A Glu113Ala mutation within a factor VIII Ca2+-binding site enhances cofactor interactions in factor Xase

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Factor VIII is a crucial cofactor in the blood coagulation cascade.
• An acidic-rich segment (residues 110-126) in the A1 domain of factor VIII is involved in calcium ion coordination.
• Calcium ions are essential for the cofactor activity of factor VIII.

Purpose of the Study:

• To investigate the role of the acidic-rich segment in factor VIII's A1 domain.
• To characterize the functional impact of specific mutations within this segment on factor VIII activity.
• To determine how mutations affect factor VIII's interaction with factor IXa and its cofactor activity.

Main Methods:

• Site-directed mutagenesis was used to replace Glu113 with Alanine (E113A).
• Specific activity was assessed using a one-stage clotting assay.
• Activation and inactivation kinetics were studied via thrombin time course experiments.
• Factor Xa generation assays were performed with synthetic phospholipid vesicles and on platelet membrane surfaces to evaluate kinetic parameters and binding affinities.

Main Results:

• The E113A mutation resulted in approximately a 2-fold increase in factor VIII specific activity.
• Activation and inactivation rates of E113A were similar to wild-type factor VIII.
• On platelet membranes, E113A factor VIIIa showed a 4-fold greater affinity for factor IXa compared to wild-type.
• E113A factor VIIIa also exhibited a 2-fold increased affinity for the platelet membrane.

Conclusions:

• Substitution of Glu113 with Alanine enhances factor VIII's specific activity.
• The increased activity is primarily attributed to enhanced factor IXa binding on the platelet membrane surface.
• This enhanced interaction may be significant for factor VIII cofactor function in physiological conditions with low factor levels.