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In vertebrate hemostasis, factor Va serves as the cofactor in the prothrombinase complex that results in a 300,000-fold increase in the rate of thrombin generation compared with factor Xa alone. Structurally, little is known about the mechanism by which factor Va alters catalysis within this complex. Here, we report a crystal structure of protein C inactivated factor Va (A1xA3-C1-C2) that depicts a previously uncharacterized domain arrangement. This orientation has implications for binding to membranes essential for function. A high-affinity calcium-binding site and a copper-binding site have both been identified. Surprisingly, neither shows a direct involvement in chain association. This structure represents the largest physiologically relevant fragment of factor Va solved to date and provides a new scaffold for the future generation of models of coagulation cofactors.

Research has been conducted on the human protein S. Experiments indicate that this protein reversibly self-associates in the absence of Ca (super)2+.

Research indicates that human bone cell phenotypic markers exhibit distinct age-related alterations, with some elderly people demonstrating significantly lower quantities of bone proteins. Immunologically purified bone preosteoblast-like cell populations were used in the research, with the findings facilitating a better understanding of the early stages of bone cell development.

Quantitative evaluation of tissue-type plasminogen activator (tPA) was done to evaluate substrate structure-efficiency correlations and to assess the specificity of tPA in hydrolyzing peptidyl fluorogenic substrates. Synthetic substrates used in the experiment contained 6-amino-1-naphthalenesulfonamide (ANSN) leaving group, Arg in the P1 position, various amino acids in the P2 and P3 positions, and different substituents in the sulfonamide moiety in the leaving group. The results showed that the factors that affect the efficiency and affinity of the substrates for tPA include hydrophobicity of the substrates, the volume of amino acids in the peptide core and the structure of the leaving group.

The membrane bound factor Xa appears to be the major activator of the vitamin K dependent zymogen, human factor VII, in vivo. The activation by Xa is anionic phospholipid (PCPS) dependent. The VIIa-tissue factor complex (factor IXa) and thrombin activate factor VII in vitro, but in vivo activation is uncertain. In the presence of PCPS, factor VIIIa has no affect on the activation by factor IXa, while factor Va decreases the activation by Xa. Factor XIa has no observable effect on factor VII activation rates.

Characterization of the positions of disulfide bonds and cysteines in bovine factor Va heavy chain reveals that 10 cysteine residues exist in the A1 and A2 domains, four of which attach to two disulfide bonds. Three main disulfide bonding forms alpha, beta and gamma loops are present in factor Va. The Cys(579) and Cys(660) residues contain a disulfide bridge while Cys(589) is a free SH.

Monoclonal antibodies use of factor Va, which inhibits factor Va cofactor activity, explains that the NH(sub 2)-terminal of the two chains of the cofactor enhances factor Va-factor Xa interaction. Phospholipid absence necessitates heavy and light chains of the cofactor in the interaction with factor Xa. The interaction of Xa with Va on phospholipid surface develops a prothrombin inducing enzymatic complex. The high affinity binding site for factor Va involves the serine protease domain of factor Xa.

The amidolytic activity and cooperative interactions during factor VIIa-tissue factor (TF) complex formation was studied in the presence of EDTA or divalent cations such as Me2+. The amidolytic activity of the factor VII-a TF complex increased in the presence of Me2+ while cooperative interactions occurred between Me2+ factor VIIa, tissue factor and substrate.

Binding experiments on factor Va reveal that two phospholipid vesicle binding sites located inside factor Va's light chain introduce the fluorescent lipophilic probe 1-azidopyrene in the presence of vesicles containing either 100% phosphatedylcholine (PC) or 75% PC and 25% phosphatidylserine (PS). The light chain's Mr equal to 46000/48000 COOH-terminal portion contain Va's C1 and C2 regions. This region dominantly influences Va binding to vesicles containing 100% PS whereas electrostatic reactions influence reactions involving vesicles containing PC.

Factor VIIa, a two chain serine protease hydrolyzes tripeptide fluorogenic substrates depending on the hydrophobicity of the P1 structure of the substrate, and its reactivity is enhanced by complex formation between factor VIIa and its initiation of the mechanism of coagulation. The configurational modifications in factor VIIa caused by TF leads to acceleration of the degradation process of the cofactor-substrate complex, and magnifies the catalytic constant of substrate hydrolysis. The volume of the components in the P structure determines substrate efficiency for factor VIIa.

Factor V, a profactor in the blood coagulation cascade, is converted to factor Va after activation by thrombin. Factor Va is composed of an amino terminal heavy chain and a carboxyl terminal light chain held together through calcium ion-mediated interactions. There are 10 cysteine residues in the heavy chain and 8 cysteine residues in the 647-residue light chain. The disulfide bond sites of bovine factor Va light chain were determined by autoradiography. Results showed that there are two free cysteine residues, at Cys 1953 and Cys2100 of the C1 and C2 domains, respectively. There are also three disulfide bonds at Cys1684-Cys1710, Cys1866-2020 and Cys2025-2180.

A study was conducted on the use of aminonaphthalenesulfonamide (ANSN) compounds as modifiable fluorescent detecting groups. These compounds possess a high quantum fluorescent yield and their sulfonyl moiety allows for numerous chemical modifications. ANSN was found to be capable of elevating the specificity of substrate hydrolysis by thrombin more than 30 times. Alterations on the peptidyl and detecting group side of the resultant substrate were also suggested to optimize the substrate's specificity for use in an assay system with a mixture of proteases.

Author(s): Saulius Butenas [1]; Kenneth G Mann [1] To the editor: Following mechanical or chemical damage of the vessel or monocyte stimulation, tissue factor is exposed to the blood and [...]

Evidence of Estrogen Receptors in Normal Human Osteoblast-Like Cells ALTHOUGH ESTROGEN DEFICIENCY is a recognized cause of postmenopausal bone loss and estrogen treatment has potent effects on bone (1), previous [...]

A study was conducted to analyze a framework characterizing the inactivation of factor Va by activated protein C. Factor Va cofactor activity was evaluated in a microtiter plate DAPA assay. Kinetic analysis and molecular weight averages were then determined using a right angle light scattering analysis method. Results indicated that the model accurately determines the rates of inactivation of factor Va and the effect of product inhibition.