Figure 3. ASM active site.

(a,b) Active sites of the holo (a) and phosphocholine bound (b) structures. Cα atoms are shown in grey ribbon. Two zinc atoms are shown as gold spheres, and one water molecule as red sphere. Phosphocholine is shown as sticks with yellow carbon, red oxygen and orange phosphorus. (c) Proposed catalytic mechanism in 2D diagram. Red arrows indicate electron relay during the cleavage. Only phosphate group in sphingomyelin are shown in details. Ceramide and choline indicate orientation of substrate in ASM.