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#bacterial cell wall

13 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

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

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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 ami

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

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Microbial Physiology

What is the role of bactoprenol in bacterial cell wall synthesis?

Peptidoglycan precursor synthesis involves cytoplasmic steps generating lipid II, composed of N-acetylmuramic acid-pentapeptide linked to undecaprenyl pyrophosphate, a 55-carbon polyisoprenoid lipid carrier historically named bactoprenol. Bactoprenol is embedded in inner membrane with long hydrophobic tail anchoring within bilayer while pyrophosphate-linked disaccharide protrudes. Its function is to shuttle hydrophilic precursor across hydrophobic membrane barrier from cytoplasmic side where it is made to periplasmic side where polymerization occurs. After transfer of precursor to growing glyc

Ref: Barreteau et al., FEMS Microbiol Rev 2008, Bacterial Cell Wall Recycling; Manat et al., Ann Rev Microbiol 2014, Lipid II Cycle.

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Microbial Physiology

What is the primary lipid structure in Gram-positive bacteria?

Gram-positive bacteria lack lipopolysaccharide and outer membrane, so their cytoplasmic membrane directly faces thick peptidoglycan. Primary lipid structure remains conventional phospholipid bilayer built from glycerol-3-phosphate esterified to straight-chain or branched fatty acids, typically phosphatidylglycerol, cardiolipin, and lysylphosphatidylglycerol that modulates surface charge to resist cationic antimicrobial peptides. These phospholipids provide fluid matrix housing respiratory complexes, transporters, and lipid II flipping machinery essential for wall synthesis. Glycerol diethers w

Ref: Alberts et al., Molecular Biology Cell, Chapter 10: Bacterial Membranes; Sohlenkamp & Geiger, FEMS Microbiol Rev 2015, Bacterial Phospholipids.

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Microbial Physiology

Which structure is unique to Gram-negative bacteria?

Gram-negative and Gram-positive cell envelope architectures diverge sharply. Gram-positive envelope comprises thick peptidoglycan multilayer with interspersed wall teichoic acids and lipoteichoic acids anchored to underlying plasma membrane, without outer membrane. Gram-negative envelope contains thin peptidoglycan layer sandwiched in periplasm plus additional outer membrane asymmetric membrane whose outer leaflet is almost exclusively lipopolysaccharide and inner leaflet phospholipid, harboring porins, Bam complex, and Lpt transport machinery. Peptidoglycan itself, plasma membrane composed of

Ref: Silhavy et al., The Bacterial Cell Envelope; Alberts et al., Molecular Biology Cell, Membrane Architecture.

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Microbial Physiology

The peptidoglycan layer in Gram-positive bacteria is:

Gram-positive bacteria lack outer membrane but retain an unusually thick peptidoglycan sacculus measuring 20 to 80 nanometers, composed of many layers of glycan strands made of alternating N-acetylglucosamine and N-acetylmuramic acid crosslinked by tetrapeptide side chains containing L-alanine, D-glutamate, meso-diaminopimelic acid or L-lysine, and D-alanine, often further cross-bridged by interpeptide bridges. Embedded within and covalently attached to this mesh are anionic wall teichoic acids, polymers of ribitol or glycerol phosphate linked via phosphodiester bonds, and lipoteichoic acids a

Ref: Silhavy et al., The Bacterial Cell Envelope; Vollmer et al., FEMS Microbiol Rev 2008, Peptidoglycan and Teichoic Acids.

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Microbial Physiology

Which enzyme is inhibited by penicillin?

Bacterial peptidoglycan final crosslinking step catalyzed by transpeptidases penicillin binding proteins class B enzymes PBP2 for elongation, PBP3 FtsI for division. They cleave C terminal D alanine D alanine dipeptide from pentapeptide side chain MurNAc L Ala D Glu mDAP D Ala D Ala, forming acyl enzyme intermediate through active site serine nucleophile, then transfer to amino group acceptor meso diamino pimelic acid or L lysine of neighboring strand creating peptide crossbridge essential for wall rigidity. Beta lactam antibiotics penicillin contain four membered ring mimicking D Ala D Ala co

Ref: Tipper & Strominger, PNAS 1965, Penicillin inhibits transpeptidase PBP crosslinking peptidoglycan.

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Microbial Physiology

The Braun’s lipoprotein (BLP) in Gram-negative bacteria serves to:

Gram negative envelope integrity depends on covalent linkage between outer membrane and peptidoglycan preventing blebbing and leakage. Braun's lipoprotein Lpp 58 amino acid mature after signal peptidase II lipid modification N acyl S diacylglycerol cysteine anchoring into inner leaflet outer membrane plus third acyl chain, most abundant protein about seven times ten to five copies per cell. C terminal lysine forms peptide bond to meso diaminopimelic acid of peptidoglycan tetrapeptide via LD transpeptidases LdtA B C catalyzing attachment post export. Two thirds of Lpp exist bound form linking l

Ref: Braun & Rehn, Annu Rev Microbiol 1969, Braun lipoprotein attaches outer membrane to peptidoglycan.

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Microbial Physiology

The O-side chain of lipopolysaccharides (LPS) in Gram-negative bacteria functions as:

Lipopolysaccharide O antigen part structural component extends from core into extracellular environment. O side chain consists tandem repeats oligosaccharide units 2 to 6 sugars with diverse linkages deoxy sugars abequose tyvelose modifying antigenicity. Synthesis via Wzy dependent, ABC transporter, or synthase pathways assembled on undecaprenyl phosphate lipid carrier then ligated to lipid A core. Because repeats exposed and highly variable between strains due to glycosyl transferase gene hypervariation and horizontal transfer, immune system recognizes them as type specific antigens. Serologi

Ref: Raetz & Whitfield, Annu Rev Biochem 2002, O-antigen polysaccharide as antigenic determinant in LPS.

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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

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

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