Identification of the active site of Citrobacter freundii β‐lactamase using dansyl‐penicillin

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Abstract
The active site sequence of a β‐lactamase encoded by chromosomal gene(s) in Citrobacter freundii GN346 was determined using dansyl‐penicillin as a fluorescent probe. The tryptic digest of the labelled enzyme gave a fluorescent peptide containing 22 amino acids. The sequence of this peptide was identical to the consensus sequence of class C β‐lactamases, Gly‐Ser‐X‐Ser‐Lys. The residue labelled was the serine adjacent to the glycine. The active site sequence corresponded to positions 46–67 of the entire sequence of the Citrobacter freundii β‐lactamase determined on the basis of the DNA sequence of the structural gene [(1986) Eur. J. Biochem. 156, 441–445]. The labelled serine corresponded to Ser‐64.