Haemadin

In molecular biology, haemadin is an anticoagulant peptide synthesised by the Indian leech, Haemadipsa sylvestris.
It adopts a secondary structure consisting of five short beta-strands, which are arranged in two antiparallel distorted sheets formed by strands beta1-beta4-beta5 and beta2-beta3 facing each other. This beta-sandwich is stabilised by six enclosed cysteines arranged in a disulfide pairing resulting in a disulfide-rich hydrophobic core that is largely inaccessible to bulk solvent. The close proximity of disulfide bonds and organises haemadin into four distinct loops. The N-terminal segment of this domain binds to the active site of thrombin, inhibiting it.
Haemadin belongs to a superfamily of protease inhibitors that also includes hirudin and antistasin.