Citric Acid Cycle (Krebs) Tournament
Ten MCAT-DoK rounds spanning enzymology, regulation, intermediate ordering, quantitative ATP and CO2 accounting, and the TCA-ETC handshake. The tournament begins with an interactive onboarding step that locates the cycle inside aerobic respiration.
Where the Citric acid cycle (TCA cycle) fits in Aerobic respiration
The TCA cycle is the central oxidation hub of aerobic respiration. Pyruvate metabolism feeds it acetyl-CoA from carbohydrates and fats, and the NADH and FADH₂ it produces drive the electron transport chain. To enter the tournament, click the highlighted Citric acid cycle box on the overview below.
Click the highlighted Citric acid cycle (TCA cycle) box to continue.
What this tournament tests
Each round maps to a distinct MCAT cognitive demand for AAMC content category 1A (Bioenergetics). The ten formats together cover Krebs enzymology, regulation, the TCA-ETC handshake, intermediate ordering, enzyme-cofactor matching, and the quantitative CO2 and ATP accounting that consistently appears on test day.
Visual Labeling
Identify the alpha-ketoglutarate dehydrogenase complex on the live Reactome diagram.
Fill in the Blank
Recall succinyl-CoA synthetase as the source of the cycle's only substrate-level phosphorylation.
Disruptor
Predict the integrated effect of Complex I inhibition (rotenone) on TCA flux.
Hot Spot
Click succinate dehydrogenase - the only TCA enzyme that doubles as an ETC complex.
Data Interpretation
Read the per-turn yield table and compute total ATP equivalents per acetyl-CoA.
Sequence Ordering
Place all 8 TCA intermediates from citrate to oxaloacetate in correct order.
Match the Pairs
Pair each Krebs enzyme to the high-energy product (NADH, FADH2, or GTP) it makes.
Numeric Input
Count CO2 released by the TCA cycle proper per original glucose molecule.
Select All That Apply
Identify which signals inhibit IDH3 / OGDH and slow the cycle.
Odd One Out
Distinguish true TCA enzymes from look-alikes that feed (or compete with) the cycle.
Public leaderboard
Your score posts to a global, persistent leaderboard scored by points first, time as tiebreaker.
The TCA cycle in 60 seconds
The citric acid cycle (also Krebs cycle, TCA cycle) takes one acetyl-CoA and oxidizes it completely to 2 CO₂, capturing the released energy as 3 NADH, 1 FADH₂, and 1 GTP per turn.
The three irreversible regulated enzymes are citrate synthase, isocitrate dehydrogenase (IDH3), and alpha-ketoglutarate dehydrogenase (OGDH) - all inhibited by NADH, succinyl-CoA, and ATP, all activated by ADP and (especially in muscle) Ca²⁺.
The cycle's only substrate-level phosphorylation is at succinyl-CoA synthetase, which converts succinyl-CoA + GDP + Pi into succinate + CoA + GTP.
Succinate dehydrogenase (SDH) is the unicorn: it is both a TCA enzyme and Complex II of the electron transport chain, embedded in the inner mitochondrial membrane. It uses FAD, not NAD+, because the standard reduction potentials don't allow NAD+ to oxidize succinate. That is also why FADH₂ yields fewer ATPs (~1.5) than NADH (~2.5) - it bypasses Complex I and pumps fewer protons.
FAQ
What is produced per turn of the TCA cycle?
Per acetyl-CoA: 3 NADH, 1 FADH2, 1 GTP (~ATP), and 2 CO2. Net ATP equivalents: ~10 per acetyl-CoA, ~20 per glucose from the TCA cycle alone.
Which TCA enzyme is also Complex II of the ETC?
Succinate dehydrogenase (SDH). It uses FAD/FADH2 instead of NAD+/NADH and feeds electrons directly to ubiquinone without pumping protons.
Do I need an account to play?
No. The tournament is fully public. You get a randomized handle and your score posts to the public leaderboard.
Where does the diagram come from?
The Reactome 'Citric acid cycle (TCA cycle)' pathway (R-HSA-71403), licensed CC BY 4.0. Quiz hotspots are overlaid on the actual entity bounding boxes.
Keep going
Glycolysis enzymology, PFK-1 regulation, hexokinase vs glucokinase kinetics, arsenate poisoning.
130+ topic games across Biochem, Bio/Molecular, Gen/Org Chem, Physics, Psych/Soc.
Diagram: Reactome Pathway Citric acid cycle (R-HSA-71403), licensed CC BY 4.0.