Conformational landscape of the mycobacterial inosine 5'-monophosphate dehydrogenase octamerization interface

Inosine 5'-monophosphate dehydrogenase (IMPDH), a key enzyme in bacterial purine metabolism, plays an essential role in the biosynthesis of guanine nucleotides and shows promise as a target for antimicrobial drug development. Despite its significance, the conformational dynamics and substrate-induced structural changes in bacterial IMPDH remain poorly understood, particularly with respect to its octameric assembly. Using cryo-EM, we present full-length structures of IMPDH from Mycobacterium smegmatis (MsmIMPDH) captured in a reaction intermediate state, revealing conformational changes upon substrate binding. The structures feature resolved flexible loops that coordinate the binding of the substrate, the cofactor, and the K⁺ ion. Our structural analysis identifies a novel octamerization interface unique to MsmIMPDH. Additionally, a previously unobserved barrel-like density suggests potential self-interactions within the C-terminal regions, hinting at a regulatory mechanism tied to assembly and function of the enzyme. These data provide insights into substrate-induced conformational dynamics and novel interaction interfaces in MsmIMPDH, potentially informing the development of IMPDH-targeted drugs.