Model of the ADAMTS13 metalloprotease domain and kinetics of S119 mutants. (A) Michaelis-Menten kinetics of WT rADAMTS13 (○), mutants S119F (▿) and S119A (♦). Initial velocity in function of FRETS-VWF73 concentration ([S]) at fixed enzyme concentration (0.1nM) is shown. (B) Surface representation of the ADAMTS13 metalloprotease model. Depicted in red and green are the S1′ and S2′ loop, respectively. The loops constitute part of the active-site cleft. Mutated residue serine 119 (magenta) is located to the bottom left, facing the C-terminus of the S1′ loop. (C) Cartoon representation zoomed into the S1′ loop (red) and inner molecule. Depicted in sticks are S119, M249 (Met-turn), and residues making up the hydrophobic patch to which the W262 indole side chain is oriented (V248, L120, and L114). Hydrogen bonds are shown in dashes. Mutation of residue S119 to phenylalanine (overlaid in orange) disrupts the hydrogen bond and causes a steric clash between the S1′ loop for all possible rotamers at fixed backbone positions (1 rotamer shown).