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Electron Transport Chain

Q: 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 a FAD-dependent dehydrogenase. The energy yield is 2 or 1.5 ATP molecules.

Electron transport chain (ETC)

For medical students2 min readUpdated 2026-10-10

The electron transport chain (ETC) is a system of protein complexes and mobile carriers located in the inner mitochondrial membrane. It facilitates the sequential transfer of electrons from primary substrates to molecular oxygen, ultimately resulting in the formation of water.

LocalizationInner mitochondrial membrane

Terminal AcceptorMolecular oxygen (forming a water molecule)

Composition4 enzyme complexes and 2 low-molecular-weight carriers

Key MetalsIron (in heme and Fe-S clusters) and Copper (in cytochrome c oxidase)

Main Protein Complexes of the ETC

The mitochondrial electron transport chain comprises four high-molecular-weight enzyme assemblies embedded within 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 site of FMN is an isoalloxazine ring linked to ribitol phosphate. In the oxidized state, the nitrogen atoms in this ring form a quinone structure, whereas reduction to $FMNH_2$ involves hydrogen addition. The enzyme oxidizes NADH to NAD⁺ and performs the first coupling site by pumping protons ($nH^+$) out of the matrix.
Complex II (succinate dehydrogenase). Contains FAD and FeS proteins. Unlike other FAD-dependent enzymes dissolved in the matrix, this complex is firmly anchored to the inner membrane. It oxidizes FAD-dependent substrates (e.g., converting succinate to fumarate). Important note: 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 relies on the reversible change in the oxidation state of iron within the heme group (from $Fe^{3+}$ to $Fe^{2+}$). This represents the second coupling site.
Complex IV (cytochrome c oxidase). Consists of cytochromes $a, a_3$ and copper ions. Through coupled changes in the oxidation states of heme iron and copper (from $Cu^{2+}$ to $Cu^{1+}$), the enzyme executes the final transfer of electrons to oxygen. It acts as a proton pump (third coupling site).

Mobile Electron Carriers

Low-molecular-weight carriers shuttle between the stationary complexes, integrating the entire system into a unified chain.

Coenzyme Q (ubiquinone). A non-protein lipid-soluble carrier. Structurally, it consists of a benzoquinone ring with a long isoprenoid tail of 10 subunits. 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 and the ring becomes aromatic, forming reduced ubiquinol ($QH_2$).
Cytochrome c. A mobile hydrophilic protein localized on the outer surface of the inner membrane. Its sole function is to intercept electrons from Complex III and deliver them to Complex IV.

Entry Pathways of Electrons into the Respiratory Chain

There are two main pathways 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). These 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). These enzymes reside 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 Step and General Scheme

Regardless of how electrons enter the chain, they ultimately converge on ubiquinone and 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.

喜び (The final reaction reduces molecular oxygen to form metabolic (endogenous) water. The equation for the terminal step is: $2e^- + 2H^+ + 1/2 O_2 \rightarrow H_2O$

Mnemonic

The electron path from NADH is "long" (passes through Complex I), whereas from FAD it is "short" (starts directly at Coenzyme Q, bypassing the first complex).

Frequently asked questions

Which substances are specific inhibitors of Complex I (NADH dehydrogenase)?

Specific inhibitors of Complex I include barbiturates, rotenone, and amobarbital (amytal). These substances interrupt the respiratory chain at the very beginning, halting oxidative phosphorylation.

Barbiturates (amobarbital) — inhibit electron transfer from FMN to coenzyme Q (ubiquinone).
Rotenone — blocks electron transfer from NADH to ubiquinone, leading to a drastic decrease or cessation of oxygen consumption.

Which poisons and inhibitors block terminal electron transfer at Complex IV?

Terminal electron transfer at Complex IV is blocked by CO, $H_2S$, and cyanides. These inhibitors target cytochrome c oxidase, which is Complex IV of the respiratory chain.

Cyanides (CN⁻) — irreversibly inhibit cytochrome c oxidase, preventing electron transfer to oxygen regardless of the substrate.
CO — inhibits cytochrome c oxidase (Complex IV).
$H_2S$ — 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 a FAD-dependent dehydrogenase. The energy yield is 2 or 1.5 ATP molecules.

What is the P/O ratio, and what are its values for NAD- and FAD-dependent substrates?

The phosphorylation ratio (P/O ratio) is the ratio of ATP synthesized to oxygen consumed. It reflects the amount of ATP generated during electron transfer along the respiratory chain.

NAD-dependent substrates (NADH) — energy yield is 3 ATP (older nomenclature) or 2.5 ATP (modern nomenclature).
FAD-dependent substrates (FADH₂) — energy yield is 2 ATP (older nomenclature) or 1.5 ATP (modern nomenclature).

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, whereas most dehydrogenases reside within the mitochondrial matrix.

What role do copper ions play in the ETC?

Copper ions ($Cu^{2+}$) are components of cytochrome c oxidase (Complex IV). By reversibly changing their oxidation state to $Cu^{1+}$, they work together with iron to ensure the final transfer 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 directly in the inner membrane. Furthermore, unlike Complexes I, III, and IV, it does not pump protons into the intermembrane space.

What happens to coenzyme Q when it is reduced?

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.

Go deeper

Mechanisms of proton gradient generation at coupling sites 1, 2, and 3.
Function 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 within FMN.
Role and structure of iron-sulfur (FeS) clusters in ETC enzymes.