THE STRUCTURE OF A 19-RESIDUE FRAGMENT FROM THE C-LOOP OF THE 4TH EPIDERMAL GROWTH FACTOR-LIKE DOMAIN OF THROMBOMODULIN

The solution structure has been determined for a 19-residue peptide that is fully folded at room temperature. The sequence of this peptide is based on the C-loop, residues 371-389, of the fourth epidermal growth factor-like domain of thrombomodulin, a protein that acts as a cofactor for the thrombin activation of protein C. Despite its small size, the peptide forms a compact structure with almost no repeating secondary structure. The results indicate the structure is held together by hydrophobic interactions, which in turn stabilize the two beta-turns in the structure. The first beta-turn in the C-loop represents a conserved motif that is found in the published structures of five other epidermal growth factor-like proteins. The critical role of Phe(376) in the stabilization tion of the first beta-turn is consistent with mutagenesis data with soluble thrombomodulin. The results also show that a small subdomain of a larger protein can fold independently, and therefore it could act as an initiation site for further folding.