TABLE 4.

Ratio of k_cat/K_m of mutant enzymes with Asp²⁷⁶ and Leu⁶⁹ substitutions and of IRT-4 and IRT-14 β-lactamases relative to that of the wild-type (pBSKS/TEM-1)^a

Substrate	kcat/Km ratio (%)
	TEM-1	Asn276Asp	M69Lb	IRT-4 Met69Leu, Asn276Asp)c	IRT-14 (Met69Leu, Arg275Gln)d
Benzylpenicillin	100	46	44	10.1	7.8
Amoxicillin	100	18	26	13.6	11.6
Ticarcillin	100	22	12	3.9	1.3
Piperacillin	100	45	52	5.5	6.5
Mezlocillin	100	32	NDe	ND	ND
Mecillinam	100	115	ND	ND	ND
Cephalothin	100	78	21	1.8	13
Cephaloridine	100	288	24	21.8	58.2
Cefoperazone	100	77	68	ND	ND

^aThe value for wild-type pBSKS/TEM-1 was set equal to 100. 

^bData are from reference 16. 

^cData are from reference 7. 

^dData are from reference 10. 

^eND, not determined.