Factor VIIIa A2 subunit residues 558-565 represent a factor IXa interactive site

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Factor VIIIa is a crucial cofactor in the intrinsic pathway of coagulation, forming the tenase complex.
• The A2 subunit of Factor VIIIa plays a role in macromolecular interactions within the tenase complex.
• Previous studies suggested the central A2 subunit region is important for these interactions.

Purpose of the Study:

• To investigate the role of the central A2 subunit region in factor Xase complex function.
• To synthesize and characterize a peptide representing residues 558-565 of Factor VIII.
• To determine the peptide's effect on factor Xa generation and factor VIIIa stability.

Main Methods:

• Chemical synthesis of a peptide (FVIII558-565) corresponding to Factor VIII residues 558-565.
• Assay of factor Xa generation in a purified system.
• Kinetic analysis of peptide inhibition and its interaction with factor IXa.
• Tryptic cleavage and scrambled sequence controls for peptide activity.
• Investigation of factor IXa-mediated cleavage of isolated Factor VIII heavy chain.

Main Results:

• The synthesized peptide FVIII558-565 inhibited factor Xa generation with a KI of 105 microM.
• Tryptic cleavage abolished peptide activity, and a scrambled sequence showed significantly reduced inhibition.
• Overlapping peptides also demonstrated inhibitory activity, confirming the importance of the scissile bond region.
• Peptide inhibition was non-competitive with factor X but overcome by increased factor IXa.
• The peptide inhibited factor IXa-dependent stabilization of factor VIIIa.

Conclusions:

• The A2 subunit sequence 558-565 is critical for cofactor-protease interaction in the intrinsic factor Xase activity.
• This peptide's ability to inhibit both factor Xa generation and factor VIIIa stabilization suggests physiological significance.
• The findings elucidate a key interaction site on Factor VIIIa essential for coagulation.