Carboxypeptidation in peptidoglycan synthesis

[CrazCo]

Hi,

 

During the transpeptidation reaction in peptidoglycan biosynthesis when the peptides are cross-linked, is the terminal D-alanine lost [i.e. in E. coli when DAP forms a peptide bond with D-ala of the new disaccharide]? Or does the terminal D-alanine remain and is later cleaved by the carboxypeptidation reaction.

 

Essentially, I'm confused by the difference between transpeptidation and carboxypeptidation.

 

If someone could help me clear it up that'd be great!

 

Thanks!

[sdcarmody]

During transpeptidation, the acceptor stem peptide terminal D-ala always cleaves. Once this happens the cross-linked stem peptides cannot serve as substrates for d-ala-d-ala carboxypeptidase.

 

 

The degree of cross-linking varies between species. Stem peptides that have not been cross-linked serve as substrates for d-ala - d-Ala carboxypeptidase, I can't remember off the top of my head, but I think there are several carboxypeptidases that are responsible for cleaving different segments of the stem peptides during peptidoglycan degradation.

Edited October 4, 2012 by sdcarmody