Crystal Structure of the Mycobacterium fortuitum Class A β-Lactamase: Structural Basis for Broad Substrate Specificity

Abstract

␤-Lactamases are the main cause of bacterial resistance to penicillins and cephalosporins. Class A ␤-lactamases, the largest group of ␤-lactamases, have been found in many bacterial strains, including mycobacteria, for which no ␤-lactamase structure has been previously reported. The crystal structure of the class A ␤-lactamase from Mycobacterium fortuitum (MFO) has been solved at 2.13-Å resolution. The enzyme is a chromosomally encoded broad-spectrum ␤-lactamase with low specific activity on cefotaxime. Specific features of the active site of the class A ␤-lactamase from M. fortuitum are consistent with its specificity profile. Arg278 and Ser237 favor cephalosporinase activity and could explain its broad substrate activity. The MFO active site presents similarities with the CTX-M type extended-spectrum ␤-lactamases but lacks a specific feature of these enzymes, the VNYN motif (residues 103 to 106), which confers on CTX-M-type extended-spectrum ␤-lactamases a more efficient cefotaximase activity.Mycobacteria are important causes of infectious diseases. Although Mycobacterium tuberculosis and Mycobacterium leprae, two slow-growing species, are responsible for the most serious diseases, some fast-growing species, such as Mycobacterium avium, Mycobacterium kansasii, Mycobacterium chelonae, and Mycobacterium fortuitum, may cause opportunistic infections among AIDS patients (24). In addition, M. fortuitum has been reported to be responsible for a wide spectrum of clinical diseases, such as skin or soft tissue infections following surgery, pulmonary infections, accidental penetrating trauma, and wounds that come in contact with soil or water contaminated with these mycobacteria, though nosocomial infections are by far the most common (41, 44).The cell wall of mycobacteria contains mycolic acid, arabinogalactan, and peptidoglycan, forming a covalent complex. The influx of small hydrophilic agents through the resulting low-permeability envelope is extremely slow and is thought to be one of the major factors involved in the resistance of mycobacteria to ␤-lactam antibiotics (21, 41). ␤-Lactamase production, catalyzing ␤-lactam antibiotic hydrolysis, appears to be the second mechanism by which mycobacteria express ␤-lactam resistance (9,21,35). Chromosomally encoded ␤-lactamases have been detected in most, but not all, mycobacterial species, including M. fortuitum and M. tuberculosis (43).The class A ␤-lactamase of M. fortuitum (MFO) is a chromosomally encoded broad-spectrum ␤-lactamase hydrolyzing both cephalosporins and penicillins. Its substrate profile includes ureidopenicillins (piperacillin, azlocillin, and mezlocillin), carbenicillin, cephalothin, cefotaxime, and cefuroxime, but not ceftazidime. Cefoxitin, dicloxacillin, imipenem, and aztreonam are poorly recognized by the enzyme, and the protein is inhibited by the penem inhibitor BRL42715 and, to a lesser extent, clavulanic acid. The specificity profile is similar overall to those of CTX-M-type enzymes; some TEM-derived extended-spectrum ␤-lactamases (ESBLs); the ...

Hydrolysis of Clavulanate by Mycobacterium tuberculosis β-Lactamase BlaC Harboring a Canonical SDN Motif

“…In contrast, the polymorphism in Bla MAb and BlaC affects the third residue (SDN instead of SDG). In mycobacteria, inhibition of SDN-containing class A ␤-lactamases has been reported for Mycobacterium smegmatis (3) and Mycobacterium fortuitum (24,25). Thus, the impact of the G 132 N substitution on clavulanate hydrolysis appears to be specific to BlaC.…”

A Structure-Based Classification of Class A β-Lactamases, a Broadly Diverse Family of Enzymes

“…Many enzymatic studies have been carried out on Mycobacterium tuberculosis (BlaC), Mycobacterium abscessus (MAB-1), and Mycobacterium fortuitum (BlaF/MFO-1) (Table 1) (4,22,23,113). The enzymes produced by these species have a broader spectrum of activity, also degrading cephalosporins, for example, and they are less sensitive to clavulanic acid; nevertheless, they are classified as functional group 2b enzymes.…”

Impact of β-Lactamase Inhibition on the Activity of Ceftaroline against Mycobacterium tuberculosis and Mycobacterium abscessus

“…Mycobacteria are intrinsically resistant to most ␤-lactams due to the production of broad-spectrum ␤-lactamases in combination with low affinity of the drug targets and limited permeability of the cell wall (12)(13)(14). The former mechanism can be counteracted by ␤-lactamase inhibitors, which display various efficiencies for enzymes from different species (15,16).…”