Hereditary thrombotic thrombocytopenic purpura is caused by mutations in ADAMTS13 resulting in defective processing of von Willebrand factor (VWF) which 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 towards multimeric VWF and FRETS-VWF73 but with abnormal kinetics having a significantly reduced kcat (0.88+/-0.04s(-1) vs 2.78+/-0.11s(-1)) and slightly smaller KM (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 (kcat=0.82+/-0.03s(-1) and KM=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.