Factor V and VIII Flashcards
What is the function of FV?
FV is a cofactor for the prothrombinase complex
What is the function of FVIII?
FVIII is a cofactor for the intrinsic tenase complex
Why is FV and FVIII important as a cofactor?
They bring together the enzyme and the substrate
they are cofactors for the prothrombinase complex and the intrinsic tenase complex
Alone, Xa has limited activity and only small amounts of thrombin are generated. The thrombin activates FV and FVIII.
Cofactors amplify the IXa and Xa activity by lowering Km and increasing Kcat by x100,000
What is the structure of FV? How is this related to its function?
A1, A2, B, A3, C1, C2
A: protein-protein interactions
B: Important in FV but not FVIII, sequentially different between them
C: Phospholipid binding
Presence of B domain is essential as this regulates the cofactor activity by blocking Xa binding - removal of B domain leads to activation of FV - this is done by thrombin
What is the structure of FVIII, and how is it related to its function?
A1, a1, A2, a2, B, a3, A3, C1, C2
A2 : binds to FIXa
FVIII is typically bound to vWF
Why must FV and FVIII be proteolytically activated?
to maintain balance of the haemostatic mechanism
too much FVIII - thrombosis
too little FV - bleeding
How is FV activated and inactivated?
Activated - cleavage by thrombin of the B domain
Inactivated- Cleavage by APC
What are the similarities between FV and FVIII?
Gene structure
Protein domain structure
Absence of either causes bleeding disorder
Both require specific cleavages by thrombin to become active cofactors
B domains both highly glycosylated
Both active cofactors assemble into PL-located complexes with an active serine protease, to cleave another zymogen
Activated protein C cleavage switches them both off
What are the differences between FV and FVIII?
High levels of FVIII associated with thrombosis but not FV.
Presence of acidic peptide sequences between domains in FVIII, not FV
Different proteolytic cleavages for activation
Completely different B domain sequences
Interaction of FVIII with carrier protein VWF
Interaction of FV with TFPI
Different genetics and clinical prevalence (haemophilia A X-linked and much more common)
What is East Texas Bleeding disorder?
B domain of FV is spliced out so binds TFPI with higher affinity - increased bleeding
How is FVIII activated and inactivated?
Activation - Thrombin cleaves at acidic peptides forming heterotrimer. alpha 3 residue is removed and there is decreased affinity for vWF leading to dissociation of FVIIIa.
Inactivation - spontaneous dissociation of A2 domain OR cleavage by APC
What is the role of the B domain in synthesis?
the B domain is crucial for export of FV and FVIII
B domain binds to export proteins LMN1 and 2 to allow export
What is the role of the B domain in synthesis?
the B domain is crucial for export of FV and FVIII
B domain binds to export proteins LMAN1 and 2 to allow export
B domain in FVIII is not functionally relevant
How does the B domain of FV regulate the cofactor activity of FV?
The b domain binds to another part of itself and blocks binding of Xa
Removal of the b domain via removal at acidic and basic regions leads to activation of FV
