Replacement of the 3-carboxylate residue in phenoxymethylpenicillin by an aldehyde group gives a good substrate for E. cloacae P99 P-lactamase with k,,, = 34 s-I, K , = 7.5 x loe5 mol dmP3 and k,,,/K, = 4.5 x lo5 dm3 mol-' s-' at pH 7. With B. cereus 569/H f3-lactamase I as a catalyst, k,,,/K, = 1.87 x lo4 dm3 mol-' s-' at pH 7 and shows a bell-shaped dependence on pH with apparent pK,s of 4.76 and 9.72. Any close proximity between the penicillin 3-aldehyde and a lysine amino group on the protein does not result in iminine formation and inhibition of the enzyme.Paper 5/00873E
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