Lp(a) consists of an LDL-like core lipoprotein molecule, containing apolipoprotein B (apo-B), to which a glycoprotein of variable molecular weight, apolipoprotein (a) [apo(a)], is covalently linked via a single cysteine–cysteine disulfide bond
[57] (
Figure 1). Lp(a) particles contain apo(a) and apo-B100 in a 1-to-1 molar ratio
[58]. The lipoprotein moiety is essentially indistinguishable from LDL regarding its physical chemical properties and consists of a hydrophobic core of esterified cholesterol and triglicerides, surrounded by a surface monolayer of phospholipids, unesterified cholesterol, and other proteins
[59]. The peculiar characteristics and the size variability of Lp(a) that is the main determinant of its plasma concentration are almost entirely accounted for by the presence of apo(a). Apo(a) is encoded by the LPA gene, located on chromosome 6q26
[60], and cloning of a c-DNA encoding apo(a) revealed a high degree of homology of this lipoprotein with the fibrinolytic proenzyme plasminogen
[61]. Both molecules contain coding sequences forming multiple triple-loop structures called kringles (K)
[62] that, due to resemblance of shape, were named after Danish pastries
[63]. The characteristic tri-looped arrangement of the kringle structure is stabilized by the presence of three internal disulfide bridges, resulting from the interaction of six conserved cysteine residues
[62]. Plasminogen contains an N-terminal tail domain that is attached to one copy each of five kringles, designated as kringle-1 through kringle-5, and a trypsin-like protease domain
[64]. In contrast to plasminogen, apo(a) lacks the tail domain and the first three kringle domains of plasminogen and instead is formed of multiple repeated copies of sequences homologous to plasminogen kringle-4 (K4) domain, followed by a single kringle-5-like domain and an inactive protease-like domain
[65]. Lp(a) contains 10 subtypes of K4 repeats (K4 type-1 to K4 type-10) that differ from each other based on aminoacidic sequence. All K4 kringle types are present in a single copy within the Lp(a) moiety, with the notable exception of K4 type-2, which is present in a variable number of identically repeated copies, usually ranging from 3 to more than 40
[58], that are encoded by the LPA gene. This important variation leads to the size heterogeneity in apo(a) isoforms found in the general population. As a rule, apo(a) isoform size is inversely related to plasma Lp(a) concentration in most populations
[66]. Kringles are ligand-binding sites and as such serve critical functions and pathobiological roles that are mediated by their lysine-binding sites (LBS). K4 type-9 forms a covalent disulfide bridge to the apo-B100 moiety of LDL and is critical in the creation of the covalent apo(a) LDL-complex whose formation is crucially initiated by noncovalent interaction between LBS of apo(a) and lysine residues of apoB100. The lysine binding site in K4 type-10 is thought to mediate the binding of Lp(a) to different substrates including fibrin, cell surface receptors, and extracellular matrix proteins
[67][68][69][70].