Virtuelle (Nano) Welten: PDB Molecule of the Month 25: Thrombin

Anticoagulants

Blood clots are not always wanted. For instance, many people take small doses of aspirin, under the direction of their doctors, to reduce the chance of the blood clots that cause heart attacks. Aspirin acts on the protein cyclooxygenase, which is important in another aspect of clot formation that uses small cell fragments called platelets. The rat poison warfarin, not commonly used these days, blocks the formation of the modified glutamate amino acids that hold calcium ions, shown on the last page. The unfortunate rats then die because of uncontrolled blood clotting. Leeches, as you might expect, also detest blood clots, because it means the end to their meal. They make special proteins that block thrombin (or other enzymes), stopping the formation of the clot. One example, a protein called hirudin, is shown here on the left (from PDB entry 2hgt). The leech protein is shown in blue-- notice how it blocks the active site of thrombin perfectly.