Kinetics of Turnover of Cefotaxime by the Enterobacter cloacae P99 and GCl -Lactamases: Two Free Enzyme Forms
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- 作者：Sanjai Kumar ; S. A. Adediran ; Michiyoshi Nukaga ; and R. F. Pratt
- 刊名：Biochemistry
- 出版年：2004
- 出版时间：March 9, 2004
- 年：2004
- 卷：43
- 期：9
- 页码：2664 - 2672
- 全文大小：129K
- 年卷期：v.43,no.9(March 9, 2004)
- ISSN：1520-4995
文摘
Third-generation cephalosporins bearing oximino side chains are resistant to hydrolysis byclass C 
-lactamases such as that from Enterobacter cloacae P99. For example, steady state parametersfor hydrolysis of cefotaxime by this enzyme are as follows: kcat = 0.41 s-1, Km = 17.2 
M, and kcat/Km= 2.3 × 104 s-1 M-1. On the other hand, however, the Ki value for cefotaxime as an inhibitor of cephalothinhydrolysis is 27 nM. The discrepancy between Km and Ki indicated that a real steady state had not beenachieved in at least one of these experiments. Analysis indicated that only two to three cefotaxime turnoversoccurred during the Ki determination. This suggested that the first few turnovers of cefotaxime by theP99 -lactamase may be different from those in the subsequent steady state. A direct pre-steady stateexperiment confirmed this hypothesis. The simplest reaction scheme that fitted the data involved replacementof the initial enzyme form, E, which bound cefotaxime tightly, with a second more weakly binding form,E', by partitioning of the acyl-enzyme intermediate during the first few turnovers. Steady state turnoverof cefotaxime then largely involved E' as the free enzyme form. E' slowly reverted to E in the post-steadystate regime. Further evidence for this scheme included quantitative analysis of the post-steady state andobservation of a difference in the catalytic activity of E and E' in 2 M ammonium sulfate. The kineticsof P99 -lactamase-catalyzed hydrolysis of an acyclic depsipeptide substrate bearing a third-generationcephalosporin side chain showed that the side chain is necessary but not sufficient for production ofresistance to -lactamase; a combination of the side chain and the dihydrothiazine ring of a cephalosporinis required. The -lactamase of E. cloacae GC1, an extended spectrum mutant of the P99 enzyme, rapidlyhydrolyzes third-generation cephalosporins, without the structural transition described above. The flexibilityof the extended 
loop of the GC1 enzyme probably leads to this situation. Conformational restriction ofthe loop in the P99 enzyme is probably responsible for the long-lived acyl-enzyme intermediate and thetransition to E' induced by cefotaxime.
-lactamases such as that from Enterobacter cloacae P99. For example, steady state parametersfor hydrolysis of cefotaxime by this enzyme are as follows: kcat = 0.41 s-1, Km = 17.2 M, and kcat/Km= 2.3 × 104 s-1 M-1. On the other hand, however, the Ki value for cefotaxime as an inhibitor of cephalothinhydrolysis is 27 nM. The discrepancy between Km and Ki indicated that a real steady state had not beenachieved in at least one of these experiments. Analysis indicated that only two to three cefotaxime turnoversoccurred during the Ki determination. This suggested that the first few turnovers of cefotaxime by theP99 -lactamase may be different from those in the subsequent steady state. A direct pre-steady stateexperiment confirmed this hypothesis. The simplest reaction scheme that fitted the data involved replacementof the initial enzyme form, E, which bound cefotaxime tightly, with a second more weakly binding form,E', by partitioning of the acyl-enzyme intermediate during the first few turnovers. Steady state turnoverof cefotaxime then largely involved E' as the free enzyme form. E' slowly reverted to E in the post-steadystate regime. Further evidence for this scheme included quantitative analysis of the post-steady state andobservation of a difference in the catalytic activity of E and E' in 2 M ammonium sulfate. The kineticsof P99 -lactamase-catalyzed hydrolysis of an acyclic depsipeptide substrate bearing a third-generationcephalosporin side chain showed that the side chain is necessary but not sufficient for production ofresistance to -lactamase; a combination of the side chain and the dihydrothiazine ring of a cephalosporinis required. The -lactamase of E. cloacae GC1, an extended spectrum mutant of the P99 enzyme, rapidlyhydrolyzes third-generation cephalosporins, without the structural transition described above. The flexibilityof the extended loop of the GC1 enzyme probably leads to this situation. Conformational restriction ofthe loop in the P99 enzyme is probably responsible for the long-lived acyl-enzyme intermediate and thetransition to E' induced by cefotaxime.