Mutation of the H-bond acceptor S119 in the ADAMTS13 metalloprotease domain reduces secretion and substrate turnover in a patient with congenital thrombotic thrombocytopenic purpura.

Hereditary thrombotic thrombocytopenic purpura is caused by mutations in a disintegrin and metalloprotease with thrombospondin motifs (ADAMTS13) resulting in defective processing of von Willebrand factor (VWF) that causes intravascular platelet aggregation culminating in thrombocytopenia with shistocytic anemia. In this study the functional and structural role of a recently identified ADAMTS13 metalloprotease domain mutation S119F was investigated. Secretion from heterologous cells was hampered but not completely eliminated. Secreted S119F was active toward multimeric VWF and FRETS-VWF73 but with abnormal kinetics, having a significantly reduced overall catalytic rate (k(cat); 0.88 +/- 0.04 s(-1) vs 2.78 +/- 0.11 s(-1)) and slightly smaller Michaelis constant (K(M); 1.4 +/- 0.2microM vs 2.3 +/- 0.3microM). A computational model of the metalloprotease domain demonstrates both steric and polar interaction effects caused by S119F. Interestingly, mutant S119A has properties similar to S119F (k(cat) = 0.82 +/- 0.03 s(-1) and K(M) = 1.1 +/- 0.1microM), allowing to assign distorted kinetics to the loss of the H-bond with conserved residue W262. We conclude that the S119-W262 H-bond is crucial for maximal turnover.