Homo sapiens (human)/Hirudo medicinalis (leech)
Blood coagulation helps the body respond to wounds and prevent life- threatening blood loss. Undesirable blood clotting can also lead to the formation of clots in internal blood vessels and prevent proper blood circulation. The result can be fatal. Blood coagulation is a complex process involving a cascade of protease coagulation factors. This process has two pathways, the extrinsic and the intrinsic pathway, and at the point where the two pathways coalesce to continue as one, is a protein called thrombin. It initiates the final stages of blood clotting, hence the name thrombosis for the formation of undesired blood clots. Thrombin, a glycoprotein with attached sugars, belongs to the family of serine proteases. It also has autocatalytic ability, that is, it can cleave itself at certain points in the polypeptide chain.
Here you can see the structure of this protein in a complex with hirudin. Hirudin is itself a protein which is 65 residues long, fairly short as far as proteins go. Hirudin is a protein that is produced by the leech Hirudo medicinalis, an eel-like organism, that can live in water and on land. In the middle ages doctors often attached leeches to the skin of patients to help "thin the blood". While not understanding the action of the leech as we do today they recognized some mechanism by which the blood-sucking action of leeches could affect the health of a patient. Hirudin and its inhibition of thrombin is that action.
Protein Data Bank (PDB)
author: Arno Paehler