#cross-linking

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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DNA-Protein Interaction analysis

In ChIP, cross-linking is used to:

In vivo DNA-protein interactions are dynamic and easily disrupted during extraction. Chromatin Immunoprecipitation utilizes formaldehyde crosslinking to generate reversible covalent methylene bridges between lysine residues of proteins and exocyclic amines of DNA bases located within two angstroms. This chemical freezing preserves native regulatory assemblies at defined time point, prevents dissociation during cell lysis, sonication and stringent washes, and maintains chromatin architecture. Reversal by heat releases DNA for analysis. Crosslinking therefore secures transient transcription fact

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Protein

Which amino acid is critical for elastin cross-linking?

Lysine is the scientifically accurate answer to this question. Within the study of Protein, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of Lysine directly address what is being asked. Among the other options, Histidine, Serine, and Alanine do not correctly answer this question because they either refer to different concepts, describe properties of other molecules or processes, or represent common misconceptions about this topic.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4

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Protein

Which enzyme catalyzes the cross-linking of collagen fibrils?

Lysyl oxidase is the accurate response regarding enzymatic activity or regulation described in this question. Enzymes are biological catalysts that accelerate reactions by lowering activation energy through specific substrate binding and transition state stabilization. In the context of Protein, Lysyl oxidase plays a specific catalytic or regulatory role determined by its active site configuration and mechanism of action. The other options (Hydroxylase, Peptidase, and Elastase) are either different enzymes with distinct substrate specificities, act through different mechanisms, or are involved

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4

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