A transpeptidase (EC 3.4.16.4) is a bacterial enzyme which cross-links the peptidoglycan chains to form rigid cell walls. This enzyme is also known by several other names including DD-peptidase, DD-transpeptidase, D-alanyl-D-alanine carboxypeptidase and serine-type D-Ala-D-Ala carboxypeptidase.

The antibiotic penicillin binds to and inhibits the activity of the transpeptidase enzyme. Because of the interaction between penicillin and transpeptidase, this enzyme is also known as "penicillin binding protein."

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