The Loop-In Binding Mode of Dihydroorotase: Implications for Ligand Binding and Therapeutic Targeting

Dihydroorotase (DHOase; EC 3.5.2.3) is a zinc-dependent metalloenzyme that plays a key role in the de novo pyrimidine biosynthesis pathway, catalyzing the reversible cyclization of N-carbamoyl aspartate to dihydroorotate. This reaction is essential for the production of uridine monophosphate, the precursor of all pyrimidine nucleotides required for DNA and RNA synthesis. Despite its conserved enzymatic function, DHOase exhibits significant structural diversity across species, particularly in its oligomeric states, gene fusion patterns, and active site architecture. A crucial structural feature of DHOase is its flexible active site loop, which undergoes dynamic conformational changes during catalysis. Previously, the loop-in conformation was associated with substrate binding, whereas the loop-out conformation was linked to product release and non-substrate ligand binding. However, recent crystallographic studies challenge this paradigm, revealing that certain non-substrate ligands and inhibitors, including malate, 5-fluoroorotate, plumbagin, 5-aminouracil, and 5-fluorouracil, interact with DHOase via a loop-in binding mechanism rather than the previously assumed loop-out mode. These findings necessitate a reassessment of the catalytic mechanism of DHOase and underscore the active site loop as a potential target for drug development. This review revisits the structural and biochemical mechanisms of DHOase, with a focus on recent crystallographic insights that redefine the loop-in binding mode for ligand interaction. By leveraging the unique conformational dynamics of the active site loop, novel inhibitors may be developed to selectively target pyrimidine biosynthesis in cancer cells and microbial pathogens. These insights emphasize the crucial role of structural biology in therapeutic design and highlight DHOase as a promising drug target.