Kringles

Figure 2.19 Organization of polypeptide chains into domains. Small protein molecules like the epidermal growth factor, EGF, comprise only one domain. Others, like the serine proteinase chymotrypsin, are arranged in two domains that are required to form a functional unit (see Chapter 11). Many of the proteins that are involved in blood coagulation and fibrinolysis, such as urokinase, factor IX, and plasminogen, have long polypeptide chains that comprise different combinations of domains homologous to EGF and serine proteinases and, in addition, calcium-binding domains and Kringle domains.

Kringle domains, which have a characteristic pattern of three internal disulfide bridges within a region of about 85 amino acid residues. 

Park, C.H., Tulinsky, A. Three-dimensional sfructure of the Kringle sequence structure of profhrombin fragment 1. Biochemistry 25 3977-3982, 1986. 

FIGURE 6.38 A sampling of proteins that consist of mosaics of individual protein modules. The modules shown include 7CG, a module containing 7-carboxyglutamate residues G, an epidermal growth-factor-like module K, the kringle domain, named for a Danish pastry ... 

NK4 (4-kringle domains of HGF) Inhibits EC differentiation and migration induced by VEGF... 

Conserved sequences that fold into large loops stabilized by three disulfide linkages. The name Kringle comes from the Scandinavian pastry that these structures resemble. They can mediate certain protein-protein interactions. 

Kinase Domain Kinase Inhibitors Kinins Kir Channels Knockout Mice Kringle Domains K+-Sparing Diuretics Kv(3 -Subunits Kv-Channels KvLQT 1 -Channels Kynurenine Pathway L-NAME... 

Lipoprotein (a) is an independent risk factor for coronary artery disease [68]. It consists of two components an LDL particle and apolipoprotein (a) which are linked by a disulfide bridge. Apo(a) reveals a genetically determined size polymorphism, resulting from a variable number of plasminogen kringle IV-type repeats [69]. Statins either do not affect Lp(a) or may even increase Lp(a) [70, 71]. In a study of 51 FH patients, treated with 40 mgd 1 pravastatin, it has been shown that the increase in Lp(a) was greatest in patients with the low molecular-weight apo(a) phenotypes [70]. 

The heterogeneity of Lp(a) is related to ultrastructural motifs in the molecule, strongly resembling the so-called kringles found in both plasminogen and other plasma proteins, such as proteases of the coagulation system (tissue type plasminogen activator and prothrombin) (Fig. 2). 

Fig. 2. Schematic representation of a typical kringle structure, plasminogen, and apolipoprotein (a). [With permission of Scanu et al. (S14).]...

Fig. 3. Proposed apo(a) kringle structure. [With permission of Guevara et al. (G28).]...

The variability in number of kringles is genetically determined. Marcovina has described 34 different apo(a) isoforms differing in molecular mass by approximately 12.54 kDa, which closely corresponds to the predicted molecular mass of one kringle unit (M12, M30) (Fig. 5). 

It is possible that the interaction between apo(a) and apo-Bl00 involves more than a simple covalent disulfide bond, such as electrostatic interactions, hydrogen bonds, and van der Waals interactions between specific amino acids and kringle-ligand complex forms (F9, G29). 

Apart from the heterogeneity of apo(a), caused by a varying number of Lpa-KIV2-kringles and the degree of glycosylation (varying up to 35%) (G26), human Lp(a) seems to exist of two species with different affinities for lysine-Sepharose (L13), but similar interaction with LDL-receptor in vitro (A15). This... 

The evolutionary hypothesis is that the ancestral molecule of apo(a) was a plasminogen-type protein, having five kringles, that emerged by a duplication event from a protein with one kringle and one serine protease domain about 300 million years ago (12). 

Apo(a) isoform size variability was subsequently found to result from differences in sizes of the hepatic apo(a) mRNA in both humans (K25) and baboons (H31) with few exceptions, apo(a) protein sizes were directly correlated to the size of the apo(a) transcript. Although studies suggested differences in the numbers of kringle IV repeats in the gene (G8, LI9), size variation in the apo(a) gene was definitively shown to result from differences in the number of tandemly... 

Fig. 8. Organization of the kringle IV repeats in apo(a). It has been determined that kringle IV repeats, which vary in amino acid sequence from the identically repeated kringle sequence (kringle IV types 1 and 3-10), are highly variable in the population, ranging from less than 10 to greater than 50. The numbers in brackets below the boxes refer to the kringle nomenclature described in McLean et at. (M24). Sequences corresponding to apo(a) kringle V and protease domains are indicated by stippled and solid boxes, respectively.

It is important to note that not all of the variability in Lp(a) levels can be explained by isoform size variability in Caucasians, only 40-70% of the variation results from differences in the numbers of kringle IV repeats in the gene (B15, B16). Interindividual variation (—22%) also occurs as a result of as yet undefined cis-acting sequences at the apo(a) locus (B15). In some cases, unrelated individuals with the same-sized apo(a) gene and protein have been shown to possess markedly different plasma Lp(a) concentrations (B15, L5) within families, however, the relationship between apo(a) gene size and plasma Lp(a) con-... 

A T-C polymorphism was described in the sequence of apo(a)-KIV10 (V6), corresponding to nucleotide 12,605 of the published cDNA sequence (M24). This variant results in the substitution of a methionine (ATG) with a threonine (ACG) at this position. No correlation was observed between the polymorphism and plasma Lp(a) levels. Although the Met-Thr substitution is present within the lysine binding pocket in KIV10, its effect on lysine binding properties of this kringle remains to be determined. 

Since apo(a) contains kringles that are almost identical to the kringles in plasminogen, with a moderate affinity to fibrin through their lysine-binding sites, the possible role of Lp(a) in fibrinolysis could be at that level (R17, R18). 

Lafferty et al. (L7) described antibodies raised against Lp(a) that cross-react with plasminogen, most of which recognize an epitope present on kringle IV of apo(a). 

G8. Gavish, D., Azrolan, N., and Breslow, J. L., Plasma Lp(a) concentration is inversely correlated with the ratio of kringle IV/kringle V encoding domains in the apo(a) gene. J. Clin. Invest. 84, 2021-2027 (1989). 

---