Apo(a) is the main constituent of lipoprotein(a) (Lp(a)). It has serine proteinase activity and is able of autoproteolysis. Inhibits tissue-type plasminogen activator 1. Lp(a) may be a ligand for megalin/Gp 330. Disulfide-linked to apo-B100. Binds to fibronectin and decorin. N- and O-glycosylated. The N-glycans are complex biantennary structures present in either a mono- or disialylated state. The O-glycans are mostly (80%) represented by the monosialylated core type I structure, NeuNAcalpha2-3Galbeta1-3GalNAc, with smaller amounts of disialylated and non-sialylated O-glycans also detected. LPA genetic variants, including variations in the number of Kringle domains, define the lipoprotein(a) quantitative trait locus (LPAQTL) and influence lipoprotein(a) levels in plasma [MIM:618807]. Depending on the individual, the encoded protein contains 2-43 copies of kringle IV-2 repeats. Often the assignement of amino acids in lipoprotein(a) is based on a long allele that contains 37 copies of the kringle-type repeats (PubMed:3670400). The reference allele represented here contains 15 copies of the kringle-type repeats. Apo(a) is known to be proteolytically cleaved, leading to the formation of the so-called mini-Lp(a). Apo(a) fragments accumulate in atherosclerotic lesions, where they may promote thrombogenesis. O-glycosylation may limit the extent of proteolytic fragmentation. Homology with plasminogen kringles IV and V is thought to underlie the atherogenicity of the protein, because the fragments are competing with plasminogen for fibrin(ogen) binding. Belongs to the peptidase S1 family. Plasminogen subfamily.