Domains of human prothrombin embedded into vesicles: relation with biological activity

Full-length cDNA cloning and protein three-dimensional structure modeling of porcine prothrombin

Prothrombin is a vitamin K-dependent serine protease and plays pivotal roles in both procoagulant and anticoagulant pathway of hemostasis. In this study, we cloned the full-length cDNA of porcine prothrombin by cDNA library screening and SMART RACE technique. The full-length cDNA is 2027 bp, with a 1869 bp Open Reading Frame (ORF) coding 623 amino acids. The deduced protein of porcine prothrombin contains signal peptide, propeptide, Gla domain, two kringle domains and trypsin domain. Porcine prothrombin shares 86.15% nucleotide similarity and 83% amino acid similarity with human prothrombin. The trypsin domain is highly conserved between the two species with 92.1% amino acid identity. Macromolecular interaction sites comparison between porcine and human prothrombin suggests that the Gla domain in porcine prothrombin contains an additional potential gamma-carboxyglutamic acid site. However, a thrombin cleavage site (Arg284-Thr285) in its light chain is lost. When thrombin heavy chain is concerned, the most important functional sites such as catalytic triad DHS, RGD site, Na+ binding site and anion-binding exosite-I and II are highly conserved. However, great differences have been observed between residues 145 and 158 of heavy chain which is associated with thrombomodulin binding. Two important limited proteolysis sites at Ala150 and Lys154 were lost in porcine sequence, which would affect epsilon-thrombin and gammaT-thrombin generation. Comparison on 3-D protein models demonstrates that these proteins are obviously different in autolysis loop (Lys145 to Gly155). Compared with that of human prothrombin, variation at critical recognition sites would likely alter its binding affinity and reaction velocity, which would contribute to coagulation disorder when porcine liver is transplanted into human body.

Effect of 4-hydroxynonenal on phosphatidylethanolamine containing condensed monolayer and on its interaction with apolipoprotein A-I

4-Hydroxynonenal (4HNE), generated during polyunsaturated fatty acid oxidation, is present in atherosclerotic lesions. As 4HNE is able to react with phosphatidylethanolamine (PE), we investigated, using AC polarography, whether it may alter the physico-chemical state of a condensed PE-containing phospholipid monolayer and its interaction with apoA-I. The stability of a phospholipid monolayer relative to potential (around the potential of zero charge) is dependent on lipid composition (PE>PC>PE/PC). ApoA-I insertion into PE/PC monolayer is easier than in PC monolayer. Pre-treatment of PE/PC monolayer by 4HNE does not alter monolayer stability, but decreases apo A-I insertion into the monolayer.

Interaction of an amphitropic protein (factor Xa) with membrane models in a complex system

Phosphatidylserine (PS) plays a crucial role, in the conversion of prothrombin into thrombin by the protease, factor Xa. Physiologically, the conversion occurs in the prothrombinase complex. The question of how water-soluble proteins that normally circulate in plasma bind remains to be unambiguously determined. We previously found that the amphitropic proteins (prothrombin, factors V and Va) penetrate into phospholipid layers. AC polarography has allowed the detection for the first time of insertion of factor Xa into condensed monolayers containing phosphatidylserine (PS) and phosphatidylcholine (PC) either 100% PS or 25% PS in the presence of Ca2+. This observation demonstrates that part of factor Xa can cross the phospholipid polar headgroup/hydrocarbon chain interface. In parallel experiments, radioactive surface measurements permitted measuring binding of tritium-labeled factor Xa onto a PS monolayer and calculate an association constant, 6x10(6) M(-1). Penetration of factor Xa into PS-containing vesicles was investigated also using photoactivable 5-[125I]iodonaphthalene-1-azide, which binds selectively to the lipid embedded domains of the protein. These experiments suggest that Factor Xa penetrates preferentially by its heavy chain, an alternative mode of binding to the commonly accepted binding via its Gla domain. Interaction of factor Xa with PS vesicles also changes its apparent K(m) for S 2222.