Measuring O-GlcNAc cleavage by OGA and cell lysates on a peptide microarray

Design of OSMI-4 Analogs Using Scaffold Hopping: Investigating the Importance of the Uridine Mimic in the Binding of OGT Inhibitors

β-N-Acetylglucosamine transferase (OGT) inhibition is considered an important topic in medicinal chemistry. The involvement of O-GlcNAcylation in several important biological pathways is pointing to OGT as a potential therapeutic target. The field of OGT inhibitors drastically changed after the discovery of the 7-quinolone-4-carboxamide scaffold and its optimization to the first nanomolar OGT inhibitor: OSMI-4. While OSMI-4 is still the most potent inhibitor reported to date, its physicochemical properties are limiting its use as a potential drug candidate as well as a biological tool. In this study, we have introduced a simple modification (elongation) of the peptide part of OSMI-4 that limits the unwanted cyclisation during OSMI-4 synthesis while retaining OGT inhibitory potency. Secondly, we have kept this modified peptide unchanged while incorporating new sulfonamide UDP mimics to try to improve binding of newly designed OGT inhibitors in the UDP-binding site. With the use of computational methods, a small library of OSMI-4 derivatives was designed, prepared and evaluated that provided information about the OGT binding pocket and its specificity toward quinolone-4-carboxamides.

Chemical Synthesis and Biological Applications of O-GlcNAcylated Peptides and Proteins

All human cells use O-GlcNAc protein modifications (O-linked N-acetylglucosamine) to rapidly adapt to changing nutrient and stress conditions through signaling, epigenetic, and proteostasis mechanisms. A key challenge for biologists in defining precise roles for specific O-GlcNAc sites is synthetic access to homogenous isoforms of O-GlcNAc proteins, a result of the non-genetically templated, transient, and heterogeneous nature of O-GlcNAc modifications. Toward a solution, this review details the state of the art of two strategies for O-GlcNAc protein modification: advances in "bottom-up" O-GlcNAc peptide synthesis and direct "top-down" installation of O-GlcNAc on full proteins. We also describe key applications of synthetic O-GlcNAc peptide and protein tools as therapeutics, biophysical structure-function studies, biomarkers, and as disease mechanistic probes to advance translational O-GlcNAc biology.

Demystifying O-GlcNAcylation: hints from peptide substrates

O-GlcNAcylation, analogous to phosphorylation, is an essential post-translational modification of proteins at Ser/Thr residues with a single β-N-acetylglucosamine moiety. This dynamic protein modification regulates many fundamental cellular processes and its deregulation has been linked to chronic diseases such as cancer, diabetes and neurodegenerative disorders. Reversible attachment and removal of O-GlcNAc is governed only by O-GlcNAc transferase and O-GlcNAcase, respectively. Peptide substrates, derived from natural O-GlcNAcylation targets, function in the catalytic cores of these two enzymes by maintaining interactions between enzyme and substrate, which makes them ideal models for the study of O-GlcNAcylation and deglycosylation. These peptides provide valuable tools for a deeper understanding of O-GlcNAc processing enzymes. By taking advantage of peptide chemistry, recent progress in the study of activity and regulatory mechanisms of these two enzymes has advanced our understanding of their fundamental specificities as well as their potential as therapeutic targets. Hence, this review summarizes the recent achievements on this modification studied at the peptide level, focusing on enzyme activity, enzyme specificity, direct function, site-specific antibodies and peptide substrate-inspired inhibitors.

Study of cross talk between phosphatases and OGA on a ZO-3-derived peptide

O-GlcNAcylation, like phosphorylation, is a dynamic and rapid posttranslational modification which regulates many cellular processes. Phosphorylation on tyrosine and O-GlcNAcylation on nearby serine or threonine residues may modulate each other. Indeed, by using a microarray with a peptide model system based on the ZO-3 protein, extensive cross talk between O-GlcNAcylation by OGT and phosphorylation by kinases was observed. However, studying the effects of kinases and OGT without the reverse processes catalyzed by phosphatases and O-GlcNAcase (OGA) does not provide a complete picture of the cross talk. The study of the missing part showed that nearby phosphorylation affects the de-O-GlcNAcylation by OGA, but not to the same extent as it affects the O-GlcNAcylation by OGT. Both the phosphorylation and de-phosphorylation processes were only slightly affected by the presence of an O-GlcNAc residue on a nearby serine.

Chemical and Biochemical Strategies To Explore the Substrate Recognition of O-GlcNAc-Cycling Enzymes

The O-linked N-acetylglucosamine (O-GlcNAc) modification is an essential component in cell regulation. A single pair of human enzymes conducts this modification dynamically on a broad variety of proteins: O-GlcNAc transferase (OGT) adds the GlcNAc residue and O-GlcNAcase (OGA) hydrolyzes it. This modification is dysregulated in many diseases, but its exact effect on particular substrates remains unclear. In addition, no apparent sequence motif has been found in the modified proteins, and the factors controlling the substrate specificity of OGT and OGA are largely unknown. In this minireview, we will discuss recent developments in chemical and biochemical methods toward addressing the challenge of OGT and OGA substrate recognition. We hope that the new concepts and knowledge from these studies will promote research in this area to advance understanding of O-GlcNAc regulation in health and disease.