Structural domains comprising this superfamily share the structure of domain D of the unusual penicillin-binding protein (PBP), Tp47. Tp47 is unusual in that it displays beta-lactamase activity, so it does not fit the classical PBP archetype, and thus it appears to represent a new class of PBP. Domain D presents the characteristic immunoglobulin fold, with two opposing beta-sheets that form the typical barrel-like structure. However, and as domain C, it contains an inserted alpha-helical turn between strands 2 and 3. Domain D is rather isolated as it only interacts with domain C via an ionic interaction between Arg-330 and Glu-404 in the linker region. This explains the large degree of motion observed for this domain.

The abundance of hydrophobic residues and the immunoglobulin fold of domain D suggests that it mediates protein-protein interactions. In addition its location, flexibility, and relative disposition indicate a function other that PBP and beta-lactamase activities PMID:12196546. The FunFams for this superfamily suggest that this domain shows haemagglutination activity.