Table 2.

Kinetic parameters of the CTX-M-4 wild-type β-lactamase and the S237A mutant enzymea

SubstrateCTX-M-4S237A mutant
RelativeVmaxbKm (μM)RelativeVmax/Km (%)cRelativeVmaxbKm (μM)RelativeVmax/Km (%)c
Cephalothin10010310010090100
Penicillin G1422664218259
Ampicillin173156.54031143
Cefuroxime55210273733612.2
Cefotaxime7629526.55041213.5
Aztreonam<0.5NDd 1.8ND
  • a Each Km and relative V max is the mean value of three separate determinations not differing by more than 10%.

  • b Values are percentages of theV max for cephalothin.

  • c Values are percentages of the relativeV max/Km ratio for cephalothin.

  • d ND, not determined.