Proteolytic interconversion and N-terminal sequences of the Citrobacter diversus major β-lactamases

The N-terminal sequences of the two major β-lactamases produced by Citrobacter diversus differed only by the absence of the first residue in form II and the loss of five amino acid residues at the C-terminal end. Limited proteolysis of the homogeneous form I protein yielded a variety of enzymatically active products. In the major product obtained after the action of papain, the first three N-terminal residues of form I had been cleaved, whereas at the C-terminal end the treated enzyme lacked five residues. However, this cannot explain the different behaviours of form I, form II and papain digestion product upon chromatofocusing. Form I, which was sequenced up to position 56, exhibited a very high degree of similarity with a Klebsiella oxytoca β-lactamase. The determined sequence, which contained the active serine residue, demonstrated that the chromosome-encoded β-lactamase of Citrobacter diversus belong to class A.