Adenosine triphosphate (ATP) serves as the primary energy currency for cells, powering nearly all biological processes. This molecule stores and releases energy through the breaking and forming of its phosphate bonds. Nicotinamide adenine dinucleotide in its reduced form, NADH, is a crucial molecule in cellular energy production, carrying high-energy electrons to convert chemical energy from food into usable ATP.
NADH’s Role in Cellular Respiration
NADH is generated during cellular respiration, a complex series of metabolic reactions that convert nutrients into ATP. It is produced in two major stages: glycolysis and the citric acid cycle, also known as the Krebs cycle. During glycolysis, a molecule of glucose is broken down into two molecules of pyruvate in the cell’s cytoplasm, yielding a net production of two NADH molecules.
Following glycolysis, pyruvate is converted into acetyl-CoA, which then enters the citric acid cycle within the mitochondria. In this cycle, acetyl-CoA is further oxidized through a series of reactions, generating more NADH, along with carbon dioxide and another electron carrier, FADH2. NADH transports these high-energy electrons to the electron transport chain, where the bulk of ATP is synthesized.
The Electron Transport Chain and ATP Synthesis
The electrons carried by NADH are delivered to the electron transport chain (ETC), a series of protein complexes embedded in the inner mitochondrial membrane. NADH donates its electrons to Complex I, initiating their movement through the chain. As these electrons move from one protein complex to the next, energy is released. This energy is then used to pump protons (hydrogen ions, H+) from the mitochondrial matrix into the intermembrane space, creating a concentration gradient.
This buildup of protons in the intermembrane space generates a proton motive force or an electrochemical gradient. The protons, driven by this gradient, then flow back into the mitochondrial matrix through a specialized enzyme called ATP synthase. The movement of protons through ATP synthase causes the enzyme to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate. This process, known as chemiosmosis or oxidative phosphorylation, is responsible for producing the majority of cellular ATP.
Calculating ATP Yield from NADH
Historically, each NADH molecule was stated to produce 3 ATP, but current understanding indicates a more accurate yield of approximately 2.5 ATP. This revised calculation stems from more precise measurements of proton pumping and ATP synthase activity. For every NADH molecule, about 10 protons are pumped across the inner mitochondrial membrane. However, approximately 4 protons are required to flow back through ATP synthase to generate one molecule of ATP. Therefore, 10 protons divided by 4 protons per ATP results in 2.5 ATP per NADH.
The actual ATP yield can vary slightly from this theoretical maximum due to several factors. For instance, the two NADH molecules produced during glycolysis in the cytoplasm cannot directly enter the mitochondria. Their electrons must be transported across the mitochondrial membrane via shuttle systems, such as the malate-aspartate shuttle or the glycerol-3-phosphate shuttle.
The malate-aspartate shuttle transfers electrons in a way that preserves the ATP yield from NADH. In contrast, the glycerol-3-phosphate shuttle transfers electrons to FADH2 within the mitochondria, which yields fewer ATP molecules compared to NADH, effectively reducing the overall ATP production from those initial glycolytic NADH molecules. Other factors contributing to variations include proton leakage across the membrane and energy used for transport, which can slightly reduce the net ATP generated.