Coagulation factor VIII

Identification and functional requirement of Cu(I) and its ligands within coagulation factor VIII

Coagulation factor VIII (FVIII) is a heterodimer consisting of a light chain of 80 kDa (domains A3-C1-C2) in a metal ion-dependent association with a 220-kDa heavy chain (domains A1-A2-B). The nature of the metal ion-dependent association between the heavy and light chains was investigated using atomic absorption spectroscopy, electron paramagnetic resonance spectroscopy (EPR), and site-directed mutagenesis and expression of the FVIII cDNA. Whereas copper ion was not detected in intact recombinant FVIII, EDTA dissociation of the chains yielded an EPR signal consistent with 1 mol of Cu(I)/mol of active protein, supporting the hypothesis that a single molecule of reduced copper ion is buried within intact FVIII and is released and oxidized upon treatment with EDTA. Cu(I), and not Cu(II), was able to reconstitute FVIII activity from dissociated chains, demonstrating a requirement for Cu(I) in FVIII function. Three potential copper ion binding sites exist within FVIII: one type-2 site and two type-1 sites. The importance of these potential copper ion ligands was tested by studying the effect of site-directed mutants. Of the two histidines that compose the type-2 binding site, the His-1957 --> Ala mutant displayed secretion, light and heavy chain assembly, and activity similar to wild-type FVIII, while mutant His-99 --> Ala was partially defective for secretion and had low levels of heavy and light chain association and activity. In contrast, FVIII having the mutation Cys-310 --> Ser within the type-1 copper binding site in the A1 domain was inactive and partially defective for secretion from the cell, and the heavy and light chains of the secreted protein were not associated. Mutant Cys-2000 --> Ser within the A3 domain displayed secretion, assembly, and activity similar to that for wild-type FVIII. These results support the hypothesis that Cu(I) is buried within the type-1 copper binding site within the A1 domain and is required for FVIII chain association and activity.

Copper-atom identification in the active and inactive forms of plasma-derived FVIII and recombinant FVIII-delta II

The plasma-derived factor VIII (pd-FVIII) circulates as different heterodimers of heavy and light chains associated by a metallic ion still present in the functional activated factor VIII trimer of molecular mass 50,000-45,000-70,000 Da. The chelation of the metal leads to the dissociation of these complexes with a concomitant loss of the procoagulant activity. Until now, this ion has not been directly identified and its role in the structure/function relationships remains unclear. We report the first determination of the nature of this metal using atomic-absorption spectroscopy with Zeeman effect. A comparative identification was also performed with the new recombinant factor VIII, FVIII-delta II. In the different active pd-FVIII heterodimers (of molecular mass ranging over 210,000-80,000-90,000-80,000 Da) and in FVIII-delta II, copper was detected. This result is consistent with sequence similarities described between FVIII and copper-binding proteins. The quantification of the copper content in FVIII-delta II and in the corresponding pd-FVIII dimer of 90,000-80,000 Da indicated, for both proteins, the presence of one copper ion/mol FVIII. Copper was also identified in the activated FVIII complex and remained in the dimer of 50,000-70,000 Da generated during FVIII inactivation. Further dissociation into isolated fragments of molecular masses 70,000 Da and 50,000 Da was concomitant with the loss of the copper ion. No copper was detected in the isolated fragment of molecular mass 45,000 Da. These results suggest that the presence of the cation is not directly related to FVIII activity but is an essential structural prerequisite for FVIII heavy-light-chain association.