TABLE 2.

Kinetic parameters of the CTX-M-4 wild-type β-lactamase and the S237A mutant enzyme^a

Substrate	CTX-M-4			S237A mutant
	Relative Vmaxb	Km (μM)	Relative Vmax/Km (%)c	Relative Vmaxb	Km (μM)	Relative Vmax/Km (%)c
Cephalothin	100	103	100	100	90	100
Penicillin G	14	22	66	42	18	259
Ampicillin	17	31	56.5	40	31	143
Cefuroxime	55	210	27	37	336	12.2
Cefotaxime	76	295	26.5	50	412	13.5
Aztreonam	<0.5	NDd		1.8	ND

^aEach K_m and relative V_max is the mean value of three separate determinations not differing by more than 10%. 

^bValues are percentages of the V_max for cephalothin. 

^cValues are percentages of the relative V_max/K_m ratio for cephalothin. 

^dND, not determined.