X-ray crystallographic analyses of complexes between bovine beta-trypsin and Schiff base copper(II) or iron(III) chelates

To establish the structural basis underlying the activity of a novel series of metal-chelate trypsin inhibitors, the structures of p-amidinosalicylide ne-L-alaninato(aqua)copper(II) (1a), m-amidinosalicylidene-L-alaninato(aqua )copper(II) (1b), bis(p-amidinosalicylidene-L-alaninato)iron(III) (2a), and bis(m-amidinosalicylidene-L-alaninato)iron(III) (2b) bound to bovine P-try psin were studied by X-ray crystallography. The amidinium group of the inhi bitor donates hydrogen bonds to Asp189, Gly219 and Ser190, as seen before i n trypsin-benzamidine complexes. The copper(II) ion of 1a is situated away from trypsin's catalytic triad residues, and is octahedrally coordinated by a Schiff base and three water molecules. In contrast, the copper(II) ion o f 1b is situated close to the catalytic triad and adopts a square pyramidal coordination geometry. The iron(III) ion of 2a is octahedrally coordinated by two Schiff base ligands and, like the copper(II) ion of la, is situated away from the catalytic triad. The p-amidinophenyl ring of a second Schiff base ligand of 2a is directed toward a hydrophobic groove formed by Trp215 and Leu99. Finally, the iron(III) ion of 2b appears to be replaced by magn esium(II), which is octahedrally coordinated by a Schiff base, Gln192 and t wo water molecules. One of the Schiff base ligands seen in the trypsin-2a c omplex or in the unbound form of 2b is replaced by water molecules and Gln1 92. His57 and Ser195 form water-mediated interactions with the magnesium(II ) ion of 2b, and Ser195 also forms a hydrogen bond with the phenolic oxygen atom of the Schiff base ligand. These structures re veal a novel mode of i nteraction between metal-chelate inhibitors and serine proteases, thus prov iding a structural basis for the development of more potent inhibitors agai nst a variety of trypsin-like enzymes.;(C), 2001 Academic Press.