Main ETC protein complexes
The mitochondrial electron transport chain comprises four high-molecular-weight enzyme assemblies embedded in the lipid bilayer of the membrane. Their function is to transfer electrons step-by-step and (in most cases) pump protons into the intermembrane space.
- Complex I (NADH dehydrogenase). Contains flavin mononucleotide (FMN) and iron-sulfur clusters (FeS). The active part of FMN is the isoalloxazine ring linked to ribitol phosphate. In the oxidized state, the nitrogen atoms in this ring form a quinoid structure, and upon reduction to $FMNH_2$, they accept hydrogen. The enzyme oxidizes NADH to NAD⁺ and carries out the first coupling site step by pumping protons ($nH^+$) out of the matrix.
- Complex II (succinate dehydrogenase). Contains FAD and FeS proteins. Unlike other FAD-dependent enzymes floating in the matrix, this complex is rigidly fixed in the inner membrane. It oxidizes FAD-dependent substrates (e.g., converts succinate to fumarate). Important feature: Complex II does not pump protons.
- Complex III ($QH_2$ dehydrogenase). Includes cytochromes $b_1, b_2, c_1$ and FeS clusters. Accepts electrons from ubiquinol. The mechanism is based on the reversible change in the oxidation state of iron in the heme (from $Fe^{3+}$ to $Fe^{2+}$). This is the second coupling site step.
- Complex IV (cytochrome c oxidase). Consists of cytochromes $a, a_3$ and copper ions. Through coupled changes in the oxidation states of iron in heme and copper (from $Cu^{2+}$ to $Cu^{1+}$), the enzyme performs the final transfer of electrons to oxygen. It acts as a proton pump (third coupling site step).
Mobile electron carriers
Low-molecular-weight carriers shuttle between the fixed complexes, linking the entire system into a single chain.
- Coenzyme Q (ubiquinone). A non-protein lipid carrier. Structurally, it consists of a benzoquinone ring with a long 10-unit isoprenoid side chain. In its oxidized form (ubiquinone), the ring contains two keto groups. Upon accepting electrons from Complexes I and II, the keto groups are reduced to hydroxyl groups, the ring becomes aromatic, and reduced ubiquinol ($QH_2$) is formed.
- Cytochrome c. A mobile hydrophilic protein localized on the outer surface of the inner membrane. Its sole task is to intercept electrons from the third complex and deliver them to the fourth.
Electron entry pathways into the respiratory chain
There are two main routes by which hydrogen (and its electrons) from primary substrates enters the ETC. The route depends on the dehydrogenase coenzyme.
- Pathway 1: Via NAD-dependent dehydrogenases (long pathway). The enzymes are localized in the mitochondrial matrix. Primary donors include pyruvate, isocitrate, $\alpha$-ketoglutarate, malate (oxidized to oxaloacetate), and glutamate. Hydrogen follows the route: Substrate $\rightarrow$ NAD⁺ $\rightarrow$ NADH formed $\rightarrow$ Complex I $\rightarrow$ Coenzyme Q.
- Pathway 2: Via FAD-dependent dehydrogenases (short pathway). The enzymes are located in the matrix or on the membrane. Substrates include succinate, $\alpha$-glycerophosphate, and acyl-CoA (converted to enoyl-CoA via ETF). Route: Substrate $\rightarrow$ FAD $\rightarrow$ Coenzyme Q. This pathway is shorter and completely bypasses the first protein complex.
Terminal stage and general scheme
Regardless of how electrons enter the chain, they ultimately converge on ubiquinone and then follow a strict, unified route to the terminal acceptor.
Flow sequence: NADH or FAD substrates $\rightarrow$ Coenzyme Q $\rightarrow$ Complex III $\rightarrow$ Cytochrome c $\rightarrow$ Complex IV $\rightarrow$ Oxygen.
The final reaction leads to the reduction of molecular oxygen and the formation of so-called metabolic (endogenous) water. The equation for the terminal stage is as follows: $2e^- + 2H^+ + 1/2 O_2 \rightarrow H_2O$
Briefly: answers to topical questions
What substances are specific inhibitors of Complex I (NADH dehydrogenase)?
Specific inhibitors of Complex I include barbiturates, rotenone, and amobarbital. These substances interrupt the respiratory chain at the very beginning, halting oxidative phosphorylation.
- Barbiturates (amobarbital, amytal) inhibit electron transfer from FMN to coenzyme Q (ubiquinone).
- Rotenone blocks electron transfer from NADH to ubiquinone, leading to a sharp decrease or cessation of oxygen consumption.
Which poisons and inhibitors block terminal electron transfer in Complex IV?
Terminal electron transfer in Complex IV is blocked by CO, H2S, and cyanides. These inhibitors act on cytochrome c oxidase, the fourth complex of the respiratory chain.
- Cyanides (CN⁻) irreversibly inhibit cytochrome c oxidase, making electron transfer to oxygen impossible regardless of the substrate.
- CO inhibits cytochrome c oxidase (Complex IV).
- H2S inhibits cytochrome c oxidase (Complex IV).
Through which shuttle mechanisms does cytosolic NADH transfer its electrons into the mitochondrial chain?
Cytosolic NADH transfers its electrons to the mitochondrial chain via two shuttle systems. Hydrogen is transported across the impermeable inner mitochondrial membrane using specific pairs of substrates.
- Malate-aspartate shuttle transfers reducing equivalents into the chain via mitochondrial NAD⁺. The energy yield is 3 or 2.5 ATP molecules.
- Glycerophosphate shuttle transfers hydrogen to FAD-dependent dehydrogenase. The energy yield is 2 or 1.5 ATP molecules.
What is the P/O ratio, and what is its value for NAD- and FAD-dependent substrates?
The phosphorylation ratio (P/O) is the ratio of ATP synthesized to oxygen consumed. It reflects the amount of ATP formed during electron transfer along the respiratory chain.
- NAD-dependent substrates (NADH): the energy yield is 3 ATP by the old nomenclature or 2.5 ATP by the modern one.
- FAD-dependent substrates (FADH₂): the energy yield is 2 ATP by the old nomenclature or 1.5 ATP by the modern one.
The electron pathway from NADH is "long" (passing through Complex I), whereas from FAD it is "short" (starting directly at Coenzyme Q and bypassing the first complex).
Class and exam questions
Where are the enzymes of the electron transport chain localized?
All major protein complexes of the respiratory chain are densely embedded in the inner mitochondrial membrane, while most dehydrogenases are located in the mitochondrial matrix.
What is the role of copper ions in the ETC?
Copper ions ($Cu^{2+}$) are part of cytochrome c oxidase (Complex IV). By reversibly changing their oxidation state to $Cu^{1+}$, they work together with iron to ensure the final release of electrons to oxygen.
How does Complex II differ from the other ETC enzymes?
Succinate dehydrogenase (Complex II) is the only Krebs cycle enzyme embedded in the inner membrane. Moreover, unlike Complexes I, III, and IV, it does not pump protons into the intermembrane space.
What happens to coenzyme Q upon reduction?
Upon accepting electrons, the two keto groups of the quinone ring are converted into hydroxyl groups. As a result, the ring becomes aromatic, forming ubiquinol.
What else to study on this topic
- Mechanisms of proton gradient generation at coupling sites 1, 2, and 3.
- The operation of membrane ATP synthase complexes and the chemiosmotic theory of oxidative phosphorylation.
- Structure of matrix dehydrogenases and the specifics of acyl-CoA oxidation via ETF.
- Chemical structure of the isoalloxazine ring in FMN.
- Role and structure of iron-sulfur (FeS) clusters in ETC enzymes.
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the "Electron Transport Chain" and 3,616 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in the "Energy Metabolism" section
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.