CRYSTAL-STRUCTURES OF APOLIPOPROTEIN(A) KRINGLE IV37 FREE AND COMPLEXED WITH 6-AMINOHEXANOIC ACID AND WITH P-AMINOMETHYLBENZOIC ACID - EXISTENCE OF NOVEL AND EXPECTED BINDING MODES

Kringles are protein modules found within a wide variety of fibrinolyt ic and coagulation-related proteins that show binding affinity for lys ine, lysine analogs and for fibrin. We report here the crystal structu res of apolipoprotein(a) kringle IV37 (apo(a) K4(37)) in its free stat e and in separate complexes with two omega-amino acids, 6-aminohexanoi c acid (6AHA) and p-aminomethylbenzoic acid (PAMBA). The structures of the unliganded form and of both complexes have been determined and re fined by restrained least-squares methods to about 2.0 Angstrom. The o verall kringle architecture is essentially identical with that determi ned in other kringles but it shows some small significant structural c hanges in the lysine binding site. There is virtually no difference in conformation between the unliganded and complexed forms, suggesting t hat apo(a) K4(37) does not undergo any conformational rearrangement up on binding. The 6AHA molecule binds to apo(a) K4(37) in a completely d ifferent way from that observed with the kringle 4 of plasminogen (PGK 4). Its amino group makes an ion pair interaction with the two asparta te residues (Asp55/Asp57) of the anionic center and its carboxylate gr oup faces out into the solvent making water-mediated contacts with the protein. The mode of binding of PAMBA resembles more that described f or 6AHA when bound to PGK4. The PAMBA molecule is bound by ion pair in teractions with the two aspartate residues (Asp55/Asp57) and with Arg7 1 from the cationic center and by van der Waals contacts. The relative importance of the cationic center from kringles for binding zwitterio nic ligands is discussed. (C) 1996 Academic Press Limited