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Apolipoprotein(a) [apo(a)] is the distinctive glycoprotein of lipoprotein Lp(a), which is disulfide linked to the apo B100 of a low density lipoprotein particle. Apo(a) possesses a high degree of sequence homology with plasminogen, the precursor of plasmin, a fibrinolytic and pericellular proteolytic enzyme. Apo(a) exists in several isoforms defined by a variable number of copies of plasminogenlike kringle 4 and single copies of kringle 5, and the protease region including the backbone positions for the catalytic triad (Ser, His, Asp). A lysinebinding site that is similar to that of plasminogen kringle 4 is present in apo(a) kringle IV type 10. These kringle motifs share some amino acid residues (Asp55, Asp57, Phe64, Tyr62, Trp72, Arg71) that are key components of their lysinebinding site. The spatial conformation and the function of this site in plasminogen kringle 4 and in apo(a) kringle IV-10 seem to be identical as indicated by (i) the ability of apo(a) to compete with plasminogen for binding to fibrin, and (ii) the neutralisation of the lysinebinding function of these kringles by a monoclonal antibody that recognises key components of the lysinebinding site. In contrast, the lysinebinding site of plasminogen kringle 1 contains a Tyr residue at positions 64 and 72 and is not recognised by this antibody. Plasminogen bound to fibrin is specifically recognised and cleaved by the tissuetype plasminogen activator at Arg561-Val562, and is thereby transformed into plasmin. A SerIle substitution at the activation cleavage site is present in apo(a). Reinstallation of the ArgVal peptide bond does not ensure cleavage of apo(a) by plasminogen activators. These data suggest that the stringent specificity of tissuetype plasminogen activator for plasminogen requires molecular interactions with structures located remotely from the activation disulfide loop. These structures ensure second site interactions that are most probably absent in apo(a).

Apolipoprotein(a): Structure-Function Relationship at the Lysine-Binding Site and Plasminogen Activator Cleavage Site