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#peptidoglycan

9 public questions tagged with this topic.

Microbial Physiology

Which type of bond in peptidoglycan is hydrolyzed by lysozyme?

Peptidoglycan glycan backbone consists of repeating beta-linked disaccharide where N-acetylglucosamine and N-acetylmuramic acid are joined via beta-1,4 glycosidic bonds between C1 of one sugar and C4 of other, creating long linear strands that run roughly perpendicular to cell long axis. This beta-1,4 linkage creates specific geometry recognized by lysozyme, a muramidase ubiquitous in innate immunity found in tears, saliva, egg white, and macrophage granules. Lysozyme's active site cleft accommodates hexasaccharide, distorts MurNAc residue into half-chair conformation, and catalyzes hydrolysis via glutamate 35 donating proton and aspartate 52 stabilizing oxocarbenium intermediate, cleaving between MurNAc and GlcNAc. Cleavage weakens sacculus, leading to osmotic lysis when combined with turgor. Beta-1,3 linkages characterize pseudomurein and cellulose-like polymers but resist lysozyme, while alpha-1,4 is found in amylose and alpha-1,6 in glycogen branching. Specificity explains why pseudomurein-containing archaea and peptidoglycan O-acetylated pathogens resist lysozyme, and why synthetic beta-1,4 fragments activate NOD2 innate signaling. Mechanistic studies show lysozyme distorts D ring into boat conformation, lowering activation energy for glycosidic cleavage, and this precise stereochemical requirement underlies why modifications like N-deacetylation, O-acetylation at C6 of MurNAc found in pathogenic Staphylococcus aureus and Neisseria gonorrhoeae confer lysozyme resistance, promoting survival within neutrophils and facilitating colonization of mucosal surfaces.

Ref: Vollmer et al., FEMS Microbiol Rev 2008, Lysozyme Specificity; Callewaert & Michiels, J Biosci 2010, Muramidases.

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Microbial Physiology

The pentapeptide bridge in Gram-positive bacteria consists mainly of:

Crosslinking between glycan strands in many Gram-positive pathogens involves pentaglycine bridge characteristic of Staphylococcus aureus, intensively studied because it influences vancomycin sensitivity. After polymerization, tetrapeptide side chains protrude from MurNAc. In S. aureus, FemXAB family nonribosomal peptidyl transferases add glycine residues one by one to epsilon amino group of L-lysine at position three, using glycyl-tRNA as donor, generating pentaglycine. This flexible extension then becomes substrate for transpeptidase that links glycine terminal to D-alanine of neighboring stem, creating pentaglycine cross-bridge. Composition varies: Streptococcus pneumoniae uses dipeptide L-Ala-L-Ala, Enterococcus uses L-Ala-L-Ala or L-Ala-L-Ser. Glycine as smallest amino acid provides flexibility facilitating long crosslinks, increasing cell wall thickness and resistance to lysozyme. Interpeptide bridges containing alanine, serine, or proline also occur, but glycine dominance in this species is textbook example of how nonribosomal amino acid incorporation shapes wall architecture and antibiotic resistance through Fem-mediated bridging and altered penicillin-binding protein 2a affinity. The pentaglycine bridge length influences susceptibility to lysostaphin, an endopeptidase from Staphylococcus simulans that specifically cleaves Gly-Gly bonds, widely used in laboratory to lyse staphylococci, and to host immunity protein FemX-mediated resistance mechanisms that replace glycine with serine in some methicillin-resistant isolates, altering bridge flexibility and vancomycin binding.

Ref: Schneider et al., Mol Microbiol 2004, FemABX Pentaglycine; Pinho et al., Nature Rev Microbiol 2013, Staphylococcal Cell Wall.

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Microbial Physiology

Which enzyme is involved in the cross-linking of peptidoglycan?

After glycan polymerization by transglycosylases elongating alternating N-acetylglucosamine and N-acetylmuramic acid strands, final strength requires crosslinking of stem peptides attached to MurNAc. Stem typically comprises L-alanine, D-glutamate, meso-diaminopimelic acid or L-lysine, and terminal D-alanyl-D-alanine dipeptide. DD-transpeptidases, members of penicillin-binding protein family including PBP1a, PBP1b, PBP2a, and PBP3, perform nucleophilic attack where serine in active site forms bond with penultimate D-alanine, releasing terminal D-alanine, then transfers acyl-intermediate to amino group of adjacent diamino acid, creating 4-3 crosslink. Some bacteria also have LD-transpeptidases forming 3-3 linkages. Lysozyme hydrolyzes glycan backbone rather than crosslinks, ATP synthase generates ATP from proton motive force, DNA gyrase introduces negative supercoiling. Inhibition of transpeptidase by beta-lactams that mimic D-Ala-D-Ala substrate leaves nascent peptidoglycan poorly crosslinked, compromising mechanical strength so that turgor pressure causes lysis. This step is crucial for shape determination and antibiotic susceptibility, explaining why PBP mutations confer resistance. Recent cryo-EM structures capture PBP2 in active conformation with nascent peptidoglycan strand threaded through donor site, revealing how transpeptidase orients peptide for crosslinking, and how beta-lactams occupy same pocket mimicking acyl-D-Ala-D-Ala, explaining structure-activity relationships used to design carbapenems and cephalosporins that evade certain beta-lactamases.

Ref: Vollmer et al., FEMS Microbiol Rev 2008, Peptidoglycan Crosslinking; Lovering et al., Ann Rev Biochem 2012, PBPs.

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Microbial Physiology

Which of the following bacterial cell wall components is absent in Archaea?

Bacterial cell wall architecture includes several hallmark polymers absent in Archaea, reflecting fundamentally different envelope synthesis pathways. Peptidoglycan, composed of alternating N-acetylglucosamine and N-acetylmuramic acid crosslinked by D-amino acid containing peptides, is universal among bacteria but never found in archaea, where pseudomurein or S-layers substitute. Outer membrane containing lipopolysaccharide, characterized by Lipid A endotoxin, core oligosaccharide, and O-antigen, is diagnostic of Gram-negative bacteria and relies on Lpx biosynthetic enzymes absent in archaea. Lipopolysaccharide itself, including its toxic Lipid A moiety, is therefore missing in archaeal membranes which instead contain ether-linked isoprenoids. Additionally, bacterial accessory polymers such as lipoteichoic acid, wall teichoic acid, and mycolic acids are not synthesized by archaea. Because Archaea lack all three listed components, their envelopes are intrinsically resistant to beta-lactams, lysozyme, and polymyxins targeting such structures. This absence underlines why antibiotics effective against bacterial walls generally fail against archaeal isolates and why archaeal lipid biosynthesis represents attractive target for specific inhibitors.

Ref: Kandler & Konig, Archaea 1998, Bacterial vs Archaeal Walls; Albers & Meyer, Nature Rev Microbiol 2011, Comparative Envelope.

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Microbial Physiology

Which polysaccharide is found in the archaeal cell wall instead of peptidoglycan?

Bacterial cell walls are built on peptidoglycan, a heteropolymer of N-acetylglucosamine and N-acetylmuramic acid crosslinked by peptide bridges sensitive to lysozyme and penicillin. Archaea do not synthesize peptidoglycan; instead, methanogenic euryarchaeota that possess a rigid wall produce pseudomurein, also called pseudopeptidoglycan, which superficially resembles peptidoglycan but differs chemically enough to confer resistance to classical wall antibiotics. Pseudomurein consists of N-acetylglucosamine linked to N-acetyltalosaminuronic acid rather than N-acetylmuramic acid, bearing L-amino acids at crosslink positions rather than D-amino acids typical of bacteria, and featuring beta-1,3 glycosidic linkages in place of beta-1,4. Teichoic acid is unique to Gram-positive walls, chitin is polymer of N-acetylglucosamine found in fungi and arthropods, not in archaea. Thus presence of pseudomurein instead of true murein defines wall chemistry in certain methanogens and explains why lysozyme and beta-lactams fail to lyse these organisms despite similar morphology. Genomic analysis shows pseudomurein biosynthetic enzymes are unrelated to bacterial Mur ligases, suggesting independent evolutionary origin, and presence of NAT instead of NAM impacts innate immune recognition because mammalian peptidoglycan recognition proteins like PGRP and NOD receptors fail to detect archaeal wall, contributing to commensal tolerance in gut methanogens.

Ref: Albers & Meyer, Nature Rev Microbiol 2011, Pseudomurein; Kandler & Konig, Archaea 1998, Cell Walls.

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Carbohydrates

Which of the following is not a component of bacterial cell walls?

Chitin is the correct choice because it does not accurately describe or belong to the category addressed in this question. In the context of Carbohydrates, the other options (Peptidoglycan, N-acetylglucosamine, and N-acetylmuramic acid) are all valid and well-established concepts. Chitin is either unrelated to the topic, describes a different biological process, or represents a common misconception. Questions framed as 'which is NOT' require students to identify the exception among otherwise correct statements, demanding comprehensive knowledge of the topic rather than recognition of a single fact.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 7

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Carbohydrates

Which sugar derivative is a component of bacterial peptidoglycan?

N-acetylmuramic acid is the correct answer as it accurately identifies the biological location, composition, or distribution described in this question. In Carbohydrates, the spatial organization and localization of molecules are critical to their function. N-acetylmuramic acid is specifically associated with the structure or compartment mentioned because of its unique biochemical properties and physiological role. The other options (Dihydroxyacetone, Trehalose, and Amylopectin) are primarily associated with different cellular compartments, tissues, or structural contexts.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 7

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Carbohydrates

Which polysaccharide is the main component of bacterial cell walls?

Peptidoglycan is the correct answer as it accurately identifies the biological location, composition, or distribution described in this question. In Carbohydrates, the spatial organization and localization of molecules are critical to their function. Peptidoglycan is specifically associated with the structure or compartment mentioned because of its unique biochemical properties and physiological role. The other options (Chitin, Glycogen, and Dextran) are primarily associated with different cellular compartments, tissues, or structural contexts.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 7

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