Final answer:
In bacterial cell wall synthesis, the pentapeptide becomes a tetrapeptide when penicillin-binding proteins remove the terminal D-alanine residue during the peptidoglycan cross-linking process, contributing to cell wall rigidity.
Step-by-step explanation:
During cell wall synthesis in bacteria, a pentapeptide becomes a tetrapeptide through the action of penicillin-binding proteins (PBPs). These enzymes remove the terminal D-alanine residue of the pentapeptide to form a tetrapeptide, which is then able to participate in the cross-linking of the peptidoglycan layer. This cross-linking process is essential for the rigidity and stability of the bacterial cell wall. Because of the unique presence of both L- and D-amino acids in the peptidoglycan structure of bacteria, many antibiotics target this process to disrupt cell wall synthesis, ultimately leading to bacterial death. For example, penicillin interferes with the cross-linking of peptidoglycan molecules by mimicking the D-alanine residue that is removed during maturation of the cell wall.