Title : Structure of
Human Acid Sphingomyelinase Reveals the Role of the
Saposin Domain in Activating Substrate Hydrolysis
Abstract :
- Acid sphingomyelinase ( ASM ) is a lysosomal phosphodiesterase that catalyzes the hydrolysis of sphingomyelin to produce ceramide and phosphocholine
- While other lysosomal sphingolipid hydrolases require a saposin activator protein for full activity, the ASM polypeptide incorporates a built-in N-terminal saposin domain and does not require an external activator protein
- Here, we report the crystal structure of human ASM and describe the organization of the three main regions of the enzyme: the N-terminal saposin domain , the proline-rich connector, and the catalytic domain
- The saposin domain is tightly associated along an edge of the large, bowl-shaped catalytic domain and adopts an open form that exposes a hydrophobic concave surface approximately 30Å from the catalytic center
- The calculated electrostatic potential of the enzyme is electropositive at the acidic pH of the lysosome, consistent with the strict requirement for the presence of acidic lipids in target membranes
- Docking studies indicate that sphingomyelin binds with the ceramide-phosphate group position ed at the binuclear zinc center and molecular dynamic simulations indicate that the intrinsic flexibility of the saposin domain is important for monomer-dimer exchange and for membrane interactions
- Overall, ASM uses a combination of electrostatic and hydrophobic interactions to cause local disruptions of target bilayers in order to bring the lipid headgroup to the catalytic center in a membrane-bound reaction