Glycopeptide transpeptidase

Ser residue in the active site of the enzyme acetylcholinesterase, irreversibly inhibiting the enzyme and preventing the transmission of nerve impulses (Fig. la). Iodoacetamide modifies Cys residues and hence may be used as a diagnostic tool in determining whether one or more Cys residues are required for enzyme activity (Fig. lb). The antibiotic penicillin irreversibly inhibits the glycopeptide transpeptidase enzyme that forms the cross-links in the bacterial cell wall by covalently attaching to a Ser residue in the active site of the enzyme (see Topic Al). 

Unlike mammalian cells, bacterial cells are surrounded by a fairly rigid cell wall, which allows the bacterium to live in many different environments. This protective cell wall is composed of carbohydrates linked together by peptide chains containing amide linkages, formed using the enzyme glycopeptide transpeptidase. 

Penicillin interferes with the synthesis of the bacterial cell wall. A nucleophilic OH group of the glycopeptide transpeptidase enzyme cleaves the P-lactam ring of penicillin by a nucleophilic acyl substitution reaction. The opened ring of the penicillin molecule remains covalently bonded to the enzyme, thus deactivating the enzyme, halting cell wall construction, and killing the bacterium. Penicillin has no effect on mammalian cells because they are surrounded by a flexible membrane composed of a lipid bilayer (Chapter 3) and not a cell wall. 

Counterpoint. Penicillinase (P-lactamase) hydrolyzes penicillin. Compare penicillinase with glycopeptide transpeptidase. 

Penicillinase, like glycopeptide transpeptidase, forms an acyl-enzyme intermediate with its substrate but transfers the intermediate to water rather than to the terminal glycine residue of the pentaglycine bridge. 

Fig. 8.18 The antibiotic penicilhn inhibits the bacterial enzyme glycopeptide transpeptidase. The transpeptidase is a serine protease involved in cross-hnking components of bacterial cell walls and is essential for bacterial growth and survival. It normally cleaves the peptide bond between two D-alanine residues in a pol5 peptide. Penicillin contains a strained peptide bond within the (3-lactam ring that resembles the transition state of the normal cleavage reaction, and thus penicillin binds very readily in the enzyme active site. As the bacterial enzyme attempts to cleave this penicillin peptide bond, penicillin becomes irreversibly covalently attached to the enzyme s active site serine, thereby inactivating the enzyme.

The inhibition of bacterial cell wall synthesis by penicillin is a classic example of a medically significant inhibition of an enzymatic reaction. Which of the following statements about the inhibition of glycopeptide transpeptidase by penicillin is true ... 

Penicillinase, like glycopeptide transpeptidase, forms an acyl-enzyme intermediate... 

The third and final stage of synthesis of cell walls occurs outside the cytoplasmic membrane. Thus, the transpepfidation reaction results in transformation of the linear glycopeptide polymer into the CTOss-Unked form. The enzyme transpeptidase, a membrane-bound enzyme, binds pentapepfide side chains by replacing terminal o-alanines. 

FIGURE 44-2 Action of fi-iactam antibiotics in SUqyhytococcus aureus. The bacterial cell wall consists of glycopeptide polymers linked via bridges between amino acid side chains. In S. aureus, the bridge is (Gly),-D-Ala between lysines. The cross-linking is catalyzed by a transpeptidase, the enzyme that penicillins and cephalosporins inhibit. 

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