Site-directed mutagenesis of beta-lactamase TEM-1. Investigating the potential role of specific residues on the activity of Pseudomonas-specific enzymes.

From sequence alignments, two groups can be defined for the carbenicillin-hydrolysing beta-lactamases (CARB enzymes). One group includes the Pseudomonas-specific enzymes PSE-1, PSE-4, CARB-3, CARB-4 and also the Proteus mirabilis GN79, for which the well-conserved residue Lys 234 in all class-A beta-lactamases is changed to an arginine residue. The second group includes the enzymes PSE-3 and AER-1 which have an arginine or a lysine residue at position 165. All these enzymes also have leucine at position 68, threonine at position 104 and glycine at position 240. We engineered these mutations into the TEM-1 beta-lactamase to study their potential role in defining the substrate profile of the CARB enzymes. The mutations K234R and E240G in TEM-1 noticeably increased the hydrolysis of carboxypenicillins relative to other penicillins by approximately sixfold and twofold, respectively. The variant E240G also demonstrated an improved rate of second-generation cephalosporin and cefotaxime hydrolysis. In contrast, the substitution of Trp165 by arginine does not extend the substrate profile to alpha-carboxypenicillins nor does it noticeably modify the kinetic behavior of the enzyme. The mutations M68L and E104T do not have a large effect on the hydrolysis rate but the mutation E104T enhances the affinity of the enzyme for third-generation cephalosporins. As the mutation K234R resulted in a severe decrease in the affinity for carboxypenicillins, the double mutant E240G/K234R was constructed in an attempt to enhance the CARB character of the enzyme. Contrary to what could be expected, the additional mutation E240G for the TEM-1 K234R enzyme increases neither the catalytic constant for the carboxypenicillins nor the affinity towards these substrates. Consequently, this study strongly suggests that the three-dimensional structures of the active site of the TEM-1 enzyme and PSE-3, PSE-4 or other related enzymes are significantly different. This probably explains the discrepancy of the substrate profile between the CARB enzymes and the TEM-1 protein variants.