ABSTRACT The “magnificent seven” human sirtuins play critical roles in various cellular processes including DNA repair, gene silencing, mitochondrial biogenesis, insulin secretion and apoptosis. They regulate a wide array of protein and enzyme targets through their NAD+-dependent deacetylase activities. Sirtuins are also thought to mediate the beneficial effects of low calorie intake to extend longevity in diverse organisms from yeast to mammals. Small molecules mimicking calorie restriction to stimulate sirtuin activity are attractive therapeutics against age-related disorders such as cardiovascular diseases, diabetes and neurodegenerative diseases. Little is known about one of the mitochondrial sirtuins, SIRT5. SIRT5 has emerged as a critical player in maintaining cardiac health and neuronal viability upon stress, and functions as tumor suppressor in a context specific manner. Much has been debated about whether SIRT5 has evolved away from being a deacetylase because of its weak catalytic activity, especially in the in vitro testing. We have, for the first time, identified a SIRT5-selective allosteric activator, nicotinamide riboside (NR). It can increase SIRT5 deacetylation efficiency with different synthetic peptide substrates as well as its endogenous cognate substrate. However, the deacylase activity of SIRT5 is insensitive to NR activation. Mechanism of activation will be further explored in three specific aims. In aim 1, our effort will be directed at the elucidation of structural determinants required for the differential NR sensitivities and the identification of allosteric binding site. In aim 2, target engagement and activation of SIRT5 in response to activator treatment in the cellular context will be investigated. In aim 3, several series of SIRT5 activators will be synthesized based on our initial screening, structure-activity relationship analysis and docking studies using a combination of chemical and enzymatic strategies. The knowledge gained in the proposed study will not only clarify our understanding of the biological functions of SIRT5, but also lead to new therapeutics for metabolic disorders and age-related diseases.

PROJECT NARRATIVE This proposal aims to study small molecules that are capable of stimulating the activity of SIRT5 that regulates mitochondrial function by removing acyl modifications from a broad array of biological targets. Given its numerous roles in many different pathologies, a great deal of interest exists in targeting SIRT5 therapeutically by small molecules. In addition, SIRT5-specific modulators could provide valuable tools in deciphering in vivo functions and regulation of SIRT5.