The metal-dependent conformational changes in factor IX associated with phospholipid binding. Studies using antibodies against a synthetic peptide and chemical modification of factor IX.

Factor IX undergoes two sequential metal-ion-dependent conformational transitions [Liebman, H.A. (1987) J. Biol. Chem. 262, 7605-7612]. The first transition is metal ion dependent, but cation nonselective. The second transition is metal ion selective for Ca(II) or Sr(II) and associated with the expression of conformational determinants necessary for phospholipid membrane binding. Using antibodies raised against a synthetic peptide containing Factor IX amino acid residues 28-46, it was demonstrated that the aromatic-amino-acid-stack domain (amino acid residues 41-46) of Factor IX is surface exposed in the metal-free conformer and internalized with the initial metal-ion-dependent conformational transition. The metal-ion-induced internalization of this region of Factor IX protects the tryptophan at position 42 from oxidation by N-bromosuccinimide. The oxidation of Factor IX tryptophan residues is associated with the rapid loss of coagulant activity, but protection of Trp42 allows for the continued expression of the Ca(II)-specific antigenic determinants on Factor IX and the retention of phospholipid binding. These results suggest that the Trp42 residue located in the aromatic amino acid stack domain of human Factor IX is internalized with the metal-ion-dependent conformational transition and is essential for the expression of the phospholipid membrane binding site on Factor IX.