Class D ß-lactamases: are they all carbapenemases?

Carbapenem-hydrolyzing class D ß-lactamases (CHDLs) are enzymes of the utmost clinical importance due to their ability to produce resistance to carbapenems, the antibiotics of last resort for the treatment of various life-threatening infections. The vast majority of these enzymes have been identified in Acinetobacter spp., notably in Acinetobacter baumannii. The OXA-2 and OXA-10 enzymes predominantly occur in Pseudomonas aeruginosa and are currently classified as narrow-spectrum class D ß-lactamases. Here we demonstrate that when OXA-2 and OXA-10 are expressed in Escherichia coli strain JM83, they produce a narrow-spectrum antibiotic resistance pattern. When the enzymes are expressed in A. baumannii ATCC 17978, however, they behave as extended-spectrum ß-lactamases and confer resistance to carbapenem antibiotics. Kinetic studies of OXA-2 and OXA-10 with four carbapenems have demonstrated that their catalytic efficiencies with these antibiotics are in the same range as those of some recognized class D carbapenemases. These results are in disagreement with the classification of the OXA-2 and OXA-10 enzymes as narrow-spectrum ß-lactamases, and they suggest that other class D enzymes that are currently regarded as noncarbapenemases may in fact be CHDLs.

A novel extended-spectrum ß-lactamase, SGM-1, from an environmental isolate of Sphingobium sp.

SGM-1 is a novel class A ß-lactamase from an environmental isolate of Sphingobium sp. containing all of the distinct amino acid motifs of class A ß-lactamases. It shares 77 to 80% amino acid sequence identity with putative ß-lactamases that are present on the chromosome of all Sphingobium species whose genomes were sequenced and annotated. Thus, SGM-type ß-lactamases are native to this genus. Antibiotic susceptibility testing classifies SGM-1 as an extended-spectrum ß-lactamase, conferring the highest level of resistance to penicillins. Although SGM-1 contains the conserved cysteine residues characteristic of class A carbapenemases, it does not confer resistance to the carbapenem antibiotics imipenem, meropenem, or doripenem but does increase the MIC of ertapenem 8-fold. SGM-1 hydrolyzes penicillins and the monobactam aztreonam with similar catalytic efficiencies, ranging from 10(5) to 10(6) M(-1) s(-1). The catalytic efficiencies of SGM-1 for cefoxitin and ceftazidime were the lowest (10(2) to 10(3) M(-1) s(-1)) among the cephalosporins tested, while the catalytic efficiencies against all other cephalosporins varied from about 10(5) to 10(6) M(-1) s(-1). SGM-1 exhibited measurable but not significant activity toward the carbapenems tested. SGM-1 also showed high affinity for clavulanic acid, tazobactam, and sulbactam (Ki < 1 µM); however, only clavulanic acid significantly reduced the MICs of ß-lactams.

Antibiotic resistance and substrate profiles of the class A carbapenemase KPC-6.

The class A carbapenemase KPC-6 produces resistance to a broad range of ß-lactam antibiotics. This enzyme hydrolyzes penicillins, the monobactam aztreonam, and carbapenems with similar catalytic efficiencies, ranging from 10(5) to 10(6) M(-1) s(-1). The catalytic efficiencies of KPC-6 against cephems vary to a greater extent, ranging from 10(3) M(-1) s(-1) for the cephamycin cefoxitin and the extended-spectrum cephalosporin ceftazidime to 10(5) to 10(6) M(-1) s(-1) for the narrow-spectrum and some of the extended-spectrum cephalosporins.

Chemical activation of MEK1--a redox trigger for evaluating the effects of phosphorylation.

An approach to generate mimics of phosphorylated serine proteins chemically through site-specific sulfonation of cysteine is presented. This chemical modification is reversible in the presence of reducing agent and therefore is analogous to the kinase/phosphatase system used in nature.