Domain C is the largest domain in this unusual penicillin-binding protein PBP), Tp47. This domain is mainly characterised by an immunoglobulin fold with two opposing beta-sheets that form the typical barrel-like structure. In contrast to the classical immunoglobulin fold, however, this has an additional beta-strand inserted after strand 3. Also, the strands are connected by rather large loops. Helices are inserted between strands 2 and 3 and between strands 4 and 5. Domain C interacts with domain B via a surface that has a slightly concave, goblet-like shape. Tp47 is unusual in that it displays beta-lactamase activity, and thus it does not fit the classical structural and mechanistic paradigms for PBPs, and thus Tp47 appears to represent a new class of PBP [1].

1: Crystal structure of the 47-kDa lipoprotein of Treponema pallidum reveals a novel penicillin-binding protein. Deka RK, Machius M, Norgard MV, Tomchick DR; J Biol Chem. 2002;277:41857-41864. PMID:12196546