Thrombomodulin structure and function.

Thrombomodulin is protein cofactor expressed on endothelial cell surfaces that modifies the substrate specificity of thrombin, apparently by an allosteric mechanism. The thrombin-thrombomodulin complex activates protein C, initiating an essential anticoagulant pathway. The cofactor function of membrane-associated thrombomodulin requires the last three of six tandemly repeated EGF-like domains (numbers 4, 5, and 6), as well as a Ser/Thr-rich spacer between EGF-like domain 6 and the transmembrane domain. The Ser/Thr-rich domain is variably modified with a chondroitin sulfate chain that influences the affinity of thrombin binding and the calcium ion dependence of cofactor function. The structure of EGF-like domain 4 has been determined by NMR spectroscopy, and the structure of a complex between thrombin and a peptide from thrombomodulin EGF-like domain 5 was determined by X-ray crystallography. These structures are small steps toward an understanding of how thrombomodulin regulates thrombin.