Penicillin 3-aldehyde is a good substrate and not an inhibitor of β-lactamases A and C

Replacement of the 3-carboxylate residue in phenoxymethylpenicillin by an aldehyde group gives a good substrate for E. cloacae P99 βlactamase with, k_cat = 34 s^-1, K_m = 7.5 × 10^-5 mol dm^-3 and k_cat/K_m = 4.5 × 10 ⁵ dm³ mol^-1 s^-1 at pH 7. With B. cereus 569/H β-lactamase I as a catalyst, k_cat/K_m = 1.87 × 10⁴ dm³ mol^-1 s^-1 at pH 7 and shows a bell-shaped dependence on pH with apparent pK^as 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.