A transpeptidase (EC 126.96.36.199, DD-peptidase, D-alanyl-D-alanine-carboxypeptidase, D-alanyl-D-alanine-cleaving-peptidase, D-alanyl-D-alanine-cleaving peptidase, Serine-type D-Ala-D-Ala carboxypeptidase, D-alanine carboxypeptidase, DD-carboxypeptidase, D-alanyl carboxypeptidase) is a bacterial enzyme that 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. In gram-positive bacteria, the peptidoglycan molecules are cross-linked by a pentapeptide bridge, whereas in gram-negative bacteria, the peptidoglycan molecules are directly covalently bound to each other.
The protein transpeptidase is also known as Novel penicillin-binding protein, which is necessary for cell wall peptidoglycan formation, and is inhibited by penicillin.
The antibiotic penicillin irreversibly binds to and inhibits the activity of the transpeptidase enzyme by forming a highly stable penicilloyl-enzyme intermediate. Because of the interaction between penicillin and transpeptidase, this enzyme is also known as "penicillin-binding protein."
See also - Vancomycin (a glycopeptide that prevents transpeptidase from cross-linking peptidoglycan molecules in a bacterial cell wall)