Crystal Structure of Earthworm Fibrinolytic Enzyme Component B: a Novel, Glycosylated Two-chained Trypsin

The earthworm fibrinolytic enzyme (EFE), belonging to a group of serine proteases with strong fibrinolytic activity, has been used in a mixture as an oral drug for prevention and treatment of thrombosis in East Asia. The EFE component b (EFE-b) is one of seven EFE components from Eisenia fetida, and among them it has nearly the highest fibrinolytic activity. Here, we report its crystal structure at a resolution of 2.06 Å. The structural analysis shows that EFE-b should be classified as a trypsin from earthworm. However, it is distinct from other trypsins. It is a two-chained protease with an N-terminal, pyroglutamated light chain and an N-glycosylated heavy chain. Furthermore, the heavy chain contains a novel structural motif, an eight-membered ring resulting from a disulfide bridge between two neighboring cysteine residues, and a cis peptide bond exists between these two cysteine residues. The crystal structure of EFE-b provides the structural basis for its high level of stability and reveals its complicated post-translational modifications in earthworm. This structure is the first reported for a glycosylated two-chained trypsin, which may provide useful clues to explain the origin and evolution of the chymotrypsin family.