A 45-year-old male admitted with severe pneumonia is found to have methicillin-resistant Staphylococcus aureus (MRSA) on blood culture. The resistance mechanism in this organism involves acquisition of the mecA gene, which encodes an altered penicillin-binding protein (PBP-2a) with significantly reduced affinity for β-lactam antibiotics. The structure marked **B** in the diagram represents the normal penicillin-binding proteins that are the target of β-lactam antibiotics. Which of the following best explains why MRSA with altered PBP-2a remains resistant to conventional β-lactams including cephalosporins, yet may respond to newer agents like ceftaroline?