The final reaction in bacterial cell wall synthesis is a cross-linking of adjacent peptidoglycan (murein) strands by a transpeptidation reaction. In this reaction, bacterial transpeptidases cleave the terminal D-alanine from a pentapeptide on one peptidoglycan strand and then cross-link it with the pentapeptide of another peptido-glycan strand. The cross-linked peptidoglycan (murein) strands give structural integrity to cell walls and permit bacteria to survive environments that do not match the organism's internal osmotic pressure.
The p-lactam antibiotics structurally resemble the terminal D-alanyl-D-alanine (D-Ala-D-Ala) in the pen-tapeptides on peptidoglycan (murein) (Fig. 45.1). Bacterial transpeptidases covalently bind the p-lactam antibiotics at the enzyme active site, and the resultant acyl enzyme molecule is stable and inactive. The intact p-lactam ring is required for antibiotic action. The p-lactam ring modifies the active serine site on transpep-tidases and blocks further enzyme function.
In addition to transpeptidases, other penicillin-binding proteins (PBPs) function as transglycosylases and carboxypeptidases. All of the PBPs are involved with assembly, maintenance, or regulation of peptidoglycan cell wall synthesis. When p-lactam antibiotics inactivate PBPs, the consequence to the bacterium is a structurally weakened cell wall, aberrant morphological form, cell lysis, and death.
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