Nicotinamide adenine dinucleotide, known as NADH, is a molecule found in all living cells. It functions as an electron carrier, playing an important role in how cells produce energy. NADH helps transport electrons generated during nutrient breakdown, contributing to the synthesis of adenosine triphosphate (ATP), the primary energy currency of the cell. This molecule is central to cellular metabolism and energy production.
The Electron Carrying Capacity
NADH carries two electrons for energy transfer within the cell. It picks up these electrons, along with a proton, from other molecules during metabolic reactions. This process creates NADH from NAD+.
When NADH gains electrons, it is in a reduced, high-energy state. When it later donates these electrons, it becomes oxidized back to NAD+. This cycle of reduction and oxidation is important for energy transfer, allowing NADH to shuttle electrons for ATP production.
Journey of Electrons
The electrons carried by NADH are destined for the electron transport chain (ETC), a series of protein complexes within the inner mitochondrial membrane. NADH delivers its two electrons to the first complex of this chain, Complex I.
These electrons then pass from one protein complex to the next along the ETC. As electrons move through these complexes, they transition from higher to lower energy states. This movement releases energy, which the complexes harness to perform work.
Energy Harvest
The energy released during electron transfer along the electron transport chain pumps protons from the mitochondrial matrix into the intermembrane space. This creates a concentration gradient of protons across the inner mitochondrial membrane, representing stored potential energy.
Protons then flow back into the mitochondrial matrix through ATP synthase, a specialized enzyme. This movement occurs down their electrochemical gradient, similar to water flowing through a turbine. The energy from this proton flow drives ATP synthase to combine adenosine diphosphate (ADP) with inorganic phosphate, synthesizing ATP. This process, known as chemiosmosis, generates the majority of cellular ATP.
NADH’s Broader Metabolic Role
NADH is generated in several metabolic pathways that break down nutrients to extract energy. For example, glycolysis in the cytoplasm breaks down glucose, producing two NADH molecules per glucose molecule.
Following glycolysis, pyruvate is processed in the mitochondria, leading into the Krebs cycle, also known as the citric acid cycle. The Krebs cycle produces a significant amount of NADH. For each glucose molecule, the Krebs cycle generates six NADH molecules. These NADH molecules then proceed to the electron transport chain, fueling cellular energy production.