Galactosylceramidase

Introduction

Galactosylceramidase (or galactocerebrosidase) is a hydrolase ^[1] that removes galactose from galactosylceramide and other sphingolipids^[2]. Galactosylceramidase in humans is encoded by the gene GALC, and mutations in this gene are associated with Krabbe disease, or globoid cell leukodystrophy^[3].

See also Lipid signaling.

Structure

X-ray diffraction data () from mouse models indicates that GALC is an estimated 77 kDa monomer consisting of a 656 residues, which form a containing 12 α-helices and 41 β-strands^[1]. Each β-strand contains three to eleven residues. The enzyme contains three major domains: a central triosephosphate isomerase (TIM) barrel, a β-sandwich domain, and a lectin domain^[3]. The TIM barrel is composed of eight parallel β-strands surrounded by α-helices and a single disulfide bridge between residues Cys 271 and Cys 378, while the β-sandwich is composed of two twisted β-sheets. The lectin domain is characterized by a calcium ion that is bound in pentagonal bipyramidal configuration^[3].

The substrate-binding site of GALC is mostly comprised of the central TIM barrel, however loops from the other two domains also appear to contribute to the binding pocket. Residues that appear to interact with the substrate by hydrogen bonding include Gly48, Thr93, Trp135, Asn181, Glu182, Glu258, Ser261, and Arg380. Of particular interest are the residues Glu258 and Glu182, which have been proposed as the active site nucleophile and proton donor respectively, due to the average distance between the carboxyl oxygens being consistent with the retaining mechanism of glycosidic bond hydrolysis.^[3]

Function

The molecular function of galactosylceramidase is hydrolysis of a O-glycosyl bond to remove galactose from Ceramide and other sphingolipids. The cellular function is the maintenance of a functional hematopoietic stem/progenitor cell niche by contributing to the control of the intracellular content of key sphingolipids^[4].

Reaction of galactosylceramide and water catalyzed by galactosylceramidase

Disease

Defects in this enzyme cause a lysosomal storage disorder known in humans as Krabbe disease (or globoid cell leukodystrophy). Krabbe disease is a neurodegenerative disorder characterized by widespread demyelination caused by reduced or mutated function of GALC^[3]. The deficiency of GALC leads to the accumulation of the neurotoxic metabolite 1-β-d-galactosylsphingosine (psychosine) in the central nervous system. Psychosine causes the destruction of epithelial actin structures and is toxic to oligodendrocytes^[5][6], by causing lipid raft clustering which leads to defective signal transduction^[7]. Besides the defective recruitment of signal molecules to lipid rafts, there is also impairment in the early steps of endocytosis and vesicle transport^[7]. Defects in the retrograde axonal transport is correlated with decreased amounts of dynein, unusual levels of post-translational tubulin modifications, and microtubule instability^[7]. GALC deficiency also causes the accumulation of lipids in "globoid" macrophages, where the medical name for the disease originated^[6]. A common authentic model for this disease is the twitcher mouse model^[5]. The only treatment currently available is an experiemental hematopoietic stem cell transplant, and gene therapies and enzyme replacements are still being researched^[6].