Electron Transport Chain, Oxidative Phosphorylation, and Mitochondrial Uncoupling

Comprehensive practice flashcards on the Electron Transport Chain (Complexes I-IV, electron carriers, standard reduction potentials), ATP synthase structure and function, cytosolic NADH shuttles, net ATP accounting from glucose, mitochondrial uncoupling (UCP-1 and BAT vs. WAT), and metabolic inhibitors.

How many molecules of $\text{NADH}$ and $\text{FADH}_2$ are generated from the complete breakdown of one glucose molecule prior to entry into the electron transport chain?

A total of $10\,\text{NADH} + 10\,\text{H}^+$ and $2\,\text{FADH}_2$ are generated: $2\,\text{NADH} + 2\,\text{H}^+$ from glycolysis, $2\,\text{NADH} + 2\,\text{H}^+$ from the pyruvate dehydrogenase (PDH) complex ($1\,\text{NADH} + \text{H}^+$ per pyruvate), and $6\,\text{NADH} + 6\,\text{H}^+$ and $2\,\text{FADH}_2$ from the Krebs cycle ($3\,\text{NADH} + \text{H}^+$ and $1\,\text{FADH}_2$ per acetyl-CoA).

How do the inner and outer mitochondrial membranes differ in their permeability to $\text{NADH}$ and $\text{FADH}_2$?

The outer mitochondrial membrane is permeable to $\text{NADH}$ and $\text{FADH}_2$, whereas the inner mitochondrial membrane is strictly impermeable to them.

What thermodynamic property dictates the directional flow of electrons through the electron transport chain?

Electrons flow spontaneously down the chain from electron carriers with a lower standard reduction potential ($E_0'$), which readily donate electrons, to carriers with a higher standard reduction potential ($E_0'$), which have a higher electron affinity.

What are the standard reduction potentials ($E_0'$) of the primary electron donor pair $\text{NAD}^+/\text{NADH}$ and the terminal electron acceptor pair $\frac{1}{2}\text{O}_2/\text{H}_2\text{O}$?

$\text{NAD}^+/\text{NADH} + \text{H}^+$ has an $E_0'$ of $-0.32\,\text{V}$, whereas $\frac{1}{2}\text{O}_2 + 2\,\text{H}^+/\text{H}_2\text{O}$ has an $E_0'$ of $+0.82\,\text{V}$.

How many electrons and protons are accepted and carried by flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)?

Both oxidized forms ($\text{FMN}$ and $\text{FAD}$) accept $2\,e^-$ and $2\,\text{H}^+$ to form their fully reduced states, $\text{FMNH}_2$ and $\text{FADH}_2$.

Identify the three redox states of Coenzyme Q (Ubiquinone) shown in this reaction pathway.

The three states are: fully oxidized ubiquinone ($\text{Q}$), the intermediate semiquinone radical ($\text{QH}^\bullet$ or $\text{Q}^{\bullet-}$), and fully reduced ubiquinol ($\text{QH}_2$).

What is the mechanism of electron transfer through iron-sulfur ($\text{Fe-S}$) centers in the electron transport chain?

Each center transfers a single electron at a time via the transition of ferric iron to ferrous iron: $\text{Fe}^{3+} + e^- \rightarrow \text{Fe}^{2+}$, which then passes the electron forward to regenerate $\text{Fe}^{3+}$.

What are the three distinct heme prosthetic groups found in cytochromes of the respiratory chain?

Heme A (found in a-type cytochromes), Iron protoporphyrin IX / Heme B (found in b-type cytochromes), and Heme C (covalently linked to cysteine residues in c-type cytochromes).

What is the enzyme name, prosthetic group composition, and proton pumping output of Complex I?

Complex I is known as NADH dehydrogenase (or NADH-Q oxidoreductase); its prosthetic groups are flavin mononucleotide ($\text{FMN}$) and multiple iron-sulfur ($\text{Fe-S}$) clusters; it pumps $4\,\text{H}^+$ from the mitochondrial matrix into the intermembrane space per $\text{NADH}$ oxidized.

What is the enzyme name, prosthetic group composition, and proton pumping output of Complex II?

Complex II is known as succinate dehydrogenase (or succinate-Q reductase); its prosthetic groups are flavin adenine dinucleotide ($\text{FAD}$) and multiple iron-sulfur ($\text{Fe-S}$) clusters; it pumps zero ($0\,\text{H}^+$) protons across the inner mitochondrial membrane.

Do electrons collected by Complex I get transferred directly to Complex II?

No; Complex I and Complex II operate in parallel and collect electrons independently from two different substrates, transferring them separately into the common ubiquinone ($\text{Q}$) pool.

What is the enzyme name, prosthetic group composition, and proton pumping output of Complex III?

Complex III is known as ubiquinone:cytochrome c oxidoreductase (or Q-cytochrome c oxidoreductase); its prosthetic groups are hemes and iron-sulfur ($\text{Fe-S}$) centers; it removes $4\,\text{H}^+$ from the matrix (two from $\text{QH}_2$ oxidation and two pumped from the matrix).

How does cytochrome c shuttle electrons between Complex III and Complex IV?

Cytochrome c initially associates with Complex III, accepts a single electron ($\text{Fe}^{3+} + e^- \rightarrow \text{Fe}^{2+}$), dissociates, diffuses through the intermembrane space to associate with Complex IV, and donates its electron to the copper binuclear centers of Complex IV.

What is the enzyme name, prosthetic group composition, and proton pumping output of Complex IV?

Complex IV is known as cytochrome oxidase (or cytochrome c oxidase); its prosthetic groups are hemes and two copper binuclear centers; it pumps $2\,\text{H}^+$ out of the mitochondrial matrix into the intermembrane space per pair of electrons transferred.

What is the chemical reaction catalyzed at Complex IV representing the terminal step of the electron transport chain?

$2\,\text{Fe}^{2+}\text{ (Complex IV)} + \frac{1}{2}\text{O}_2 + 2\,\text{H}^+ \rightarrow 2\,\text{Fe}^{3+}\text{ (Complex IV)} + \text{H}_2\text{O}$ (or $4\,e^- + 4\,\text{H}^+ + \text{O}_2 \rightarrow 2\,\text{H}_2\text{O}$).

How many total protons are pumped out of the matrix per molecule of $\text{NADH}$ and $\text{FADH}_2$ through the respiratory chain?

$\text{NADH}$ results in $10\,\text{H}^+$ pumped ($4\,\text{H}^+$ by Complex I, $4\,\text{H}^+$ by Complex III, and $2\,\text{H}^+$ by Complex IV); $\text{FADH}_2$ results in $6\,\text{H}^+$ pumped ($0\,\text{H}^+$ by Complex II, $4\,\text{H}^+$ by Complex III, and $2\,\text{H}^+$ by Complex IV).

How many protons must flow back into the matrix to synthesize one molecule of ATP via ATP synthase, and what are the resulting ATP yields for $\text{NADH}$ and $\text{FADH}_2$?

Every $4\,\text{H}^+$ translocated through ATP synthase yields $1\,\text{ATP}$. Consequently, $10\,\text{H}^+$ from $\text{NADH}$ yields $2.5\,\text{ATP}$, and $6\,\text{H}^+$ from $\text{FADH}_2$ yields $1.5\,\text{ATP}$.

What are the structural components and functional roles of the $F_0$ and $F_1$ domains of ATP synthase depicted here?

The membrane-embedded $F_0$ domain facilitates $\text{H}^+$ translocation, which rotates the c-subunit ring; this drives rotation of the central shaft ($\gamma, \epsilon$) and induces conformational changes in the catalytic $F_1$ domain where each $\alpha\text{-}\beta$ pair synthesizes $\text{ATP}$ from $\text{ADP} + \text{P}_i$.

Which shuttle mechanism is depicted, in which tissues is it active, and what is its ATP yield per cytosolic $\text{NADH}$?

This is the malate-aspartate shuttle, active in the liver, kidneys, and heart; it transfers reducing equivalents to matrix $\text{NAD}^+$, generating mitochondrial $\text{NADH}$ and yielding $2.5\,\text{ATP}$ per cytosolic $\text{NADH}$ ($5\,\text{ATP}$ per glucose).

Which shuttle mechanism is depicted, in which tissues is it used, and how does it deliver electrons into the electron transport chain?

This is the glycerol-3-phosphate shuttle, used in skeletal muscle and the brain; cytosolic glycerol 3-phosphate dehydrogenase uses $\text{NADH}$ to reduce dihydroxyacetone phosphate to glycerol 3-phosphate, which is reoxidized by a mitochondrial inner membrane enzyme transferring electrons to $\text{FAD}$ to form $\text{FADH}_2$, yielding $1.5\,\text{ATP}$ per cytosolic $\text{NADH}$ ($3\,\text{ATP}$ per glucose) by feeding directly into the ubiquinone ($\text{Q}$) pool.

What is the total net ATP yield from the complete aerobic oxidation of one molecule of glucose?

The yield is $32\,\text{ATP}$ if the malate-aspartate shuttle is used (e.g., in hepatocytes of the liver) or $30\,\text{ATP}$ if the glycerol-3-phosphate shuttle is used (e.g., in skeletal muscle and brain).

What transport proteins coordinate the exchange of ATP, ADP, and inorganic phosphate across the inner mitochondrial membrane as shown in the diagram?

Adenine nucleotide translocase acts as an antiporter transporting $\text{ATP}^{4-}$ out to the intermembrane space and $\text{ADP}^{3-}$ into the matrix; phosphate translocase acts as a symporter importing $\text{H}_2\text{PO}_4^-$ along with an $\text{H}^+$ into the matrix.

What is mitochondrial uncoupling, and what is its physiological purpose?

Uncoupling is the disruption or collapse of the mitochondrial proton gradient by providing an alternate pathway for $\text{H}^+$ to re-enter the matrix bypassing the $F_0F_1$ ATP synthase complex; this releases the stored potential energy directly as heat (non-shivering thermogenesis) rather than synthesizing ATP.

What is UCP-1 (thermogenin) and how is it activated in brown adipose tissue (BAT)?

UCP-1 is an inner mitochondrial membrane protein abundant in BAT; when body temperature drops, cold stimulates $\beta$-adrenergic hormones to release free long-chain fatty acids from triacylglycerols, which bind the intermembrane face of UCP-1 to activate proton leak into the matrix.

What anatomical locations show active brown adipose tissue (BAT) on a PET scan following cold exposure in adult humans?

BAT appears predominantly in the neck and upper chest areas upon exposure to cold conditions.

Why do large hibernating mammals like wild boars lack functional UCP-1, and how do they adapt to generate heat?

They lost UCP-1 while historically inhabiting tropical or subtropical climates where non-shivering thermogenesis was unneeded; they compensate by building nests for birth, bearing large litters, shivering, and huddling together with littermates.

What was 2,4-dinitrophenol (DNP) used for in the early 1900s, and what is its molecular mechanism of action?

DNP was used as a weight-loss drug; it is a chemical uncoupler that acts as a hydrophobic proton carrier, dissolving in the inner mitochondrial membrane and dissipating the proton gradient, which prevents ATP synthesis and releases metabolic energy as extreme heat.

Which specific respiratory chain inhibitors target electron transfer from the iron-sulfur ($\text{Fe-S}$) centers of Complex I to ubiquinone?

Rotenone, Amytal, Piericidin A, and Myxothiazol.

What is the site and mode of action of Antimycin A?

Antimycin A inhibits Complex III by blocking electron transfer from cytochrome $b$ to cytochrome $c_1$.

How do cyanide ($\text{CN}^-$) and carbon monoxide ($\text{CO}$) inhibit the electron transport chain?

Both inhibit Complex IV (cytochrome oxidase); cyanide binds to the $\text{Fe}^{3+}$ state of heme, while carbon monoxide binds to the $\text{Fe}^{2+}$ state.

Which inhibitors block ATP synthase at the $F_0$ domain versus the $F_1$ domain?

Oligomycin, Venturicidin, and DCCD (dicyclohexylcarbodiimide) inhibit the $F_0$ domain (blocking proton flow); Aurovertin inhibits the catalytic $F_1$ domain.

What compound selectively inhibits adenine nucleotide translocase to block ATP-ADP exchange across the inner mitochondrial membrane?

Atractyloside.

In an oxygraph assay with mitochondria respiring on fumarate and ADP, why does adding an uncoupler like DNOC cause a sharp increase in the rate of $\text{O}_2$ consumption that is unaffected by oligomycin but blocked by KCN?

DNOC dissipates the proton gradient by uncoupling electron transfer from ATP synthesis, causing maximum uninhibited electron flow and continuous $\text{O}_2$ consumption regardless of ATP synthase inhibition by oligomycin; adding KCN completely stops $\text{O}_2$ consumption because it blocks Complex IV, the terminal site of oxygen reduction.