The plasmid-mediated TEM-1 and TEM-2 β-lactamases are the most commonly encountered among Gram-negative bacteria. They belong to molecular class A, and differ by one amino acid at position 39: TEM-1 have a glutamine and TEM-2 a lysine. Kinetic parameters (k c a t and K m ) and catalytic efficiency (k c a t /K m ) of TEM-1 and TEM-2 β-lactamases are slightly, but significantly different. For all antibiotics except methicillin and cefazolin, the catalytic efficiency values of TEM-2 are clearly greater than that of TEM-1. Molecular modelling of TEM-2, when compared to that of TEM-1, showed an additional ionic bond between Lys-39 and Glu-281.