Title : The structure and catalytic mechanism of human
sphingomyelin phosphodiesterase like 3a--an
acid sphingomyelinase homologue with a novel nucleotide hydrolase activity
Abstract :
- Human sphingomyelinase phosphodiesterase like 3a (SMPDL3a ) is a secreted enzyme that shares a conserved catalytic domain with human acid sphingomyelinase ( aSMase ), the enzyme carrying mutations causative of Niemann-Pick disease
- We have solved the structure of SMPDL3a revealing a calcineurin-like fold
- A dimetal site , glycosylation pattern and a disulfide bond network are likely to be conserved also in human aSMase
- We show that the binuclear site of SMPDL3a is occupied by two Zn(2+) ions and that excess Zn(2+) leads to inhibition of enzyme activity through binding to additional sites
- As an extension of recent biochemical work we uncovered that SMPDL3a catalyses the hydrolysis of several modified nucleotides that include cytidine 5'-diphosphocholine, cytidine diphosphate ethanolamine and ADP-ribose, but not the aSMase substrate, sphingomyelin
- We subsequently determined the structure of SMPDL3a in complex with the product 5'-cytidine monophosphate ( CMP ), a structure that is consistent with several distinct coordination modes of the substrate/product in the active site during the reaction cycle
- Based on the structure of CMP complexes, we propose a phosphoryl transfer mechanism for SMPDL3a
- Finally, a homology model of human aSMase was constructed to allow for the mapping of selected Niemann-Pick disease mutations on a three-dimensional framework to guide further characterization of their effects on aSMase function
- DATABASE: Structural data are available in the PDB database under the accession numbers 5EBB and 5EBE