Enzymatic Inactivation and Beta-Lactamases

One of the most clinically important resistance strategies is to destroy or chemically alter the antibiotic before it can act. Beta-lactamases — enzymes that hydrolyse the beta-lactam ring of penicillins, cephalosporins, and carbapenems — are the archetype, but bacteria also produce enzymes that modify aminoglycosides and other drugs, neutralizing them without changing the drug's target.

Definition

Enzymatic inactivation is resistance achieved by enzymes that chemically destroy or modify an antibiotic so that it can no longer bind its target; beta-lactamases are the prototypical example, hydrolysing the beta-lactam ring shared by penicillins, cephalosporins, monobactams, and carbapenems.

Scope

This topic covers enzymatic resistance: the hydrolytic destruction of beta-lactams by beta-lactamases, the major schemes for classifying these enzymes, and the broader category of drug-modifying enzymes such as aminoglycoside-modifying enzymes. Target alteration and efflux are treated in a companion topic. The treatment is mechanistic and microbiological rather than clinical, and includes no dosing or therapy guidance.

Core questions

• How do enzymes neutralize an antibiotic without altering its target?
• What reaction do beta-lactamases catalyse, and on which drugs?
• How are beta-lactamases classified, and why does classification matter?
• What other drug classes are inactivated by modifying enzymes?

Key concepts

• Beta-lactamase hydrolysis
• Serine versus metallo-beta-lactamases
• Ambler molecular classes A-D
• Bush-Jacoby functional groups
• Extended-spectrum beta-lactamases
• Carbapenemases
• Aminoglycoside-modifying enzymes
• Beta-lactamase inhibitors

Mechanisms

Enzymatic resistance neutralizes the drug itself. Beta-lactamases hydrolyse the four-membered beta-lactam ring that is essential for these antibiotics' activity, abolishing their ability to inhibit penicillin-binding proteins. They are grouped two complementary ways: the Ambler molecular scheme by amino-acid sequence (classes A, C, and D use an active-site serine, while class B are metallo-enzymes that require zinc), and the Bush-Jacoby scheme by functional substrate and inhibitor profile. Some beta-lactamases have narrow substrate ranges, while extended-spectrum beta-lactamases and carbapenemases hydrolyse broader sets of beta-lactams, including agents once considered stable. Other drug classes are inactivated by modification rather than cleavage: aminoglycoside-modifying enzymes add chemical groups (by acetylation, phosphorylation, or adenylylation) that prevent the drug from binding the ribosome. Beta-lactamase inhibitors counter some of these enzymes by binding the enzyme rather than the bacterial target (Bush & Bradford, 2016; Bush & Jacoby, 2010; Ramirez & Tolmasky, 2010).

Clinical relevance

Beta-lactamase type largely determines which beta-lactams an organism can resist, and the spread of extended-spectrum beta-lactamases and carbapenemases is central to understanding multidrug-resistant Gram-negative infections; classification is reference knowledge for interpreting resistance phenotypes. This entry describes the enzymology and does not provide treatment, agent-selection, or dosing recommendations.

Epidemiology

Beta-lactamases are numerous and widely distributed, with thousands of variants described, and many are carried on mobile genetic elements that facilitate global spread. Extended-spectrum beta-lactamases and carbapenemases have disseminated worldwide among Enterobacterales and other Gram-negative bacteria, while aminoglycoside-modifying enzymes are similarly broadly distributed (Bush & Jacoby, 2010; Munita & Arias, 2016).

Evidence & guidelines

The classification and mechanistic accounts here follow widely cited reviews of beta-lactamases and modifying enzymes (Bush & Bradford, 2016; Bush & Jacoby, 2010; Ramirez & Tolmasky, 2010). The entry is educational and issues no clinical guidelines.

History

A penicillin-destroying enzyme was described in bacteria before penicillin came into wide clinical use, and as new beta-lactams were introduced, bacteria responded with an expanding diversity of beta-lactamases. Ambler's molecular classification and the Bush-Jacoby functional scheme provided complementary frameworks for organizing these enzymes, and the later emergence of extended-spectrum beta-lactamases and carbapenemases marked successive waves of resistance to broader beta-lactams (Bush & Jacoby, 2010; Bush & Bradford, 2016).

Key figures

• Karen Bush
• George A. Jacoby
• Richard P. Ambler
• Marcelo E. Tolmasky

Seminal works

• bush-jacoby-2010
• bush-bradford-2016
• ramirez-tolmasky-2010

Frequently asked questions

What do beta-lactamases do?

They hydrolyse the beta-lactam ring of penicillins, cephalosporins, and related antibiotics, destroying the structure the drug needs to inhibit bacterial cell-wall synthesis.

How are beta-lactamases classified?

By two complementary schemes: the Ambler molecular classes A-D based on protein sequence (serine versus metallo-enzymes), and the Bush-Jacoby functional groups based on substrate and inhibitor profiles.