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Structure of saposin B in complex with chloroquine
Structural highlights
DiseaseSAP_HUMAN Defects in PSAP are the cause of combined saposin deficiency (CSAPD) [MIM:611721; also known as prosaposin deficiency. CSAPD is due to absence of all saposins, leading to a fatal storage disorder with hepatosplenomegaly and severe neurological involvement.[1] [2] Defects in PSAP saposin-B region are the cause of leukodystrophy metachromatic due to saposin-B deficiency (MLD-SAPB) [MIM:249900. MLD-SAPB is an atypical form of metachromatic leukodystrophy. It is characterized by tissue accumulation of cerebroside-3-sulfate, demyelination, periventricular white matter abnormalities, peripheral neuropathy. Additional neurological features include dysarthria, ataxic gait, psychomotr regression, seizures, cognitive decline and spastic quadriparesis. Defects in PSAP saposin-C region are the cause of atypical Gaucher disease (AGD) [MIM:610539. Affected individuals have marked glucosylceramide accumulation in the spleen without having a deficiency of glucosylceramide-beta glucosidase characteristic of classic Gaucher disease, a lysosomal storage disorder.[3] [4] Defects in PSAP saposin-A region are the cause of atypical Krabbe disease (AKRD) [MIM:611722. AKRD is a disorder of galactosylceramide metabolism. AKRD features include progressive encephalopathy and abnormal myelination in the cerebral white matter resembling Krabbe disease.[5] Note=Defects in PSAP saposin-D region are found in a variant of Tay-Sachs disease (GM2-gangliosidosis). FunctionSAP_HUMAN The lysosomal degradation of sphingolipids takes place by the sequential action of specific hydrolases. Some of these enzymes require specific low-molecular mass, non-enzymic proteins: the sphingolipids activator proteins (coproteins). Saposin-A and saposin-C stimulate the hydrolysis of glucosylceramide by beta-glucosylceramidase (EC 3.2.1.45) and galactosylceramide by beta-galactosylceramidase (EC 3.2.1.46). Saposin-C apparently acts by combining with the enzyme and acidic lipid to form an activated complex, rather than by solubilizing the substrate. Saposin-B stimulates the hydrolysis of galacto-cerebroside sulfate by arylsulfatase A (EC 3.1.6.8), GM1 gangliosides by beta-galactosidase (EC 3.2.1.23) and globotriaosylceramide by alpha-galactosidase A (EC 3.2.1.22). Saposin-B forms a solubilizing complex with the substrates of the sphingolipid hydrolases. Saposin-D is a specific sphingomyelin phosphodiesterase activator (EC 3.1.4.12). Publication Abstract from PubMedChloroquine (CQ) has been widely used in the treatment of malaria since the 1950s, though toxicity and resistance is increasingly limiting its use in the clinic. More recently, CQ is also becoming recognized as an important therapeutic compound for the treatment of autoimmune disorders and has shown activity as an anticancer agent. However, the full extent of CQ pharmacology in humans is still unclear. Herein, we demonstrate that the lysosomal protein saposin B (sapB), critical for select lipid degradation, binds CQ with implications for both CQ function and toxicity. Using isothermal titration calorimetry (ITC) and fluorescence quenching experiments, CQ was shown to bind to the dimeric form of sapB at both pH 5.5 and pH 7.4 with an average binding affinity of 2.3x104 M-1 . X-ray crystallography confirmed this, and the first complete crystal structure of sapB with a bound small molecule (CQ) is reported. The results suggest that sapB might play a role in mitigating CQ-based toxicity and that sapB might itself be overwhelmed by CQ causing impaired lipid degradation. The Lysosomal Protein Saposin B Binds Chloroquine.,Huta BP, Mehlenbacher MR, Nie Y, Lai X, Zubieta C, Bou-Abdallah F, Doyle RP ChemMedChem. 2015 Nov 30. doi: 10.1002/cmdc.201500494. PMID:26616259[6] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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