Macromolecular Machineries in Cell‐Wall Recycling

Abstract

Peptidoglycan, the major constituent of bacterial cell wall, is a huge polymeric macromolecule composed of glycan strands covalently linked by interconnecting peptide stems and critical for survival of bacteria. Peptidoglycan is constantly edited, biosynthesised and degraded, during essential bacterial processes such as division, elongation or insertion of the cellular machinery (e.g. flagella, pili) into cell wall. Degradation fragments are recovered, especially in Gram‐negative bacteria, by active transport and recycled to rebuild the wall. In some important Gram‐negative pathogens, β‐lactam resistance is directly linked to peptidoglycan recycling. The recycling process requires orchestration of different lytic enzymes and transporters from periplasm, the inner membrane, to cytoplasm. Recent structural and functional studies have provided relevant insights into enzymatic regulation, substrate specificity and activity of the lytic machineries involved in recycling with relevant implications in antibiotics resistance. Key Concepts Peptidoglycan recycling is an essential process in Gram‐negative bacteria that recovers self‐degradation products of cell wall in the cytoplasm to reutilise them to rebuild the wall. PG recycling is important in cell‐wall biosynthesis and in bacterial communication in many bacteria, and in antibiotics resistance in some pathogens from Enterobacteriaceae and Pseudomonadaceae families. PG recycling involves a large number of enzymes and transporters spanning from periplasm, inner and outer membranes, and cytoplasm. Lytic transglycosylases (LTs) are periplasmic and, typically, modular enzymes that initiate PG recycling by non‐hydrolytic degradation of glycan chains. They are classified in six families. Several LTs are found in each Gram‐negative bacterium. The Slt structure reveals how, under the effect of β‐lactam antibiotics, the cell wall is repaired by degradation of aberrant nascent PG chains and how it initiates the resistance phenotype. MltF is a modular membrane‐bound LT that experiences allosteric activation triggered by muropeptides. PG amidases are present in periplasm and cytoplasm. Activity in periplasm is regulated by cellular location, protein–protein interactions and oligomeric state. An activation mechanism has been discovered for cytosolic enzymes. Anhydro‐muropeptides are the key molecules to activate the AmpR to induce AmpC β‐lactamase in response to β‐lactam challenge. Regulation (in space and time) of the catalytic processes as well as of interaction between different partners is essential in PG recycling enzymes.