Can you identify the chemical structures in the citric acid cycle? Follow the TCA cycle from citrate through its seven subsequent intermediates, then complete the eighth reaction that regenerates citrate from oxaloacetate and acetyl-CoA.
What Is the TCA Cycle?
The tricarboxylic acid (TCA) cycle, also called the citric acid cycle or Krebs cycle, is a central pathway in aerobic metabolism. In eukaryotes, most reactions take place in the mitochondrial matrix; succinate dehydrogenase is embedded in the inner mitochondrial membrane. The cycle oxidizes an acetyl group and regenerates oxaloacetate so that another turn can begin.
Each turn generates 3 NADH, 1 FADH2, 1 GTP (or ATP, depending on the organism and enzyme isoform), and 2 CO2 per acetyl-CoA. These reduced cofactors can subsequently feed electrons into the respiratory chain.
Interactive TCA Cycle Quiz: Identify All 8 Reactions
Begin with citrate at the top. Choose the correct product for each reaction by comparing three chemical structures and metabolite names. The vertical arrows show the reaction order, while the return path connects oxaloacetate at the bottom to citrate at the top. Acetyl-CoA must join oxaloacetate in the citrate synthase reaction: the return arrow does not represent a direct isomerization.
The quiz tests eight enzyme-catalyzed reactions. Step 8 asks which product is formed when oxaloacetate reacts with acetyl-CoA. Click Check answers to see your score and explanations.
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The 8 Steps of the Citric Acid Cycle
| Step | Enzyme | Main transformation |
|---|---|---|
| 1 | Aconitase | Citrate is isomerized to isocitrate |
| 2 | Isocitrate dehydrogenase | Oxidative decarboxylation; NADH and CO2 formed |
| 3 | α-Ketoglutarate dehydrogenase complex | Succinyl-CoA formed; NADH and CO2 released |
| 4 | Succinyl-CoA synthetase | Succinate formed; GTP or ATP produced |
| 5 | Succinate dehydrogenase | Fumarate formed; FAD reduced to FADH2 |
| 6 | Fumarase | Water added to form L-malate |
| 7 | Malate dehydrogenase | Oxaloacetate formed; NADH produced |
| 8 | Citrate synthase | Oxaloacetate + acetyl-CoA → citrate + CoA-SH |
How to Distinguish TCA Cycle Structures
Citrate and isocitrate differ in the position of a hydroxyl group. α-Ketoglutarate is a five-carbon α-keto dicarboxylate. Succinyl-CoA contains a thioester linked to coenzyme A. Succinate is a saturated four-carbon dicarboxylate, whereas fumarate has a carbon–carbon double bond. L-Malate contains a hydroxyl group, and oxaloacetate contains a keto group.
When preparing structure images, keep the same ionization convention across related compounds. The names in this quiz refer to biochemical metabolites; the exact drawn protonation states can depend on pH. Use the biologically relevant L-malate stereoisomer, and distinguish fumarate from maleate.
Frequently Asked Questions
How many reactions are in the TCA cycle?
The conventional TCA cycle has eight enzyme-catalyzed reactions, including citrate formation by citrate synthase.
Is the TCA cycle the same as the Krebs cycle?
Yes. TCA cycle, citric acid cycle, and Krebs cycle are names for the same pathway.
Why does oxaloacetate return to citrate?
Oxaloacetate is regenerated near the end of the cycle. Citrate synthase condenses it with a new acetyl-CoA molecule to form citrate, allowing another turn.
How much ATP does one turn produce?
One turn produces one GTP or ATP directly by substrate-level phosphorylation, along with three NADH and one FADH2. Those reduced cofactors support additional ATP generation through oxidative phosphorylation.
More Interactive Biochemistry Quizzes
Practice the preceding pathway with our Glycolysis Quiz: All 10 Steps with Chemical Structures. For a historical perspective on glucose metabolism, see Has Glycolysis Changed in 50 Years?.
References
Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 8th ed. W. H. Freeman; 2021.