Glycosphingolipids (GSLs) are a specialized class of membrane lipids composed of a ceramide backbone and a carbohydrate-rich head group. GSLs populate lipid rafts of the cell membrane of eukaryotic cells, and serve important cellular functions including control of cell–cell signaling, signal transduction and cell recognition. Of the hundreds of unique GSL structures, anionic gangliosides are the most heavily implicated in the pathogenesis of lysosomal storage diseases (LSDs) such as Tay-Sachs and Sandhoff disease. Each LSD is characterized by the accumulation of GSLs in the lysosomes of neurons, which negatively interact with other intracellular molecules to culminate in cell death. In this review, we summarize the biosynthesis and degradation pathways of GSLs, discuss how aberrant GSL metabolism contributes to key features of LSD pathophysiology, draw parallels between LSDs and neurodegenerative proteinopathies such as Alzheimer’s and Parkinson’s disease and lastly, discuss possible therapies for patients.

Bacterial glycosyltransferases often synthesize the same glycan linkages as mammalian glycosyltransferases, yet they usually have very little sequence identity. Nevertheless, the properties, folding, substrate specificities and catalytic mechanisms of these enzyme proteins may have significant similarity. Thus bacterial glycosyltransferases can be utilized for the enzymatic synthesis of both bacterial and mammalian types of complex glycan structures. A comparison is made here between mammalian and bacterial enzymes that synthesize epitopes found in mammalian glycoproteins, and those found in the O antigens of Gram negative bacteria. These epitopes include Thomsen-Friedenreich (TF or T) antigen, blood group O, A and B, type 1 and 2 chains, Lewis antigens, sialylated and fucosylated structures and polysialic acids. Many different approaches can be taken to investigate the substrate binding and catalytic mechanisms of glycosyltransferases, including crystal structure analyses, mutations, comparison of amino acid sequences, NMR and mass spectrometry. Knowledge of the protein structures and functions helps to design glycosyltransferases for specific glycan synthesis and to develop inhibitors. The goals are to develop new strategies to reduce bacterial virulence and to synthesize vaccines and other biologically active glycan structures.

Cells undergo proliferation and apoptosis, migration and differentiation via a number of cell surface receptors, most of which are heavily glycosylated. This review discusses receptor glycosylation and the known roles of glycans on the functions of receptors expressed in diverse cell types. We included growth factor receptors that have an intracellular tyrosine kinase domain, growth factor receptors that have a serine/threonine kinase domain, and cell-death-inducing receptors. N- and O-glycans have a wide range of functions including roles in receptor conformation, ligand binding, oligomerization, and activation of signaling cascades. A better understanding of these functions will enable control of cell survival and cell death in diseases such as cancer and in immune responses.

The opportunistic pathogen Pseudomonas aeruginosa produces two major cell surface lipopolysaccharides, characterized by distinct O antigens, called common polysaccharide antigen (CPA) and O-specific antigen (OSA). CPA contains a polymer of d -rhamnose ( d -Rha) in α1-2 and α1-3 linkages. Three putative glycosyltransferase genes, wbpX , wbpY , and wbpZ , are part of the CPA biosynthesis cluster. To characterize the enzymatic function of the wbpZ gene product, we chemically synthesized the donor substrate GDP- d -Rha and enzymatically synthesized GDP- d -[ ³ H]Rha. Using nuclear magnetic resonance (NMR) spectroscopy, we showed that WbpZ transferred one d -Rha residue from GDP- d -Rha in α1-3 linkage to both GlcNAc- and GalNAc-diphosphate-lipid acceptor substrates. WbpZ is also capable of transferring d -mannose ( d -Man) to these acceptors. Therefore, WbpZ has a relaxed specificity with respect to both acceptor and donor substrates. The diphosphate group of the acceptor, however, is required for activity. WbpZ does not require divalent metal ion for activity and exhibits an unusually high pH optimum of 9. WbpZ from PAO1 is therefore a GDP- d -Rha:GlcNAc/GalNAc-diphosphate-lipid α1,3- d -rhamnosyltransferase that has significant activity of GDP- d -Man:GlcNAc/GalNAc-diphosphate-lipid α1,3- d -mannosyltransferase. We used site-directed mutagenesis to replace the Asp residues of the two DXD motifs with Ala. Neither of the mutant constructs of wbpZ (D172A or D254A) could be used to rescue CPA biosynthesis in the Δ wbpZ knockout mutant in a complementation assay. This suggested that D172 and D254 are essential for WbpZ function. This work is the first detailed characterization study of a d -Rha-transferase and a critical step in the development of CPA synthesis inhibitors. IMPORTANCE This is the first characterization of a d -rhamnosyltransferase and shows that it is essential in Pseudomonas aeruginosa for the synthesis of the common polysaccharide antigen.