Summary
Almost two decades ago an anticoagulant function of factor V (FV) was discovered,
as an anticoagulant cofactor for activated protein C (APC). A natural mutant of FV
in which the R506 inactivation site was mutated to Gln (FVLeiden) was inactivated slower by APC, but also could not function as anticoagulant cofactor
for APC in the inactivation of activated factor VIII (FVIIIa). This mutation is prevalent
in populations of Caucasian descent, and increases the chance of thrombotic events
in carriers. Characterisation of the FV anticoagulant effect has elucidated multiple
properties of the anticoagulant function of FV: 1) Cleavage of FV at position 506
by APC is required for anticoagulant function. 2) The C-terminal part of the FV B
domain is required and the B domain must have an intact connection with the A3 domain
of FV. 3) FV must be bound to a negatively charged phospholipid membrane. 4) Protein
S also needs to be present. 5) FV acts as a cofactor for inactivation of both FVa
and FVIIIa. 6) The prothrombotic function of FVLeiden is a function of both reduced APC cofactor activity and resistance of FVa to APC
inactivation. However, detailed structural and mechanistic properties remain to be
further explored.
Keywords
Activated protein C - anticoagulant - APC resistance - factor V - protein S