Conformational Changes in Fragments D and Double-D from Human Fibrin(ogen) upon Binding the Peptide Ligand Gly-His-Arg-Pro-Amide,

Abstract

The structure of fragment double-D from human fibrin has been solved in the presence and absence of the peptide ligands that simulate the two knobs exposed by the removal of fibrinopeptides A and B, respectively. All told, six crystal structures have been determined, three of which are reported here for the first time: namely, fragments D and double-D with the peptide GHRPam alone and double-D in the absence of any peptide ligand. Comparison of the structures has revealed a series of conformational changes that are brought about by the various knob−hole interactions. Of greatest interest is a moveable “flap” of two negatively charged amino acids (Glu^β397 and Asp^β398) whose side chains are pinned back to the coiled coil with a calcium atom bridge until GHRPam occupies the β-chain pocket. Additionally, in the absence of the peptide ligand GPRPam, GHRPam binds to the γ-chain pocket, a new calcium-binding site being formed concomitantly.