Apolipoprotein(a): structure-function relationship at the lysine-binding site and plasminogen activator cleavage site.
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 plasminogen-like kringle 4 and single copies of kringle 5, and the protease region including the backbone positions for the catalytic triad (Ser, His, Asp). A lysine-binding 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 lysine-binding 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 lysine-binding function of these kringles by a monoclonal antibody that recognises key components of the lysine-binding site. In contrast, the lysine-binding 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 tissue-type plasminogen activator at Arg561-Val562, and is thereby transformed into plasmin. A Ser-Ile substitution at the activation cleavage site is present in apo(a). Reinstallation of the Arg-Val peptide bond does not ensure cleavage of apo(a) by plasminogen activators. These data suggest that the stringent specificity of tissue-type 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).
Anglés-Cano
Eduardo
E
Institut National de la Santé et de la Recherche, Médicale, Faculté de Médecine Xavier-Bichat, Paris, France.
Rojas
Gertrudis
G
D001054:Apolipoproteins A / Q000737:chemistry* / Q000378:metabolism*; D001665:Binding Sites; D020134:Catalytic Domain; D006801:Humans; D008239:Lysine / Q000378:metabolism; D010960:Plasminogen Activators / Q000378:metabolism; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship
D001054:Apolipoproteins A; D010960:Plasminogen Activators; D008239:Lysine