Sulfo-hirudin complexed to Thrombin. Thrombin is a serine protease that catalyses the conversion of soluble fibrinogen to insoluble fibrin during the blood clothing process. Hirudin has been found to be a natural Thrombin inhibitor, enabling the formation of blood clots. Annotation 1 shows a salt-bridge interaction between the two structures at the sulfate-group of Hirudin and the lysine 81residue of Thrombins heavy chain. Annotation 2 shows a hydrogenbond between the phenolic-group of tyrosin 76 of Thrombin and an oxygen of the corresponding sulfate-group of Hirudin. Annotation 3 shows a hydrogen-network between nearby water molecules sourounding the sulfate group of hirudin. Annotation 4 shows the sodium-ion site, which is an allosteric binding-site on Thrombin. When sodium is present, hirudin can bind at the allosteric binding-site. Annotation 5 shows the nickel-ion binding-site, which contributes to the stabilization of the complex.
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