Nonallosteric Sirtuin Enzyme Activation: Characterization of Hit Compounds

Mammalian sirtuins (SIRT1-SIRT7) are a family of nicotinamide adenine dinucleotide (NAD+)-dependent protein deacylases that play critical roles in lifespan and age-related diseases. The physiological importance of sirtuins has stimulated intense interest in designing sirtuin-activating compounds. However, except for allosteric activators of SIRT1-catalyzed reactions that are limited to particular substrates, a general framework for the design of sirtuin-activating compounds has been lacking. Recently, we introduced a general mode of sirtuin activation that is distinct from the known modes of enzyme activation, establishing biophysical properties of small molecule modulators that can, in principle, result in enzyme activation for various sirtuins and substrates. Here, we characterize small molecules reported in the literature to activate the SIRT3 enzyme using a variety of computational, biochemical and biophysical techniques including protein-ligand docking, molecular dynamics simulation, nonlinear reaction dynamics simulation, kinetic assays and thermodynamic assays with multiple substrates and protocols. In particular, we identify the mechanism of action of the compound honokiol on the human SIRT3 enzyme, modeling its effect on active site conformational degrees of freedom and demonstrating how it nonallosterically activates the human SIRT3 enzyme under physiologically relevant conditions. We show that honokiol constitutes a hit compound for the design of a new generation of nonallosteric activators that can activate SIRT3 through the proposed mechanism-based mode of activation.