Mitochondrial Electron Transport, ATP Synthesis, and Fatty Acid Metabolism

Why does FADH2 produce less ATP than NADH

FADH2 donates electrons to Complex II instead of Complex I. Because Complex II does not pump protons, fewer total protons are translocated across the inner mitochondrial membrane. Consequently, less proton motive force is generated and less ATP is synthesized. NADH produces about 2.5 ATP while FADH2 produces about 1.5 ATP.

Which ETC complexes are inhibited by rotenone, antimycin A, and cyanide

Rotenone inhibits Complex I. Antimycin A inhibits Complex III. Cyanide inhibits Complex IV.

Why does NADH accumulate when Complex III is inhibited

Complex III inhibition blocks electron flow through the ETC. NADH can no longer donate electrons through Complex I because downstream electron acceptors remain reduced. NADH oxidation stops and NADH accumulates.

Why can succinate partially restore oxygen consumption during rotenone inhibition

Rotenone blocks Complex I, preventing NADH oxidation. Succinate donates electrons directly to Complex II through succinate dehydrogenase, bypassing Complex I. Electrons can therefore continue through Complexes III and IV to oxygen.

What is the function of ATP/ADP translocase

ATP/ADP translocase exports ATP from the mitochondrial matrix into the cytoplasm while simultaneously importing ADP into the matrix for ATP synthesis.

Why is ATP/ADP exchange electrogenic

ATP carries one more negative charge than ADP. Export of ATP therefore results in a net movement of negative charge out of the matrix.

How is phosphate transported into the mitochondrial matrix

Phosphate is transported through a Pi/H+ symporter that couples phosphate import to proton movement down the proton gradient.

How do uncouplers decrease ATP synthesis

Uncouplers dissipate the proton gradient by transporting protons across the inner mitochondrial membrane independently of ATP synthase. Electron transport continues, but ATP synthase no longer has sufficient proton motive force to synthesize ATP.

What are the approximate P/O ratios for NADH and FADH2

NADH produces about 2.5 ATP. FADH2 produces about 1.5 ATP.

How many protons are pumped during oxidation of one NADH

10 protons are pumped total: Complex I pumps 4 H+, Complex III pumps 4 H+, and Complex IV pumps 2 H+.

How many acetyl-CoA molecules are produced from palmitate (C16:0)

8 acetyl-CoA molecules are produced. Formula: n/2.

How many β-oxidation cycles occur during palmitate degradation

7 cycles occur. Formula: (n/2) − 1.

What is the total ATP yield from complete oxidation of palmitate

106 ATP are produced after subtracting the 2 ATP activation cost.

Why do fatty acids produce more ATP per carbon than carbohydrates

Fatty acids are more chemically reduced and contain more C-H bonds and fewer oxygen atoms. Oxidation therefore releases more electrons, generating more NADH and FADH2 for oxidative phosphorylation.

What molecule remains after oxidation of odd-chain fatty acids

Propionyl-CoA remains after oxidation of odd-chain fatty acids.

What cofactor is reduced during the first step of β-oxidation

FAD is reduced to FADH2 by acyl-CoA dehydrogenase.

Why is decarboxylation of malonyl-CoA important in fatty acid synthesis

Decarboxylation generates a reactive enolate/carbanion intermediate that attacks the growing acyl chain. CO2 loss also provides thermodynamic driving force for carbon-carbon bond formation.

Why does pyrophosphate hydrolysis drive biosynthetic reactions forward

Hydrolysis of pyrophosphate into two inorganic phosphates is highly exergonic. Removal of PPi shifts equilibrium toward product formation and makes the overall reaction more thermodynamically favorable.

What equation relates free energy and reduction potential

ΔG°′ = -nFΔE°′

If ΔE°′ is positive, is the reaction favorable

Yes. A positive ΔE°′ produces a negative ΔG°′, indicating a thermodynamically favorable reaction.

Why do β-keto acids readily undergo decarboxylation

Decarboxylation forms a resonance-stabilized enolate intermediate. This stabilization lowers the activation energy and makes CO2 loss favorable.

How do nearby positive charges affect Fe-S cluster reduction potentials

Positive charges stabilize the reduced state of the Fe-S cluster, making reduction more favorable and shifting the reduction potential to more positive values.

How does solvent exposure affect Fe-S cluster reduction potential

Increased solvent exposure destabilizes the reduced state and shifts the reduction potential to more negative values.

What is the role of the TPP thiazolium ring

The positively charged thiazolium ring stabilizes carbanion intermediates through resonance delocalization.

What does electron sink mean in TPP chemistry

An electron sink stabilizes negative charge by delocalizing electrons through resonance across the thiazolium ring.

Which pentose phosphate pathway enzyme uses TPP

Transketolase.

Why is a modified TPP lacking the positive charge nonfunctional

Without the positively charged thiazolium ring, the cofactor cannot stabilize carbanion intermediates. Electron sink function is lost and catalysis fails.

What intermediate is stabilized during Schiff-base catalysis

A carbanion or enamine intermediate is stabilized. The imine formed with lysine acts as an electron sink.

Why must citrate first be converted into isocitrate before oxidation

Citrate contains a tertiary alcohol that cannot be oxidized because the alcohol-bearing carbon lacks a hydrogen atom. Aconitase rearranges citrate into isocitrate, which contains an oxidizable secondary alcohol.

What bond provides the thermodynamic driving force in citrate synthase

The thioester bond of acetyl-CoA provides the thermodynamic driving force.

Why is citrate synthase essentially irreversible

The reaction is coupled to highly favorable thioester hydrolysis, producing a large negative ΔG under physiological conditions.

Which citric acid cycle enzymes catalyze oxidation reactions

Isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinate dehydrogenase, and malate dehydrogenase.

Where does glycolysis occur

Glycolysis occurs in the cytosol.

Where does β-oxidation occur

β-oxidation occurs in the mitochondrial matrix.

Where does the electron transport chain occur

The electron transport chain occurs in the inner mitochondrial membrane.

Where does fatty acid synthesis occur

Fatty acid synthesis occurs in the cytosol.

Why does insulin activate acetyl-CoA carboxylase

Insulin signals energy abundance and high glucose availability. Acetyl-CoA carboxylase catalyzes the committed step of fatty acid synthesis, so activation promotes energy storage as fatty acids.

Why does keto-breath occur during fasting

During fasting, excess acetyl-CoA is converted into ketone bodies. Acetoacetate spontaneously decarboxylates into acetone, a volatile molecule exhaled in the breath.

Why do desaturase enzymes require O2

Desaturases oxidize saturated carbon-carbon bonds to form double bonds. Molecular oxygen acts as the terminal electron acceptor during this oxidation reaction.

Why can dehydrases not replace desaturases

Dehydrases generate double bonds through elimination of water from alcohol-containing substrates, whereas desaturases oxidize saturated hydrocarbons directly. The mechanisms and substrate requirements are fundamentally different.

What intermediate forms after malonyl-CoA decarboxylation

An enolate/carbanion intermediate forms.

What intermediate forms during citrate synthase catalysis before hydrolysis

Citryl-CoA forms before hydrolysis.

What intermediate forms before decarboxylation during isocitrate dehydrogenase catalysis

Oxalosuccinate forms before decarboxylation.

What intermediate forms during transketolase catalysis

A TPP-bound activated ketol intermediate stabilized by the thiazolium electron sink forms.

Why are thioesters considered high-energy compounds

Sulfur overlaps poorly with the carbonyl π-system, so thioesters possess limited resonance stabilization. Hydrolysis products are more resonance stabilized, making hydrolysis highly exergonic.

Explain why cyanide poisoning rapidly stops ATP synthesis.

Cyanide inhibits Complex IV, preventing oxygen from accepting electrons. Electron transport ceases, proton pumping stops, and the proton gradient collapses. NADH accumulates because it can no longer be oxidized. Without proton motive force, ATP synthase cannot synthesize ATP, leading to rapid cellular energy failure.