D-Alanine carboxypeptidase

This enzyme [EC 3.4.16.4], also known as serine-type D-alanyl-D-alanine carboxypeptidase, catalyzes the hydrolysis of D-alanyl-D-alanine to yield two D-alanine. This enzyme comprises a group of membrane-bound, bacterial enzymes of the peptidase family Sll. They are distinct from the zinc D-alanyl-D-alanine carboxypeptidase [EC 3.4.17.14]. The enzyme also hydrolyzes the D-alanyl-D-alanine peptide bond in the polypeptide of the cell wall. In addition, the enzyme will also catalyze the transpeptidation of peptidyl-alanyl moieties that are A-acetyl-substituents of D-alanine. The protein is inhibited by j8-lactam antibiotics, which acylate the active-site seryl residue. 

D-Alanine carboxypeptidases (CPases) catalyze removal of the terminal D-alanyl residue from uncross-linked pentapeptides, and the resultant tetrapeptide cannot serve as substrate for transpeptidation. In this way, CPases may regulate the degree of cross-linking. Some CPases catalyze transpeptidation reactions between side... 

The interactions of 8-lactams with PBPs indicate that these compounds are structural analogues of R-D-alanyl-D-alanine, the natural substrate of peptidoglycan transpeptidases and D-alanine carboxypeptidases, where R is the remainder of the pentapeptide. The mechanisms of the transpeptidase and carboxypeptidase reactions are thought to involve formation of an acyl-enzyme intermediate that can react with either a primary amine (e.g., an a-amino group) to form a peptide bond, or with water to form a carboxylic acid. In both reactions D-alanine is released before the acyl enzyme is formed. When a S-lactam antibiotic enters the binding site, the /1-lactam bond is hydrolyzed, and the resulting acyl group reacts with the active-site... 

Transpeptidase and D-alanine Carboxypeptidase Penicillin-sensitive Enzymic Reactions , Federation Proceedings, 26, 9-22... 

Figure 8.10. Dreiding stereomodels of acyl-D-alanyl-D-alanine and a 7a-acetyl-cephem-3-carboxylic acid (right). The a-position at C7 is substituted with a relatively bulky group, which causes steric hindrance for transpeptidase and carboxypeptidase enzymes. (Photograph...

VanX is a carboxypeptidase that hydrolyzes the amide bond of a dipeptide, D-alanyl-D-alanine (D-Ala-D-Ala). This enzyme is essential for vancomycin-resistant enterococci (VRE), because it produces peptidoglycan precursors terminating in D-alanyl-D-lactate (D-Ala-D-Lac) in place of D-Ala-D-Ala, resulting in a 100-fold decrease in the affinity with vancomycin.Scheme 5 depicts a proposed mechanism of VanX, where the active-site structure is similar to those of CPDA and thermolysin. The Zn -bound H2O activated by Glul81 attacks... 

DD-carboxypeptidases - catalyse the cleavage of the terminal D-alanine from pentapeptide side chains by a hydrolysis reaction. (Scheme 1). (///) endopeptidases - catalyse the hydrolysis of interpeptide links formed by transpeptidation. 

There is a basic similarity in the carboxypeptidase and transpeptidase reactions in that in both cases the carbonyl of the penultimate D-alanine is transferred to an exogeneous nucleophile. If the latter is water then hydrolysis results, whereas if it is an amino group of another peptide then aminolysis occurs and the product is a crosslinked dimer of the two peptides. 

Note that all have V-tcrminal sarcosine and are therefore resistant to amino-peptidases. The second peptide has C-terminal D-Phe whereas the third has C-terminal (3-diphenyl-alanine. Both are resistant to carboxypeptidases. All three compounds are antagonists of angiotensin II. 

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