New Steps in NAD+ Metabolism

University Of Iowa, Iowa City IA

Investigators

Abstract

Nicotinamide adenine dinucleotide (NAD+) and its reduced and phosphorylated forms, NADH, NAD+P and NADPH, are among the most fundamentally important co-enzymes in all living cells. From the study of NAD+, the nature of cell-free fermentation was demonstrated, functions of co-enzymes in enzyme-mediated transformations were elucidated, vitamins were identified, biosynthetic pathways discovered, and important new mechanisms of gene regulation uncovered. NAD+ is now recognized not only as a co-enzyme for hydride transfer enzymes but also as a consumed substrate of enzymes found in all three domains of life. NAD+-consuming enzymes include ADPribose transferases (ARTs), poly(ADPribose) polymerases (PARPs), sirtuins, and cADPribose synthases, which produce nicotinamide plus and an ADPribosyl product in the course of modifying proteins, forming ADPribose polymers, deacetylating protein lysine residues, and cyclizing ADPribose. Thus, living cells have developed a variety of strategies to salvage NAD+ precursors and to synthesize NAD+ de novo to keep up with the vital demands of hydride transfer enzymes and NAD+ consuming enzymes. Though many of the most famous vitamin, pathway and enzyme discoveries in the NAD+ field are associated with classic works from 1905 to 1958, a series of creative, original and potentially transformative discoveries in the last 5 years have led to recognition of nicotinamide riboside and nicotinic acid riboside as additional NAD+ precursors in yeast that are converted to NAD+ via the nicotinamide riboside kinase pathway and also by nucleoside-splitting coupled to nicotinamide and nicotinic acid salvage. In work that upsets the accepted view of NAD+ metabolism as a stagnant set of long-known reactions, new data indicate that there is a specific transporter for nicotinamide riboside import and there is a novel pathway induced by vitamins and regulated by glucose that breaks down NAD+ to nicotinamide riboside and nicotinic acid riboside. Aims: 1) The specific transporter for nicotinamide riboside will be identified and characterized, making use of a novel and general method to determine the copy number of any yeast protein. 2) A novel pathway that degrades NAD+ to nicotinamide riboside and nicotinic acid riboside will be identified and the mechanism of its regulation by nicotinic acid, nicotinamide and glucose will be determined. 3) A 25 minute family educational experience using baker's yeast to demonstrate basic principals of bioscience will be developed and implemented at the Montshire Museum of Science. This museum enrichment activity will develop expertise among museum staff, provide bioscience knowledge and encourage inquiry to museum visitors of all ages, and encourage science as a career choice to young, mostly rural, museum visitors. Scientific Impact: The function of redox enzymes, sirtuins and ADPribose transfer enzymes depends on import and metabolism of NAD+ precursors, regulation of which is under nutritional and developmental control in all eukaryotes examined. Discovery and characterization of new, regulated steps in yeast NAD+ metabolism will enlighten the process by which this model eukaryote alters its metabolic biochemistry in response to changing nutritional conditions. Broader Impact: Dating back to the characterization of the first enzymes and co-enzymes, yeast has been tremendously important in elucidation of the chemical and biological principles of life. By developing and implementing a museum-based learning experience on yeast as a living organism, this project will enrich the biology education of rural children, while also showing that graduate students of diverse backgrounds and learning styles make critical contributions to advanced knowledge.