Ambler class D β-Lactamase (oxacillinase) (OXA)

Ambler class D β-lactamases, commonly known as oxacillinases (OXA), are bacterial enzymes that confer resistance to beta-lactam antibiotics—one of the most important antibiotic classes in clinical medicine. These serine-dependent hydrolytic enzymes inactivate beta-lactam antibiotics by cleaving the characteristic four-atom beta-lactam ring, rendering the drugs ineffective against bacterial infection. Originally characterized by their preferential activity against oxacillin, OXA enzymes have emerged as a major clinical problem, particularly in *Acinetobacter baumannii* and *Enterobacteriaceae* species implicated in hospital-acquired infections. The enzyme family exhibits remarkable genetic and biochemical diversity, with over 400 variants displaying variable substrate specificities ranging from narrow-spectrum activity against penicillins to expanded-spectrum activity including carbapenem hydrolysis. Many OXA variants are encoded on plasmids, enabling rapid dissemination of resistance genes across bacterial populations through horizontal gene transfer. Therapeutic strategies targeting these enzymes focus on beta-lactamase inhibitors (such as avibactam, tazobactam, and clavulanate) that form stable complexes with the enzyme's active site serine, preventing it from hydrolyzing co-administered beta-lactam antibiotics. The continuing emergence of new OXA variants with expanded resistance profiles, particularly those capable of hydrolyzing last-resort carbapenem antibiotics, represents a significant challenge to antimicrobial therapy and underscores the critical need for novel therapeutic approaches and stewardship strategies.

Other names

OxacillinaseOXA-type β-lactamaseOXA enzymeClass D β-lactamaseSerine β-lactamase (functional classification)

Ambler class D β-lactamases hydrolyze beta-lactam antibiotics through a serine-dependent catalytic mechanism: 1. An active site serine (Ser-67 in standard OXA numbering) acts as a nucleophile and adds to the carbonyl of the beta-lactam antibiotic 2. This forms an acyl-enzyme intermediate following departure of the nitrogen from the beta-lactam ring 3. A water molecule is activated as a nucleophile to attack the acyl-enzyme intermediate, restoring the free serine and producing the hydrolyzed (inactivated) antibiotic 4. A carbamate anion formed by post-translational modification of an active site lysine (through reaction with carbon dioxide) serves as the general base that activates both the serine during acylation and water during deacylation The enzyme's catalytic activity is entirely dependent on carbamate anion formation; absence of this modification renders the enzyme catalytically inactive.