Dissecting the Mechanisms Underlying Substrate Recognition and Functional Regulation of O-GlcNAc Cycling Enzymes.

Protein O-linked β-N-acetylglucosamine (O-GlcNAc) modification, known as O-GlcNAcylation, is an essential post-translational modification (PTM) that plays critical roles in regulating various cellular processes, ranging from transcription and signal transduction to protein degradation. O-GlcNAcylation levels are dynamically regulated by a single pair of human enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Dysregulation of O-GlcNAcylation has been implicated in many diseases, including cancer, diabetes, neurodegeneration, and cardiovascular disorders. In the past decade, remarkable progress has been achieved regarding the structures of OGT and OGA proteins, as well as a series of innovative chemical and engineered tools that inhibit or induce the activities of these enzymes. While initial studies mainly focused on the catalytic domains of these enzymes, recent research has begun to uncover the structural and functional roles of non-catalytic regions. Notably, domains such as OGT's tetratricopeptide repeat (TPR) and intervening domain (Int-D), as well as OGA's stalk domain and pseudo histone acetyltransferase (pHAT) domain, have emerged as critical contributors to enzyme functions. This Account discusses recent progress in studying these essential enzymes, especially highlighting their unique structural features and intrinsic flexibility as potential mechanisms underlying their substrate recognition and functional regulation. New perspectives and research directions are also discussed. Such information is expected to facilitate the rational design of novel modulators of OGT and OGA to enable more specific functional control and potential treatment of disease.