NAD plays a central role in maintaining cellular energy balance, mainly by participating in cellular respiration processes and regulating metabolic pathways, as follows:

Participate in cellular respiration

Glycolysis process: Glycolysis occurring in the cytoplasm is the initial stage of glucose metabolism. During this process, 3-phosphoglyceraldehyde dehydrogenase catalyzes the conversion of 3-phosphoglyceraldehyde to 1,3-diphosphoglycerate, while reducing NAD ⁺ to NADH. This step of the reaction not only allows glycolysis to continue, but also produces the high-energy electron carrier NADH that cells can utilize. The electrons carried by NADH will subsequently enter mitochondria to participate in oxidative phosphorylation and generate ATP for the cells.

Tricarboxylic acid cycle: In the mitochondrial matrix, the tricarboxylic acid cycle is a critical link in cellular respiration. During the cycling process, multiple enzymatic reactions such as isocitrate dehydrogenase and α - ketoglutarate dehydrogenase will reduce NAD ⁺ to NADH, while producing carbon dioxide and some high-energy intermediates. The NADH produced by these reactions will transfer electrons to the electron transport chain, further promoting the synthesis of ATP.

Oxidative phosphorylation: On the inner membrane of mitochondria, NADH transfers its carried electrons to the electron transport chain, which in turn transfers electrons to oxygen through a series of electron transporters. At the same time, it pumps protons from the mitochondrial matrix to the membrane gap, forming a proton gradient. The potential energy of proton gradient drives ATP synthase to synthesize ATP, which is the main way for cells to produce energy. In this way, NADH plays a crucial role in converting the chemical energy generated in glycolysis and the tricarboxylic acid cycle into active chemical energy in ATP during oxidative phosphorylation.

Regulating metabolic pathways

Regulating sugar metabolism: The ratio of NAD ⁺/NADH can affect the activity of key enzymes in the sugar metabolism pathway. For example, when the intracellular NADH level increases, it inhibits the activity of pyruvate dehydrogenase complex, reduces the entry of pyruvate into the tricarboxylic acid cycle, and thus slows down the oxidative degradation of sugars. At the same time, high NADH levels also promote the activity of certain enzymes in the gluconeogenesis pathway, allowing cells to synthesize glucose using non sugar substances to maintain stable blood glucose levels and ensure that cells have sufficient energy sources.

Regulating fat metabolism: NAD also plays an important role in fat metabolism. β - oxidation is the main pathway for the breakdown metabolism of fatty acids, during which fatty acids are gradually broken down into acetyl CoA, while producing NADH and FADH ₂. NADH can generate ATP through the electron transport chain, providing energy to cells. In addition, the NAD ⁺/NADH ratio can also affect the balance between fat synthesis and breakdown. When cellular energy is sufficient, high NAD ⁺ levels promote the activity of fatty acid synthase, converting excess acetyl CoA into fatty acids for storage; When the cellular energy is insufficient, low NAD ⁺ levels activate hormone sensitive lipase, promote fat breakdown, release fatty acids for beta oxidation, and provide energy to the cell.

Regulating mitochondrial function: NAD ⁺ can also regulate mitochondrial function by activating Sirtuins protein. SIRT3 is a deacetylase in mitochondria that deacetylates enzymes involved in mitochondrial energy metabolism, thereby enhancing their activity and improving mitochondrial respiratory function and ATP production efficiency. In addition, SIRT3 can regulate antioxidant enzymes in mitochondria, reduce oxidative stress damage to mitochondria, maintain normal structure and function of mitochondria, and ensure stable cellular energy metabolism.