NAD, or nicotinamide adenine dinucleotide, is the basic coenzyme or molecule powering many cellular reactions. In cellular respiration, or the process of producing energy in the cells, this compound combines with two hydrogen atoms. Building up molecules in this way is called anabolism. The compound takes one hydrogen atom and the electron from the other hydrogen atom to turn into high-energy NADH. NADH then travels with the extra electrons and drops them off, releasing energy in a process called catabolism, when a molecule breaks down to produce energy.
Citric Acid Cycle The citric acid cycle starts with the combination of acetyl-CoA (2C) and oxaloactate (4C) to generate citrate (6C). Through a series of eight reactions, two CO2 molecules are released, and oxaloacetate is regenerated. The citric acid cycle does not directly generate much energy. Each turn of the cycle generates one ATP via substrate-level phosphorylation and a GTP intermediate, for a total of two pyruvates per glucose molecule. The value of the citric acid cycle is its ability to generate high-energy electrons that are carried by NADH and FADH2. For each molecule of acetyl-CoA that enters the cycle, three NADH and one FADH2 are produced by two to account for the fact that the cycle turns twice per molecule of glucose. These enzymes then transport the electrons to the electron transport chain on the inner mitochondrial membrane, where more ATP is produced via oxidative phosphorylation. At the end of the citric acid cycle, oxaloacetate is regenerated in anticipation of the next round.