and 244 were targeted to verify their combinatorial effects on substrate specificity and enzyme activity and to probe for cooperativity between these residues. Selection for hydrolysis of an extended-spectrum cephalosporin, cefotaxime (CTX), led to the identification of a variety of novel mutational combinations. In vivo survival assays and in vitro characterization demonstrated a general tendency toward increased CTX and decreased penicillin resistance. Although selection was undertaken with CTX, productive binding (K M ) was improved for all substrates tested, including benzylpenicillin for which catalytic turnover (k cat ) was reduced. This indicates broadened substrate specificity, resulting in more generalized (or less specialized) variants. In most variants, the G238S mutation largely accounted for the observed properties, with additional mutations acting in an additive fashion to enhance these properties. However, the most efficient variant did not harbor the mutation G238S but combined two neighboring mutations that acted synergistically, also providing a catalytic generalization. Our exploration of concurrent mutations illustrates the high tolerance of the TEM-1 active site to multiple simultaneous mutations and reveals two distinct mutational paths to substrate spectrum diversification.