Every protein in your cell has the potential to be fined-tuned by chemical modifications. Phosphorylation of serine, threonine, and tyrosine gets most of the attention since it has been studied for decades, but another modification happens just as frequently—one that we as a field are only just now beginning to functionally decode.
O-GlcNAc transferase, or OGT, attaches a single sugar molecule to hundreds of proteins in a given human cell. In doing so, OGT controls how those proteins behave…in a glucose-responsive manner! That means that OGT becomes overactive in metabolic diseases that have too much (or too little) sguar, including cancer, and diabetes (hyperactive OGT) and neurodegeneration (hypoactive OGT). These functional changes in OGT activity have consequences for the hundreds of pathways that its substrate proteins make up. That makes it one of the most compelling enzyme targets in modern chemical biology, in our opinion in the Fehl Lab!
What OGT Actually Does
OGT catalyzes the addition of N-acetylglucosamine (GlcNAc) to serine and threonine residues on proteins. This modification—O-GlcNAcylation—is reversible. A second enzyme, O-GlcNAcase (OGA), removes the sugar. Together, they function like a molecular switch, cycling GlcNAc on and off proteins in response to changing cellular conditions, such as high vs. low glucose.
What makes OGT unusual is its substrate range. Most enzymes work within a narrow, well-defined set of targets. OGT modifies more than a thousand proteins—transcription factors, metabolic enzymes, cytoskeletal proteins, cell cycle components. It is not a specialist. It is a broad regulator that reads the metabolic state of the cell and translates that information into protein-level changes.
By the numbers, OGT is one, if not THE, most promiscuous enzymes in humans. I can’t think of another that has up to 10,000 substrates (according to the fantastic O-GlcNAc Databank at https://www.oglcnac.mcw.edu/overview/).
The sugar donor OGT uses is UDP-GlcNAc, a metabolite produced through the hexosamine biosynthesis pathway. That pathway sits at the intersection of glucose, glutamine, fatty acid, and nucleotide metabolism. When nutrients are abundant, UDP-GlcNAc levels rise, OGT becomes more active, and more proteins get glycosylated. When nutrients are scarce, the opposite happens. In a very real sense, OGT is a nutrient sensor.
The Structural Basis of OGT Function
OGT is encoded by a single gene in humans, but its architecture is anything but simple. The enzyme carries a long series of tetratricopeptide repeat (TPR) units at its N-terminus and a catalytic domain at its C-terminus. Those TPR repeats are not just scaffolding—they mediate protein-protein interactions, helping OGT find and engage its substrates. Different substrates interact with different regions of the TPR array, which partly explains how one enzyme can modify so many different proteins while still maintaining some degree of selectivity.
The catalytic domain houses the active site where UDP-GlcNAc binds and the sugar transfer reaction takes place. Structural studies have shown this site to be well-defined, which matters for drug discovery. A tractable active site is a meaningful starting point.
There is also a short isoform called sOGT that lacks most of the TPR repeats and localizes to the mitochondria, while the full-length form handles cytoplasmic and nuclear substrates. This compartmentalization adds another layer of regulation that researchers are still working to fully characterize.
Why OGT Dysregulation Matters in Disease
Cancer
In many cancers, O-GlcNAcylation levels are elevated—and not by coincidence. Tumor cells consume glucose at high rates, which feeds the hexosamine pathway and drives up UDP-GlcNAc. Elevated OGT activity then stabilizes oncoproteins, promotes proliferation, and helps tumor cells evade apoptosis. Several transcription factors central to cancer biology, including MYC and p53, are O-GlcNAcylated, and the modification alters how they function. Reducing OGT activity in cancer cell models generally slows growth, which has made the enzyme an attractive target.
Diabetes and Metabolic Disease
The hexosamine pathway has long been implicated in insulin resistance. When glucose flux through the pathway is chronically elevated—as it is in type 2 diabetes—OGT-driven glycosylation of insulin signaling proteins disrupts normal signaling. IRS-1, a key node in that pathway, is one example of a protein whose function is altered by O-GlcNAcylation. The relationship is complex and context-dependent, but the key point is that OGT sits at the interface of nutrient sensing and metabolic regulation, and that interface breaks down in metabolic disease.
Neurodegeneration
The connection between O-GlcNAcylation and neurodegeneration is particularly striking because of tau. Tau is the protein that aggregates into neurofibrillary tangles in Alzheimer’s disease, and O-GlcNAcylation and phosphorylation compete for the same sites on it. When O-GlcNAcylation decreases, tau becomes hyperphosphorylated and more prone to aggregation. This has led to the hypothesis that boosting OGT activity—or inhibiting OGA to prevent sugar removal—could be protective in Alzheimer’s. The picture is nuanced, but it illustrates how centrally OGT sits within a disease-relevant regulatory network. Other proteins, including alpha-syncuclein in Parkinson’s disease and TDP43 in ALS are also O-GlcNAcylated, expanding the roles of this key sugar in neurodegeneration. This relationship becomes more complicated, because here we actually want to INCREASE O-GlcNAc (vs. decrease O-GlcNAc in hyperglycemic diseases), so some tissue specificity will be needed when developing OGT inhibitors. The likely way is to ensure these drugs do NOT enter the brain 🙂
The Case for an O-GlcNAc Transferase Inhibitor
Given OGT’s role in cancer and metabolic disease, there has been sustained interest in developing a selective O-GlcNAc transferase inhibitor. The logic in cancer is clear: if tumor cells depend on elevated O-GlcNAcylation to sustain their proliferative and survival programs, blocking OGT should disrupt those programs.
Several small molecule inhibitors have been developed and characterized. OSMI-1 and its more potent successors—OSMI-2, OSMI-3, and OSMI-4—are among the most widely used research tools. These compounds bind the active site of OGT and compete with UDP-GlcNAc. They have been valuable for probing OGT function in cells, but they remain research tools rather than clinical candidates.
The challenge in developing a clinical O-GlcNAc transferase inhibitor comes down to selectivity and tolerability. Because OGT modifies so many proteins, broad inhibition could have wide-ranging effects on normal cell function. The goal is to find compounds potent and selective enough to be therapeutically useful—or to identify disease contexts where OGT inhibition is particularly well-tolerated or particularly effective.
One emerging strategy is to target OGT indirectly by disrupting its interactions with specific adapter proteins that direct it toward disease-relevant substrates. This would allow more targeted interference without globally suppressing O-GlcNAcylation across the proteome. One of our projects in the Fehl Lab is working on just this goal!
Chemical Biology Approaches to Studying OGT
Drugging OGT requires first understanding it—and that requires tools capable of detecting and tracking O-GlcNAcylation across the proteome. Chemical biology has produced several.
Metabolic labeling with unnatural sugars is one approach. Cells take up modified GlcNAc analogs bearing bioorthogonal handles, which OGT incorporates into its substrates. Researchers can then attach fluorescent dyes or affinity tags to those handles, enabling visualization or enrichment of O-GlcNAcylated proteins. This approach has been used to map the O-GlcNAc proteome and to study how glycosylation shifts in response to metabolic perturbations. Our Photo-GlcNAc probes, which are light-controlled, spatiotemporal tools to activeate OGT, can help us understand these dynamics.
Chemoenzymatic methods offer another route. Engineered versions of OGA can transfer modified sugars onto O-GlcNAcylated proteins, enabling selective labeling of the modified proteome. We apply chemoenzymatic labeling to find new roles of OGT and O-GlcNAc proteins in a variety of tissues, cell types, and conditions.
Machine learning is increasingly useful for predicting OGT substrates and identifying the sequence and structural features that make a site a good candidate for glycosylation. Combining computational predictions with experimental validation accelerates the work of mapping how OGT activity changes in disease states. We are increasingly using ML approaches to understand our datasets!
This is precisely the kind of work happening at the Fehl Lab, where chemical biology strategies and machine learning come together to study carbohydrate-linked proteins and their role in cellular metabolism.
Open Questions and Where the Field Is Heading
OGT is not a simple enzyme with a simple story. It sits at the convergence of metabolism, signaling, and gene regulation, and its dysregulation threads through some of the most prevalent diseases of our time. Developing the tools to study it precisely—and eventually the molecules to modulate it therapeutically—is one of the more important problems in chemical biology right now. If you want to follow that work, the Fehl Lab is a good place to start.
Despite real progress, many fundamental questions about OGT remain open. How does the enzyme select among its thousands of substrates? What governs the stoichiometry of modification at any given site? How does O-GlcNAcylation interact with phosphorylation, ubiquitination, and other modifications at a systems level? Can we safely inhibit OGT in the periphery, avoiding CNS effects?
Answering these questions will require better tools, better models, and tighter integration between chemistry and biology. More potent and selective O-GlcNAc transferase inhibitors will be part of that effort—both as research tools and as potential therapeutics.
These are our favorite things to think about and work on in the Fehl Lab, so stay tuned for our upcoming publications!
