Q.25 The formation of peptide cross-links between adjacent glycan chains in cell wall synthesis is called
- (A) Transglycosylation
- (B) Autoglycosylation
- (C) Autopeptidation
- (D) Transpeptidation
Peptide Cross-Links in Bacterial Cell Wall: Transpeptidation
Peptide cross-links between adjacent glycan chains strengthen bacterial peptidoglycan during cell wall synthesis. The correct answer is (D) Transpeptidation, the enzymatic process forming these amide bonds.
Core Mechanism
Bacterial cell walls feature peptidoglycan with alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) glycan chains cross-linked by peptides. Transpeptidation by penicillin-binding proteins (PBPs) replaces D-alanine with a peptide from adjacent chains, creating a meshwork.
This step follows transglycosylation, which polymerizes glycan strands.
Correct Answer: (D) Transpeptidation
Penicillin-sensitive transpeptidases catalyze cross-link formation: the terminal glycine or meso-diaminopimelic acid attacks the penultimate D-Ala, releasing D-Ala. This covalent linkage confers rigidity against turgor pressure.
Option Analysis
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(A) Transglycosylation: Incorrect; this glycosyltransferase reaction elongates glycan chains via NAG-NAM polymerization, not peptide bonds.
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(B) Autoglycosylation: Incorrect; rare self-glycosylation process unrelated to cell wall cross-linking.
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(C) Autopeptidation: Incorrect; non-standard term, possibly confused with autolysins that hydrolyze peptides.
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(D) Transpeptidation: Correct; precise peptide transfer between glycan strands by PBPs.
Option Reaction Type Target Bond Cell Wall Role (A) Transglycosylation Glycan extension β-1,4 glycosidic Chain polymerization (B) Autoglycosylation Self-modification Glycoprotein Protein decoration (C) Autopeptidation N/A None Not applicable (D) Transpeptidation Peptide transfer Amide cross-link Structural integrity Biotech Relevance
Beta-lactam antibiotics like penicillin target transpeptidases, halting cross-linking for bacterial lysis. Key for microbiology exams and antibiotic resistance studies.
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