Researchers Reveal How Gatekeeper Enzyme UGCG Controls Glycosphingolipid Synthesis

Glycosphingolipids (GSLs) are sugar-containing membrane lipids that help organize cell surfaces and regulate signaling, immune recognition, differentiation and neuronal function.

Human cells produce several hundred GSL species through stepwise modification of their sugar groups, and this diversity begins with a single committed reaction, in which uridine diphosphate glucose (UDP-glucose) ceramide glucosyltransferase (UGCG) transfers glucose from UDP-glucose to ceramide, producing glucosylceramide, which serves as the starting point for most complex GSLs.

By controlling how much ceramide enters this pathway, UGCG influences the scale and composition of the downstream GSL network. However, how human UGCG performs catalysis and responds to regulatory molecules has not been well understood.

Now, a team led by H. Eric Xu from the Shanghai Institute of Materia Medica of the Chinese Academy of Sciences, along with collaborators from Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, and Lingang Laboratory, has revealed the molecular basis of human UGCG function by defining how UGCG recognizes its two substrates, catalyzes glucose transfer, and is regulated by clinically used and clinical-stage inhibitors.

The study was published in Nature.

Using single-particle cryo-electron microscopy, the researchers determined the structures of full-length human UGCG in eight functional states at 2.9–3.4 Å resolution. These structures captured ligand-free, nucleotide-bound, lipid-engaged, and inhibitor-bound states.

They showed a compact three-pass transmembrane architecture that anchors a GT-A catalytic core at the membrane interface. This arrangement creates a bipartite active site for two chemically different substrates: UDP-glucose enters from the cytosolic side, while ceramide is recruited laterally from the membrane through a hydrophobic tunnel. Their paths converge at a shared reaction center.

In addition, the researchers uncovered a new catalytic mechanism based on these structures. Classical GT-A glycosyltransferases commonly use divalent metal ions to stabilize the negatively charged diphosphate of nucleotide-sugar substrates. UGCG instead creates a positively charged protein environment around UDP-glucose, and Arg272 and Arg275 directly stabilize its diphosphate group.

Biochemical assays showed similar catalytic turnover with or without Mg2+ or Mn2+ under the tested conditions, supporting metal-independent catalysis. Asp236 lies between the donor and acceptor sites and is required for efficient catalysis. Structural and functional analyses supported an inverting glucose-transfer mechanism that aligns UDP-glucose and ceramide for reaction.

Moreover, the researchers found that substrate recognition is coupled with local structural changes. A C-terminal β-hairpin responds to donor binding and helps shape the UDP-glucose pocket. Tyr196 changes orientation when the membrane-facing ceramide tunnel becomes engaged. Evolutionary analysis showed that Tyr196 is a primate-associated feature, while many non-primate mammals carry serine at the same position. Replacing human Tyr196 with serine modestly increases ceramide engagement and catalytic turnover, suggesting an additional regulatory layer.

The inhibitor-bound structures explained how drugs regulate the GSL gatekeeper. Eliglustat and ibiglustat enter the membrane-embedded ceramide tunnel and block lipid-substrate access. In contrast, miglustat mainly occupies the hydrophilic UDP-glucose-binding pocket. Functional assays supported these distinct competition modes. Thus, chemically different compounds can inhibit the same enzyme by targeting different parts of its substrate-recognition system.

The study establishes a molecular framework for how UGCG controls the committed entry step of human GSL biosynthesis. The newly defined catalytic center, lipid-entry tunnel, and inhibitor-binding sites provide a basis for developing UGCG modulators with improved potency, selectivity, and precision.