The active site of thrombin is altered upon binding to thrombomodulin. Two distinct structural changes are detected by fluorescence, but only one correlates with protein C activation.
The association of thrombin with thrombomodulin, a non-enzymatic endothelial cell surface receptor, alters the substrate specificity of thrombin. Complex formation converts thrombin from a procoagulant to an anticoagulant enzyme. Structure-function analysis of this change in specificity is facilitated by the availability of two soluble proteolytic derivatives of thrombomodulin, one consisting of the six repeated growth factor-like domains of thrombomodulin (GF1-6) and the other containing only the fifth and sixth such domains (GF5-6). Both derivatives can bind to thrombin and block fibrinogen clotting activity, though only the larger GF1-6 can stimulate the activation of protein C. To ascertain whether the substrate specificity change from fibrinogen to protein C is accompanied by structural changes in the active site of the enzyme, fluorescent dyes were positioned at different locations within the active site. A 5-dimethylaminonaphthalene-1-sulfonyl (dansyl) dye was covalently attached to the active site serine to form dansyl-thrombin, while either a fluorescein or an anilinonaphthalene-6-sulfonic acid (ANS) dye was attached covalently to the active site histidine of thrombin via a D-Phe-Pro-Arg linkage. The environment of the dansyl dye was altered in a similar fashion when either GF1-6 or GF5-6 bound to thrombin, since a similar reduction in dansyl emission intensity was elicited by these two thrombomodulin derivatives (25 and 32%, respectively). These spectral changes, and all others in this study, were saturable and reached a maximum when the ratio of thrombomodulin derivative to thrombin was close to 1. The environments of the fluorescein and ANS dyes were also altered when GF1-6 bound to thrombin because binding resulted in emission intensity changes of -13% and +18%, respectively. In contrast, no fluorescence changes were observed when the fluorescein and ANS thrombin derivatives were titrated with GF5-6. Thus, the structure of the active site was altered by thrombomodulin both immediately adjacent to the active site serine and also more than 15 A away from it. However, the structural change far from Ser-195 was only elicited by thrombomodulin species that stimulate thrombin-dependent activation of protein C.
Ye
J
J
Department of Chemistry and Biochemistry, University of Oklahoma, Norman 73019.
Esmon
N L
NL
Esmon
C T
CT
Johnson
A E
AE
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002417:Cattle; D004789:Enzyme Activation; D005453:Fluorescence; D006801:Humans; D008969:Molecular Sequence Data; D011486:Protein C / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D018179:Receptors, Thrombin; D012680:Sensitivity and Specificity; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D013917:Thrombin / Q000378:metabolism*
D011486:Protein C; D011956:Receptors, Cell Surface; D018179:Receptors, Thrombin; D013917:Thrombin