An in silico and in vitro approach to elucidate the impact of residues flanking the cleavage scissile bonds of FVIII

Coagulation Factor VIII is activated by an ordered limited thrombin proteolysis with different catalytic efficiency at three P1 Arginine residues: Arg(759)> Arg(1708)>Arg(391), indicating the flanking residues of the latter to be less optimal. This study aimed to investigate, in silico and in vitro, the impact of possessing hypothetically optimized residues at these three catalytic cleavage sites. The structural impact of the residues flanking Arginine cleavage sites was studied by in silico analysis through comparing the cleavage cleft of the native site with a hypothetically optimized sequence at each site. Moreover, recombinant FVIII proteins were prepared by replacing the sequences flanking native thrombin cleavage sites with the proposed cleavage-optimized sequence. FVIII specific activity was determined by assessing the FVIII activity levels in relation to FVIII antigen levels. We further investigated whether thrombin generation could reflect the haemostatic potential of the variants. Our in silico results show the impact of the residues directly in the cleavage bond, and their neighboring residues on the insertion efficiency of the loop into the thrombin cleavage cleft. Moreover, the in vitro analysis shows that the sequences flanking the Arg(1708) cleavage site seem to be the most close to optimal residues for achieving the maximal proteolytic activation and profactor activity of FVIII. The residues flanking the scissile bonds of FVIIII affect the cleavage rates and modulate the profactor activation. We were able to provide insights into the mechanisms of the specificity of thrombin for the P1 cleavage sites of FVIII. Thus, the P4-P2´ residues surrounding Arg(1708) of FVIII have the highest impact on rates of thrombin proteolysis which contributes to thrombin activation of the profactor and eventually to the thrombin generation potential.