Q.14

The final stage of peptidoglycan (PG) biosynthesis is marked by PG cross-link formation

and remodeling. Which of the following enzymes does NOT take part in these processes?

(A) Transglycosidases

(B) Transpeptidases

(C) Transaminases

(D) DD-carboxypeptidases

Peptidoglycan Biosynthesis Final Stage: Enzymes Involved in Cross-Linking and Remodeling

The final stage of peptidoglycan (PG) biosynthesis involves critical steps like glycan chain polymerization, peptide cross-linking, and remodeling to strengthen the bacterial cell wall. Transglycosidases, transpeptidases, and DD-carboxypeptidases play key roles, while transaminases do not participate.

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Correct Answer

C) Transaminases is the enzyme that does NOT take part in PG cross-link formation and remodeling. These processes rely on specific penicillin-binding proteins (PBPs) and hydrolases, excluding transaminases which function in amino acid metabolism elsewhere.

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Role of Transglycosidases

Transglycosidases polymerize glycan chains by linking GlcNAc-MurNAc disaccharides in lipid II precursors, forming long strands essential for PG structure. Found in class A PBPs or standalone like RodA, they enable sidewall elongation during cell growth.

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Role of Transpeptidases

Transpeptidases catalyze peptide cross-links between D-Ala-D-Ala of donor stems and mDAP of acceptor stems, creating the 4→3 linkages that provide PG rigidity. Primarily class A and B PBPs, they are major antibiotic targets like penicillin.

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Role of DD-Carboxypeptidases

DD-carboxypeptidases trim excess D-Ala residues from pentapeptides, regulating cross-link levels and preparing stems for remodeling or insertion. They contribute to PG maturation by hydrolyzing terminal D-Ala-D-Ala bonds.

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Why Not Transaminases?

Transaminases transfer amino groups in amino acid synthesis or metabolism, such as alanine to alpha-ketoglutarate, but play no direct role in periplasmic PG cross-linking or remodeling. PG peptide stems use pre-synthesized D-amino acids from cytoplasmic steps.