Characterization of OXA-29 from Legionella (Fluoribacter) gormanii: molecular class D beta-lactamase with unusual properties.

Class D β-lactamases are a group of structurally related active site serine enzymes that share a common ancestry with the high-molecular-weight class C penicillin-binding proteins (18). Class D β-lactamases usually exhibit a preference for penam substrates, including oxacillin and related compounds (hence their eponyms of oxacillinases or OXA-type β-lactamases), and with few exceptions are poorly inhibited by clavulanic acid, being classified in group 2d of the functional classification of β-lactamases (7, 21). Unlike other β-lactamases, class D enzymes are inhibited by chloride ions and tend to exhibit “burst” kinetics, with initial hydrolysis rates declining more rapidly than can be explained by substrate depletion (16). Recently the structure of OXA-10 has been solved, providing major insights into the structural and mechanistic features of these enzymes (12, 19, 25). The structural data suggested that the catalytic mechanism of class D β-lactamases is different from that of other serine β-lactamases and also provided some explanation of the mechanism of chloride inhibition typical of these enzymes (12, 19, 25). Another remarkable feature of OXA-10, which could apply to other class D β-lactamases as well, is the tendency to form dimers in the presence of some divalent cations and to exhibit modified kinetic behavior upon dimerization (25). Currently, some 30 different class D β-lactamases have been described (1, 2, 5, 10, 21, 23, 27, 35), and several groups have been identified within this class on the basis of the degree of structural relatedness among the various members (21, 32). Most known class D β-lactamases are encoded by genes carried on mobile elements, while a minority are encoded by genes apparently resident in some microbial genomes (21). From a clinical standpoint, the relevance of class D β-lactamases is essentially dependent on their occurrence as acquired enzymes in clinical isolates of various gram-negative pathogens, such as pseudomonads, acinetobacters, and enteric bacteria (7, 17, 21). The finding of class D β-lactamases that are active on oxyimino-cephalosporins or carbapenems (1, 2, 5, 10, 21, 27) has further enhanced their medical interest. In this work we report the characterization of a new class D β-lactamase determinant from Legionella (Fluoribacter) gormanii, the product of which, named OXA-29, is quite divergent from and exhibits some unusual features compared to other class D enzymes.