# Conformational Dynamics of the Active Site Loop in Dihydroorotase Highlighting the Limitations of Loop-In Structures for Inhibitor Docking

## TL;DR

The study shows that the active site loop in Dihydroorotase (DHOase) can adopt different conformations, which affects inhibitor binding and drug design.

## Contribution

The study identifies new inhibitors of DHOase and reveals that loop conformation is not always ligand-dependent, impacting inhibitor docking.

## Key findings

- Loop conformation in DHOase varies across types and is not always influenced by ligand binding.
- S. cerevisiae DHOase consistently adopts the loop-in state, limiting inhibitor docking.
- Docking experiments confirmed that the loop-in state prevents effective inhibitor binding.

## Abstract

Dihydroorotase (DHOase) catalyzes the reversible cyclization of N-carbamoyl-L-aspartate to dihydroorotate, a key step in de novo pyrimidine biosynthesis. A flexible active site loop in DHOase undergoes conformational switching between loop-in and loop-out states, influencing substrate binding, catalysis, and inhibitor recognition. In this study, we identified 5-fluoroorotate (5-FOA) and myricetin as inhibitors of Saccharomyces cerevisiae DHOase and systematically analyzed 97 crystal structures and AlphaFold 3.0 models of DHOases from 16 species representing types I, II, and III. Our results demonstrate that loop conformation is not universally ligand-dependent and varies markedly across DHOase types, with type II enzymes showing the greatest flexibility. Notably, S. cerevisiae DHOase consistently adopted the loop-in state, even with non-substrate ligands, restricting accessibility for docking-based inhibitor screening. Docking experiments with 5-FOA and myricetin confirmed that the loop-in conformation prevented productive active-site docking. These findings highlight the importance of selecting appropriate loop conformations for structure-based drug design and underscore the need to account for loop dynamics in inhibitor screening.