[1]) is a bacterial enzyme that catalyzes the transfer of the R-L-αα-D-alanyl moiety of R-L-αα-D-alanyl-D-alanine carbonyl donors to the γ-OH of their active-site serine and from this to a final acceptor.

[2] It is involved in bacterial cell wall biosynthesis, namely, the transpeptidation that crosslinks the peptide side chains of peptidoglycan strands.

[3] The antibiotic penicillin irreversibly binds to and inhibits the activity of the transpeptidase enzyme by forming a highly stable penicilloyl-enzyme intermediate.

The first step involves the cleavage of the D-alanyl-D-alanine bond of a peptide unit precursor acting as carbonyl donor, the release of the carboxyl-terminal D-alanine, and the formation of the acyl-enzyme.

[6] The center of the catalytic cleft is occupied by the Ser35-Thr36-Thr37-Lys38 tetrad, which includes the nucleophilic Ser35 residue at the amino-terminal end of helix α2.

The backbone NH group of the essential Ser35 residue and that of Ser216 downstream from the motif Lys213-Thr214-Gly215 occupy positions that are compatible with the oxyanion hole function required for catalysis.

[6] The enzyme is classified as a DD-transpeptidase because the susceptible peptide bond of the carbonyl donor extends between two carbon atoms with the D-configuration.

Penicillin is a cyclic analogue of the D-Ala-D-Ala terminated carbonyl donors, therefore in the presence of this antibiotic, the reaction stops at the level of the serine ester-linked penicilloyl enzyme.

[8] The interference with the enzyme processes responsible for cell wall formation results in cellular lysis and death due to the triggering of the autolytic system in the bacteria.