Question

What is meant by the ATP currency exchange ratio? Why does the oxidation of mitochondrial FADH \(_{2}\) generate one less ATP than oxidation of mitochondrial NADH?

Short answer

Question: Explain why the oxidation of mitochondrial FADH₂ generates one less ATP than NADH. Answer: The oxidation of mitochondrial FADH₂ generates one less ATP than NADH because FADH₂ enters the electron transport chain (ETC) at Complex II, bypassing Complex I. This causes fewer protons to be pumped into the intermembrane space, resulting in a smaller proton gradient. Consequently, fewer protons are available for ATP synthesis through ATP synthase, making FADH₂ generate one less ATP than NADH during oxidative phosphorylation.

Step-by-step solution

Defining ATP currency exchange ratio

The ATP currency exchange ratio refers to the number of ATP (adenosine triphosphate) molecules generated or consumed during a biological reaction. This ratio reflects the energy changes during the reaction under study, whether it is energy generation, storage, or consumption. ATP is the primary molecule responsible for energy storage and transport within cells.

Understanding oxidation of mitochondrial FADH₂ and NADH

NADH (Nicotinamide adenine dinucleotide) and FADH₂ (Flavin adenine dinucleotide) are two important molecules that carry electrons during cellular respiration. The main goal during cellular respiration is to generate ATP, the cell's energy currency, from the breakdown of glucose or other substrates. Both NADH and FADH₂ transfer their electrons to the electron transport chain (ETC) within the mitochondria, and this electron flow transports protons across the inner mitochondrial membrane, creating a proton gradient. Proton gradients are then utilized by ATP synthase, directly involved in the production of ATP.

Comparing the oxidation of mitochondrial FADH₂ and NADH

The key difference between FADH₂ and NADH is that NADH enters the ETC at a higher energy level (Complex I) than FADH₂ (Complex II). This means that FADH₂ contributes fewer protons to the proton gradient compared to NADH. Thus, the oxidation of mitochondrial FADH₂ generates one less ATP than NADH. While the oxidation of NADH generates approximately 2.5 to 3 ATP per molecule, the oxidation of FADH₂ generates around 1.5 to 2 ATP.

Why FADH₂ generates one less ATP

Due to FADH₂ entering the ETC at Complex II, it bypasses Complex I (also known as NADH dehydrogenase). Thus, FADH₂ skips the first site of proton pumping out of the mitochondrial matrix, resulting in fewer protons being pumped into the intermembrane space. Because of this, the proton gradient generated from the oxidation of FADH₂ is smaller, and fewer protons are available for ATP synthesis through ATP synthase. This is the reason why FADH₂ generates one less ATP than NADH during oxidative phosphorylation.