Abstract
<title>Abstract</title> <p>Sterile alpha and Toll/interleukin-1 receptor motif-containing protein 1 (SARM1) is an oligomeric NAD⁺-consuming enzyme that drives programmed axonal degeneration. SARM1 activity is controlled by opposing metabolites: NAD⁺ stabilizes an inactive conformation, whereas nicotinamide mononucleotide (NMN) promotes activation. Orthosteric base-exchange inhibitors (BEIs) were developed to suppress SARM1 activity, but several studies have reported paradoxical activation at subinhibitory concentrations. Here, we integrate biochemical kinetics, binding measurements, structural analysis, and cellular assays to evaluate a two-state Monod-Wyman-Changeux framework for SARM1 regulation. The data support a model in which SARM1 favors an inactive T state without activating ligand, NMN stabilizes an active R state, and NAD⁺ both allosterically favors the inactive state and serves as substrate. The transition occurs concertedly through a minimal dimeric cooperative unit that generates two catalytic sites. This framework explains NAD⁺-dependent substrate inhibition, state-dependent cooperativity, and low-dose activation by orthosteric BEIs, with implications for targeting oligomeric enzymes with concerted transitions.</p>