D-aminopeptidase (Swiss:Q9ZBA9) is a dimeric enzyme with each monomer being composed of three domains. Domain B is organised to form a beta barrel made up of eight antiparallel beta strands. It is connected to domain A, the catalytic domain, by an eight-residue sequence, and also interacts with both domains A and C via non-covalent bonds. Domain B probably functions in maintaining domain C in a good position to interact with domain A [1].

1: Crystal structure of a D-aminopeptidase from Ochrobactrum anthropi, a new member of the 'penicillin-recognizing enzyme' family. Bompard-Gilles C, Remaut H, Villeret V, Prange T, Fanuel L, Delmarcelle M, Joris B, Frere J, Van Beeumen J; Structure Fold Des 2000;8:971-980. PMID:10986464