"The realization that c works in this manner will guide our thinking about sleep, oxidative stress and metabolism, and eventually may serve as a step toward finding better treatments for metabolic diseases," said the senior author.
Nocturnin is part of the circadian clock that alters the metabolism and behavior of living organisms to match the body's needs at different times of the day. For example, Nocturnin levels fluctuate throughout the day, dramatically peaking when the body first awakens. Nocturnin is also a critical regulator of metabolism; compared to regular mice, mice lacking the enzyme make less insulin, are protected from fatty liver disease and are less susceptible to weight gain.
The precise function of Nocturnin inside cells has remained unclear, however. For many years, the enzyme was thought to turn on and off cellular metabolism by degrading certain cellular messages made of ribonucleic acid, or mRNAs. Last year, however, three groups of researchers discovered that Nocturnin is incapable of degrading RNAs.
The researchers discovered that Nocturnin plays a far more direct role in metabolism than previously appreciated. Rather than degrading mRNAs, the enzyme regulates specific metabolites that help energy production and protect cells from damage. The study determined that Nocturnin is located in the cell's energy-producing structures, the mitochondria, suggesting that this is where the enzyme performs its function.
The team found that Nocturnin removes a phosphate group from two molecules important in metabolism, called NADP+ and NADPH. These molecules allow the cell to modulate the levels of reactive oxygen species, which function both as harmful agents causing damage and as signaling molecules controlling metabolism and fat storage. The researchers conclude that Nocturnin is the first known enzyme to perform this reaction on NADP+ and NADPH inside mitochondria.
Removing phosphate groups from NADP+ and NADPH produces two different but equally important molecules, NAD+ and NADH, which are essential for the function of metabolic enzymes -- the molecular machines that produce energy by breaking down energy-rich biomolecules such as glucose.
Nocturnin upregulation when an animal first awakens might therefore kick the body's energy production into high gear by providing more NAD+ and NADH. "It is tempting to propose that one physiologic function of Nocturnin could be to maximize available NAD+ and NADH for energy generation in a search for food, using the elevated blood sugar that animals have at the time of awakening," the senior author said.
The authors also deciphered the crystal structure of human Nocturnin bound to NADPH, showing at the atomic level how the reaction mediated by Nocturnin occurs. NADPH fits perfectly into Nocturnin's active site so that the enzyme can easily remove the molecule's phosphate group.
Finally, the researchers determined that the fruit fly version of Nocturnin, known as Curled, is also unable to cleave RNA. Instead, Curled uses the same mechanism as human Nocturnin and targets NADP+ and NADPH. The Curled gene was first described over 100 years ago, its biochemical mechanism was a mystery until now.."