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Quizzes > Science > Biology

Citric Acid Cycle Quiz: Test Your TCA Cycle Mastery

Think you can ace this TCA cycle quiz? Take on the Krebs cycle questions now!

Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration for Citric Acid Cycle Quiz on a coral background

Ready to unlock metabolism's core? Dive into our citric acid cycle quiz designed to challenge both students and pros with TCA cycle quiz scenarios - trace each reaction from citrate formation all the way to ATP generation. Discover tricks to recall enzyme names, spot regulation points, and understand energy yields - essential tools for anyone tackling metabolic pathways. You'll tackle essential citric acid cycle questions, reinforce mnemonics for key enzymes, and strengthen your mastery in this biochemistry cycle quiz. Whether you're prepping for an exam or aiming to boost your lab credentials, click into our tca cycle quiz for a rapid-fire review or explore a fun Krebs cycle quiz to connect glycolysis and Krebs steps. Dive in now - test your knowledge and ace the cycle in minutes!

Which compound is formed by the condensation of acetyl-CoA and oxaloacetate at the start of the TCA cycle?
Citrate
Isocitrate
Succinyl-CoA
Oxalosuccinate
The first step of the TCA cycle is catalyzed by citrate synthase, which combines acetyl-CoA and oxaloacetate to form citrate in an irreversible reaction. This condensation is the committed step of the cycle. Citrate then proceeds through successive transformations to generate energy carriers. More on citrate synthase
Which enzyme catalyzes the isomerization of citrate to isocitrate?
Aconitase
Isocitrate dehydrogenase
Citrate synthase
Fumarase
Aconitase catalyzes the reversible isomerization of citrate to isocitrate via cis-aconitate. This two-step process involves dehydration and rehydration. Aconitase ensures the hydroxyl group is correctly positioned for subsequent oxidation. Learn more about aconitase
Which step in the citric acid cycle directly produces GTP (or ATP)?
Succinyl-CoA synthetase
Succinate dehydrogenase
Malate dehydrogenase
?-Ketoglutarate dehydrogenase
Succinyl-CoA synthetase (also known as succinate thiokinase) converts succinyl-CoA to succinate, coupling the reaction to substrate-level phosphorylation of GDP to GTP (or ADP to ATP). This is the only TCA cycle step that yields a nucleoside triphosphate directly. The GTP produced can be readily converted to ATP. Details on succinyl-CoA synthetase
During the oxidative decarboxylation of isocitrate, which molecule is released?
CO2
H2O
NADH
FADH2
Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to ?-ketoglutarate, releasing one molecule of CO2. Simultaneously, NAD+ is reduced to NADH. This step is one of the three irreversible oxidation reactions in the cycle. Read about isocitrate dehydrogenase
Which of the following is a potent allosteric inhibitor of isocitrate dehydrogenase in the TCA cycle?
ATP
ADP
NADH
NAD+
Isocitrate dehydrogenase is feedback inhibited by high levels of ATP, which signals abundant energy supply. This allosteric regulation slows the cycle when the cell’s energy charge is high. NADH also inhibits the enzyme but ATP is the primary allosteric effector. Cycle regulation details
Which TCA cycle enzyme catalyzes the most exergonic reaction under standard conditions?
?-Ketoglutarate dehydrogenase
Citrate synthase
Malate dehydrogenase
Fumarase
The ?-ketoglutarate dehydrogenase complex catalyzes the oxidative decarboxylation of ?-ketoglutarate to succinyl-CoA, with a standard ?G°' of about –33 kJ/mol, making it the most exergonic step. This irreversibility helps drive the cycle forward. The enzyme is structurally and mechanistically similar to pyruvate dehydrogenase. Learn more about ?-ketoglutarate dehydrogenase
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Study Outcomes

  1. Recall the TCA Cycle Sequence -

    List and describe each step from acetyl-CoA entry to oxaloacetate regeneration, ensuring you can name all key intermediates and reactions.

  2. Analyze Enzyme Functions -

    Explain the role and mechanism of critical enzymes (e.g., citrate synthase, isocitrate dehydrogenase) in driving TCA cycle flux and regulation.

  3. Calculate Energy Yield -

    Determine the net ATP (or GTP), NADH, and FADHâ‚‚ produced per acetyl-CoA molecule processed through the cycle.

  4. Apply Regulatory Principles -

    Identify how allosteric effectors like ATP, NADH, and calcium modulate TCA cycle enzyme activity under different metabolic states.

  5. Integrate Metabolic Pathways -

    Understand how the citric acid cycle interfaces with glycolysis, fatty acid oxidation, and amino acid catabolism to maintain cellular energy balance.

Cheat Sheet

  1. Acetyl-CoA Entry and Citrate Formation -

    The condensation of acetyl-CoA with oxaloacetate, catalyzed by citrate synthase, is the cycle's committed step and drives the reaction forward. This exergonic reaction also frees CoA for fatty acid oxidation, highlighting the link between lipid and carbohydrate metabolism. Mastering this step is crucial for any citric acid cycle quiz.

  2. Aconitase-Mediated Isomerization -

    Aconitase converts citrate to isocitrate via cis-aconitate, enabling subsequent oxidative decarboxylation. Remember the mnemonic "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate" to recall all eight intermediates in order. This rearrangement step is key for understanding how the Krebs cycle quiz questions often test stereochemistry and enzyme specificity.

  3. Oxidative Decarboxylation Steps -

    Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase catalyze two successive decarboxylations, producing NADH and releasing CO₂. These rate-limiting steps are highly regulated by ATP/ADP ratios and calcium ions, reinforcing the cycle's integration with cellular energy status. Highlight these enzymes when tackling questions on metabolic control in your TCA cycle quiz.

  4. Substrate-Level Phosphorylation and Electron Carriers -

    Succinyl-CoA synthetase converts succinyl-CoA to succinate, directly generating GTP (or ATP) via substrate-level phosphorylation. Downstream, succinate dehydrogenase produces FADHâ‚‚, linking the cycle to the electron transport chain. Knowing the exact yield - 3 NADH, 1 FADHâ‚‚, and 1 GTP per acetyl-CoA - is essential for scoring high on any citric acid cycle quiz.

  5. Cycle Regulation and Allosteric Control -

    Key enzymes - citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase - are inhibited by high NADH and ATP levels, while ADP and Ca²⺠act as activators. This feedback ensures the cycle responds dynamically to cellular energy demands, a common topic in biochemistry cycle quizzes. Understanding these control points boosts confidence and accuracy on exam questions.

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