Spectrophotometric evidence for involvement of aromatic residues in the interaction of ferreascidin with the ferric ion

Abstract

The iron(III)-binding properties of pyrocatechol and ferreascidin, a blood protein isolated from a stolidobranch ascidian (Pyura stolonifera), have been characterized by spectrophotometric titrations with metal ion in the presence of nitrilotriacetate (NTA) to maintain solubility under the conditions used (pH 7.0, I0.2). Such studies of the model system have signified that the formation of the [Fe(NTA)(pyrocatechol)] complex is governed by a stability constant of 10 M. Corresponding studies of the interaction of iron(III) with the ascidian protein have provided spectrophotometric evidence for the coordination of each ferric ion to one tyrosine and two (3,4-dihydroxyphenyl)-alanine (DOPA) residues, the interaction being characterized by an effective stability constant of 4 × 10 M under the conditions studied (pH 7.0, I 0.2). Finally, a case is made for potential biological relevance of the iron(III)-ferreascidin interaction in the curing of this DOPA protein for the repair of wounds and/or securement of the ascidian to its substratum.