Beta-lactamase class A and class C (Class A β-lactamase (often abbreviated as Class A BL) and class C β-lactamase (Class C BL); there is no single unified abbreviation for both classes together.)

Class A and class C beta-lactamases are two major molecular classes of enzymes produced by bacteria that confer resistance to beta-lactam antibiotics (including penicillins, cephalosporins, and related drugs). Both are serine hydrolases: they utilize an active site serine to break the characteristic four-membered beta-lactam ring of these antibiotics, disabling their antibacterial activity. Class A beta-lactamases include enzymes such as TEM, SHV, and CTX-M, many of which are plasmid-encoded, spread easily, and may confer extended-spectrum resistance (ESBLs). They are often inhibited by clavulanic acid but can evolve inhibitor resistance. Class C beta-lactamases (often called AmpC enzymes) are typically chromosomally encoded cephalosporinases that confer resistance primarily to cephalosporins and are often not inhibited by traditional beta-lactamase inhibitors. Both enzyme classes use a conserved mechanism involving acylation of a serine residue, followed by hydrolysis of the beta-lactam ring, and play a critical role in global antibiotic resistance in clinically important Gram-negative bacteria.

Other names

Class A serine β-lactamaseClass C serine β-lactamaseSerine β-lactamase (encompassing A, C, and D)Penicillinase (for some Class A enzymes)Cephalosporinase (for many Class C enzymes)ESBL (Extended-spectrum beta-lactamase, often for Class A variants like TEM, SHV, CTX-M)

Hydrolysis of beta-lactam ring: The enzyme attacks the amide bond of the beta-lactam ring, breaking antibiotic structure and inactivating antibacterial activity. Serine-based catalysis: Both A and C use a nucleophilic serine (Ser70) for acylation and breakdown of beta-lactam antibiotics.