Citric Acid Cycle and Oxidative Phosphorylation (1D) - Bio & Biochem

Which electron transport chain complex accepts electrons from FADH $_2$ generated by succinate dehydrogenase?

Complex II (succinate dehydrogenase). Complex II directly feeds electrons from FADH $_2$ into the ubiquinone pool without pumping protons itself.
Which direction do protons flow through ATP synthase to drive ATP formation in mitochondria?

Intermembrane space $\rightarrow$ matrix. Protons flow down their electrochemical gradient from intermembrane space to matrix, driving ATP synthase's conformational changes for ATP production.
Which electron transport chain complexes pump protons across the inner mitochondrial membrane?

Complexes I, III, and IV. These complexes use electron transfer energy to translocate protons into the intermembrane space, creating the electrochemical gradient.
What is the primary cellular location of the citric acid cycle in eukaryotes?

Mitochondrial matrix. The citric acid cycle occurs in the mitochondrial matrix where enzymes and substrates are concentrated for efficient metabolic processing.
Which citric acid cycle enzyme oxidizes malate to oxaloacetate and produces NADH?

Malate dehydrogenase. This enzyme regenerates oxaloacetate by oxidizing malate, reducing NAD $^+$ to NADH in the final step of the cycle.
Which citric acid cycle enzyme catalyzes citrate to isocitrate via a dehydration/rehydration isomerization?

Aconitase. Aconitase isomerizes citrate to isocitrate through cis-aconitate intermediate, preparing for subsequent decarboxylation.
What are the net products of one turn of the citric acid cycle per acetyl-CoA?

$3$ NADH, $1$ FADH $_2$ , $1$ GTP, $2$ CO $_2$ . One acetyl-CoA through the cycle yields reduced cofactors for ETC, GTP for energy, and CO $_2$ as waste.
Which citric acid cycle enzyme hydrates fumarate to malate?

Fumarase (fumarate hydratase). Fumarase adds water across the double bond of fumarate, stereospecifically forming L-malate in a reversible hydration reaction.
What are the net products of pyruvate dehydrogenase per pyruvate molecule?

$1$ acetyl-CoA, $1$ NADH, $1$ CO $_2$ . Pyruvate is oxidatively decarboxylated, yielding acetyl-CoA for the TCA cycle, NADH as an electron carrier, and CO $_2$ as a byproduct.
What is the final electron acceptor of the mitochondrial electron transport chain?

O $_2$ (reduced to H $_2$ O). Oxygen accepts electrons from cytochrome c oxidase, forming water and preventing electron backup in the chain.
What are the two mobile electron carriers that shuttle electrons between ETC complexes?

Coenzyme Q (ubiquinone) and cytochrome $c$ . These carriers facilitate electron transfer: ubiquinone between I/II and III, cytochrome c between III and IV.
Which citric acid cycle step produces the first NADH and releases CO $_2$ (oxidative decarboxylation of isocitrate)?

Isocitrate dehydrogenase. This enzyme catalyzes the rate-limiting oxidative decarboxylation of isocitrate to $alpha$ -ketoglutarate, generating NADH and releasing CO $_2$ .
What happens to oxidative phosphorylation if O $_2$ is absent and the ETC cannot pass electrons to Complex IV?

ETC stops, NADH accumulates, ATP synthase halts. Without O $_2$ as terminal acceptor, electron flow halts, preventing NADH oxidation and proton gradient maintenance for ATP synthesis.
Identify the total reduced electron carriers produced per glucose from two citric acid cycle turns (excluding PDC).

$6$ NADH and $2$ FADH $_2$ . Two acetyl-CoA from one glucose enter two cycles, each producing 3 NADH and 1 FADH $_2$ , totaling these carriers.
What is the overall purpose of the electron transport chain in oxidative phosphorylation?

Create a proton-motive force by pumping H $^+$ . The ETC transfers electrons from reduced carriers to O $_2$ , using energy to pump protons and establish a gradient for ATP synthesis.
Which citric acid cycle step converts $\text{alpha}$ -ketoglutarate to succinyl-CoA while producing NADH and CO $_2$ ?

$\text{alpha}$ -ketoglutarate dehydrogenase complex. The complex performs oxidative decarboxylation, analogous to pyruvate dehydrogenase, producing succinyl-CoA, NADH, and CO $_2$ .
Which citric acid cycle enzyme produces FADH $_2$ by oxidizing succinate to fumarate?

Succinate dehydrogenase (Complex II). As part of the ETC, this enzyme oxidizes succinate to fumarate, reducing FAD to FADH $_2$ for electron transfer.
Using $2.5$ ATP per NADH and $1.5$ ATP per FADH $_2$ , what ATP is produced from $3$ NADH and $1$ FADH $_2$ ?

$9.0$ ATP. Each NADH yields approximately 2.5 ATP and each FADH $_2$ yields 1.5 ATP through oxidative phosphorylation efficiency.
Which citric acid cycle step produces GTP (or ATP) by substrate-level phosphorylation?

Succinyl-CoA synthetase (succinate thiokinase). This enzyme cleaves the thioester bond of succinyl-CoA, coupling it to phosphorylation of GDP to GTP or ADP to ATP.
Which citric acid cycle enzyme catalyzes the formation of citrate from oxaloacetate and acetyl-CoA?

Citrate synthase. This enzyme catalyzes the irreversible condensation reaction, committing acetyl-CoA to the citric acid cycle.
What is the name of the enzyme that uses the proton gradient to synthesize ATP?

ATP synthase (Complex V). ATP synthase harnesses the proton gradient's energy via rotational catalysis to phosphorylate ADP to ATP.
What molecule condenses with acetyl-CoA to form citrate in the first step of the citric acid cycle?

Oxaloacetate. Oxaloacetate serves as the acceptor for the acetyl group from acetyl-CoA, initiating the cycle by forming citrate.
Which electron transport chain complex accepts electrons from NADH?

Complex I (NADH dehydrogenase). Complex I oxidizes NADH to NAD $^+$ , transferring electrons to ubiquinone while pumping protons across the membrane.
What enzyme complex converts pyruvate to acetyl-CoA and links glycolysis to the citric acid cycle?

Pyruvate dehydrogenase complex (PDC). This complex decarboxylates pyruvate, linking glycolysis to the TCA cycle by producing acetyl-CoA for entry into the cycle.

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