THE STRUCTURE OF RECOMBINANT PLASMINOGEN KRINGLE-1 AND THE FIBRIN BINDING-SITE

The structure of recombinant (Hoover et al. Biochemistry, 1993; 32: 10 936-10944) plasminogen (PG) kringle 1 (K1) has been determined and ref ined at 2.48 Angstrom resolution to a crystallographic R value of 0.15 9. In addition, 71 water molecules and two chloride ions have been loc ated. The folding of PGK1 is very similar to that of PGK4. The lysine/ fibrin binding site, however, differs from that of both PGK4 and tissu e-type PG activator (t-PA) K2 at the cationic centre. Although PGK1 ca n potentially have a doubly charged cationic centre utilizing Arg34 an d Arg71, the side chain of Arg34 is outside of Arg71 in a solvent regi on and its guanidino group is flexibly disordered. Moreover, site spec ific mutagenesis studies show unequivocally that Arg34 can be changed to glutamine without affecting the binding ability of PGK1. Thus, PGK1 only has Arg71 at the cationic site, PGK4 has Lys35/Arg71 and t-PAK2 has only Lys33. The cationic site differences may result in subtle res ponses in the binding affinities of the kringles. The two chloride ion s are located in the lysine binding site and effectively compensate th e positive charges of the region. They also appear to be involved inte rmolecularly in a complex way in the crystal structure. Such intermole cular anionic interactions are also found in PGK4 and t-PAK2.