Question

No exchange. Mice that are completely lacking ATP-ADP translocase \(\left(\mathrm{ANT}^{-} / \mathrm{ANT}^{-}\right)\) can be made by the knockout technique. Remarkably, these mice are viable but have the following pathological conditions: (1) high serum levels of lactate, alanine, and succinate; (2) little electron transport; and (3) a six- to eightfold increase in the level of mitochondrial \(\mathrm{H}_{2} \mathrm{O}_{2}\) compared with that in normal mice. Provide a possible biochemical explanation for each of these conditions.

Short answer

The lack of ATP-ADP translocase impairs oxidative phosphorylation, reduces electron transport, increases reliance on glycolysis, and elevates ROS levels.

Step-by-step solution

Understand Role of ATP-ADP Translocase

ATP-ADP translocase is an important membrane protein that facilitates the exchange of ATP and ADP across the mitochondrial inner membrane. This transport is crucial for allowing ATP produced in the mitochondria to be available for cellular processes in the cytosol and ensuring ADP is available in the mitochondria for further ATP synthesis.

Explanation for High Serum Levels of Lactate, Alanine, and Succinate

Without functional ATP-ADP translocase, ATP cannot be efficiently exported from the mitochondria to the cytosol, and ADP cannot enter the mitochondria. As a result, oxidative phosphorylation is impaired, forcing cells to rely more on anaerobic glycolysis for ATP. This switch increases pyruvate conversion to lactate (lactic acid fermentation) and amino acid synthesis pathways, resulting in elevated levels of lactate, alanine, and succinate in serum.

Explanation for Reduced Electron Transport

The absence of ATP-ADP translocase disrupts oxidative phosphorylation, particularly the ATP production pathway. Without recycling ADP back into the mitochondria, the ATP synthase activity is reduced, leading to increased back-pressure on the electron transport chain (ETC) and consequently decreased electron flow through complexes I-IV, causing less electron transport.

Explanation for Increased Mitochondrial Hydrogen Peroxide Levels

Because the electron transport chain activity is impaired and electrons are not efficiently passed along the chain, there is an increased chance of electron leakage. These leaked electrons can react with molecular oxygen, leading to the formation of reactive oxygen species (ROS), such as hydrogen peroxide ( H_2O_2 ), increasing the mitochondrial load of H_2O_2 significantly compared to normal mice.

Key concepts

Oxidative Phosphorylation

Oxidative phosphorylation is a critical energy-producing process that occurs in the mitochondria. It involves a series of chemical reactions that produce ATP, the primary energy currency of cells. This process is powered by the electron transport chain (ETC), which consists of protein complexes I-IV that pass electrons derived from nutrients through a series of redox reactions. These reactions are coupled with the translocation of protons across the mitochondrial inner membrane, creating a proton gradient.

• This gradient is used by ATP synthase, a protein complex that synthesizes ATP from ADP and inorganic phosphate, utilizing the flow of protons back into the mitochondrial matrix.
• The critical role of ATP-ADP translocase is to shuttle ATP out of the mitochondria into the cytoplasm and transport ADP back into the mitochondria to replenish the cycle.

If ATP-ADP translocase is lacking, as in the knockout mice, oxidative phosphorylation becomes inefficient due to the impaired exchange of ATP and ADP. This inefficiency in ATP production shifts the cell's energy reliance to alternative pathways, impacting overall cellular energy metabolism.

Anaerobic Glycolysis

Anaerobic glycolysis is a metabolic pathway that allows cells to derive energy from glucose even when oxygen is scarce or when oxidative phosphorylation is not fully functional. It involves the breakdown of glucose into pyruvate, yielding a small amount of ATP. Without enough oxygen, or in conditions where oxidative processes are impaired, pyruvate is converted into lactate, a process known as lactic acid fermentation. This is a quick way to produce ATP without requiring oxygen.

• In the case of ATP-ADP translocase deficiency, where oxidative phosphorylation is impaired, cells increasingly rely on anaerobic glycolysis for energy.
• This reliance leads to the accumulation of lactate, along with other byproducts like alanine and succinate, contributing to the high serum levels observed in the knockout mice.

Though anaerobic glycolysis is less efficient in terms of ATP yield compared to oxidative phosphorylation, it serves as a vital backup process ensuring that some ATP is available.

Reactive Oxygen Species

Reactive oxygen species (ROS) are highly reactive molecules containing oxygen. They are natural byproducts of normal oxygen metabolism and have important roles in cell signaling and homeostasis. However, excessive ROS can cause oxidative stress, damaging cellular components like DNA, proteins, and lipids.

• In the context of impaired electron transport in mitochondria, as seen with the absence of ATP-ADP translocase, there is an increased probability of electron leakage from the ETC.
• These electrons can react with oxygen to form ROS, including hydrogen peroxide (\(H_2O_2\)).

In the knockout mice, the disrupted electron flow in their mitochondria leads to a significant increase in the production of ROS. This heightened level of hydrogen peroxide can overwhelm the cell's antioxidant defenses, contributing to cellular damage and highlighting the delicate balance needed in mitochondrial oxidative processes.