Sandbox Reserved 313: Difference between revisions

'''Sphingomyelin phosphodiesterase (Sphingomyelinase):''' (SMase) is an enzyme which catalyzes the hydrolysis of sphingomyelin (SM) to ceramide and phosphocholine.  This enzyme also has hemolytic activity where red blood cells are ruptured and hemoglobin is released into the blood plasma. <ref name="gp">PMID: 16595670 </ref>.  This enzyme has become the object of renewed interest since the discovery of the sphingomyelin [[signal transduction pathway]] which is involved in apoptosis. This pathway is initiated by a neutral sphingomyelinase  hydrolysis of sphingomyelin in the plasma membrane to generate ceramide. Ceramide acts as a secondary messenger which causes the stimulation of the cascade effect of kinases and transcription factors which activate programmed cell death<ref name="gp2">PMID: 7544586  </ref>.     

''Acid sphingomyelinase (aSMase)'' - aSMase is a soluble lysosomal hydrolase with an optical acivity at pH 5. Acid-SMase has two enzymatic forms: one is targeted to the endo-lysosomal compartment where it coordinates Zn, and the other can be realeased extracellularly through the golgi secretory pathway<ref name="gp4">PMID: 21098024  </ref>. A deficiency in results in the lysosomal storage disorder Niemann-Pick disease.  Acid-SMase normally metabolizes sphingomyelin which is found in every cell of the body. When aSMase is lacking in the cell, SM builds up and eventually results in cell death<ref name="gp5">PMID: 18567738  </ref>.

''Secretory sphingomyelinase (S-SMase)'' - S-Smase is found in human vascular endothelial cells and arises from the acid-SMase gene through differential protein trafficking of a precurser.  The precurser can be targeted to the Gogli secretory pathway or lysosomes. S-SMase hydrolyses (SM) found in plasma membranes and lipoprotein molecules but has also been found to play an important role in intracellular or paracrine ceramide second messenger signaling pathways.  S-SMase is activated by normal levels of Zn2+ and operates generally at acidic pH levels, but can hydrolyze atherogenic lipoproteins at neutral pH<ref name="gp6">PMID: 11001566  </ref>.

''Neutral Mg2+-independent sphingomyelinases'' - Thee activities of magnesium independent nSMase are found predominantly in the cytosol and are not very well known<ref name="gp3">PMID: 12401200 </ref>.   

Recently, in the 1980's, the primary structure of sphingomyelinase was determined by cloning the first N-SMases from ''Bacillus cereus'' and ''Staphylococcus aureus'' and by the subsequent sequencing of their cDNAs <ref name="gp7">PMID: 2127932  </ref>.   

The crystal structure of sphingomyelinase has been solved using the bacterium  ''Listeria ivanovii'' and ''Bacillus cereus(Bc-SMase)'' to gain further insight into its catalytic activities <ref name="gp7">PMID: 16595670  </ref>.  The overall structure of Bc-SMase has been determined to consist of a β-sandwich with α/β motifs<ref name="gp">PMID: 16595670 </ref>.  Through SMase structure identification, it has been determined to be a member of the DNA I-like superfamily having geometrically identical amino acid residues as the enzymes in this superfamily.  The only different with SMase, is that it has a unique hydrophobic beta-hairpin structure. The crystal structure revealed that this unique beta-hairpin region has two solvent exposed aromatic amino acids, Trp-284 and Phe-285, on the top which bind to the cell surfaces to catalyze hemolysis.  The hydrolysis and hemolytic activity of Bc-SMase occur in a metal ion dependent manner.  Bc-Smase is found in complex with divalent metal ions, Co2+, Mg2+ or Ca2+ in the central cleft of this enzyme. The central cleft acts as an active site, because it contains Glu-53, Asp-195 and His-296 which are involved in Bc-SMase hydrolytic activity.  In the central cleft, bound Ca2+ displays a hepta-coordination pattern which is different from the Co2+ and Mg2+ bound forms which are in a double-hexa-coordination forming a double octahedral bi-pyramid.  Mg2+ binds to Glu-53 and is required for SM hydrolytic activity and Mg2+ with Ca2+ are required for hemolytic activity.

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It has been proposed that the mechanism of Bc-SMase is similar to that of bovine pancreatic DNase 1 because Bc-SMase and bovine DNase 1 are homologous proteins which both have conserved alleged catalytic amino acid residues and a similar molecular structure<ref name="gp">PMID: 16595670  </ref>. . The proposed hydrolytic activity of Bc-SMAse cloned from ''Bacillus cereus'' is coordinated by essential water bridged divalent metal ions.  Two metal ions which are bound to the Glu-53 and <scene name='Sandbox_Reserved_313/His-296/2'>His-296</scene> residues in the central cleft which orientate the substrate in the active site.  

The divalent cation which is linked to His-296 provides a general base water and a phosphate from sphingomelin binds to the central cleft at the site of the water bridged double metal ions.  The divalent metal ion which is located at Glu-53 then binds to sphingomelin by directly interacting with the amide oxygen and the O-4 ester oxygen.