Substitution of aspartic acid‐217 of Citrobacter freundii cephalosporinase and properties of the mutant enzymes

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Abstract
On the assumption that Asp‐217 of a Citrobacter freundii cephalosporinase forms a salt‐bridge with the conserved Lys‐67, Asp‐217 was changed to glutamic acid, threonine or lysine. The mutant enzymes retained about the same level of activity as that of the wild‐type enzyme, and the participation of Asp‐217 in the salt‐bridge was ruled out. However, the mutations resulted in an increase in hydrolytic activity toward oxyimino‐cephalosporins such as cefuroxime, cefmenoxime and ceftazidime, suggesting a possible mechanism of the bacterial resistance to the novel β‐lactams by a single mutation in cephalosporinases.