19.6: Electron Transport Chain: Complex I and II

The mitochondrial electron transport chain (ETC) is the main energy generation system in the eukaryotic cells. However, mitochondria also produce cytotoxic reactive oxygen species (ROS) due to the large electron flow during oxidative phosphorylation. While Complex I is one of the primary sources of superoxide radicals, ROS production by Complex II is uncommon and may only be observed in cancer cells with mutated complexes.

ROS generation is regulated and maintained at moderate levels necessary for normal cellular signaling processes in a healthy cell. However, cancer cells possess a higher antioxidant capacity, enabling ROS maintenance at a level that triggers pro-tumorigenic pathways without causing cancer cell death. Thus, cancer cells have an altered redox environment, with a high ROS production rate counterbalancing a high ROS scavenging rate. This unique feature of cancer cells makes them more sensitive to alteration in ROS levels than normal cells. The inhibitory compounds that hamper regular electron flow in the ETC can also trigger the mitochondrial cell death pathway. For instance, ETC inhibitors, such as metformin, resveratrol, and fenretinide, disrupt the normal functioning of the respiratory complexes. This induces elevated ROS production to a level that exceeds the antioxidant capacity of the cancer cells, resulting in their death.

Complex I is inhibited by metformin, an AMP-activated protein kinase that blocks mitochondrial respiratory functions and induces programmed cell death in several types of cancer cells, including pancreatic and breast cancer cells. Mutations in complex II, although rare, can lead to tumors of the carotid body-sensory organ of the peripheral nervous system.

Besides cancer, abnormal activity or deficiency in electron transport chain complexes has been linked to human neurodegenerative diseases. For example, in Parkinson's disease, there is a lack of function of complex I. Similarly, defects in complex II have been linked to Huntington's disease.

The mitochondrial inner membrane constitutes a series of five multi-subunit enzyme complexes responsible for transport of electrons from high-energy carriers, NADH, and FADH₂, in an energetically downhill sequence, to a low-energy electron acceptor- oxygen.

The first complex-NADH-Q oxidoreductase, is the largest enzyme complex in the series, transferring electrons from NADH to coenzyme Q.

This L-shaped complex includes 45 different subunits, of which the mitochondrial genome encodes seven. Its major catalytic components are the NADH-binding site, the primary electron acceptor- FMN, and multiple iron-sulfur clusters.

The second complex is part of both the citric acid cycle and the electron transport chain. It transports electrons from succinate to FADH₂ and finally to coenzyme Q via iron-sulphur clusters. This complex is therefore known as the succinate-Q reductase.

It is a nuclear-encoded tetramer with two hydrophilic subunits - A and B. Subunit-A is a flavoprotein with FAD cofactor and a succinate binding site. Subunit-B is an iron-sulfur protein with three iron-sulfur clusters. The other two subunits - C and D are hydrophobic integral-membrane proteins that contain a Q-binding site.

• Electron Transport Chain (ETC) - This is the main energy generation system of eukaryotic cells.
• Reactive Oxygen Species (ROS) - Cytotoxic substances produced by mitochondria during oxidative phosphorylation.
• Complex I and II - These are parts of the ETC, they facilitate energy production and ROS creation.
• Antioxidant Capacity - The ability of a cell to maintain ROS levels, which is higher in cancer cells.
• ETC Inhibitors - Substances that disrupt the ETC's normal functions, prompting mitochondrial cell death.

• Define ETC – Explain what it is (e.g., electron transport chain).
• Contrast Complex I and II – Explain key differences (e.g., ROS production).
• Explore ROS Maintenance in Normal and Cancer Cells – Enhanced antioxidant capacity in cancer cells (e.g., higher ROS scavenging rate).
• Explain Role of ETC Inhibitors – They inhibit normal electron flow in ETC, inducing tunnel death in cancer cells.
• Apply the Role of ETC in other Diseases – Relate ETC dysfunction to human neurodegenerative diseases.

• What is the Electron Transport Chain and how does it function?
• What are the differences between Complex I and II in the ETC?
• How does a cancer cell's antioxidant capacity differ from that of a healthy cell?

• Students – Understanding ETC helps grasp complex biological concepts related to energy production.
• Educators – Provides a clear framework for understanding and teaching cellular energy generation and proton gradient.
• Researchers – ETC's role in energy generation is vital for advanced life sciences research, particularly in cancer and neurodegenerative diseases.
• Science Enthusiasts – Offers insights into the complex workings of cellular respiration and energy production.