Biochem mnemonic guide
How to Memorize the Krebs Cycle (Without Hating Biochem)
The Krebs cycle has eight intermediates, eight enzymes, and enough cofactors to lose an exam over. Here are the mnemonics that actually work — plus a spaced-repetition deck to lock them in.
The citric acid cycle (Krebs cycle, TCA cycle) is one of the highest-yield topics for the MCAT, USMLE Step 1, biochemistry finals, and MBBS pre-clinical exams. It's also one of the most-cited 'I studied this ten times and still can't remember it' topics on r/premed and r/medicalschool. The issue is that it's not one thing to memorize — it's the sequence, the enzymes, the cofactors, and the regulation, and they all show up on the same question.
This guide walks through the mnemonics that survive on Reddit study threads year after year (because they actually work), how to pair them with active recall, and a starter Ellie deck you can generate in one click.
Mnemonic for the 8 intermediates in order
The classic sentence is: 'Citrate Is Krebs' Special Substrate For Making Oxaloacetate.' The first letters map to the intermediates in order:
- Citrate — C
- Isocitrate — I
- α-Ketoglutarate — K (Krebs')
- Succinyl-CoA — S (Special)
- Succinate — S (Substrate)
- Fumarate — F (For)
- Malate — M (Making)
- Oxaloacetate — O
Mnemonic for the enzymes
Match each intermediate transition to its enzyme. The tricky ones are the two decarboxylations (isocitrate dehydrogenase and α-ketoglutarate dehydrogenase) and the substrate-level phosphorylation step (succinyl-CoA synthetase). Reddit threads on r/medicalschool consistently recommend the mnemonic 'Can I Keep Selling Sex For Money, Officer?' which follows the same sequence:
- Citrate synthase → makes citrate
- Aconitase → converts citrate to isocitrate
- Isocitrate dehydrogenase → first NADH, releases CO2 (rate-limiting)
- α-Ketoglutarate dehydrogenase → second NADH, releases CO2 (thiamine-dependent)
- Succinyl-CoA synthetase → GTP produced (substrate-level phosphorylation)
- Succinate dehydrogenase → makes FADH2 (also Complex II of ETC)
- Fumarase → converts fumarate to malate
- Malate dehydrogenase → third NADH, regenerates oxaloacetate
ATP yield — the number that shows up on every exam
One full turn of the Krebs cycle produces 3 NADH, 1 FADH2, 1 GTP (equivalent to ATP), and releases 2 CO2. Because glucose gives 2 acetyl-CoA molecules (one from each pyruvate), each glucose provides 2 turns. Total per glucose: 6 NADH, 2 FADH2, 2 GTP.
With oxidative phosphorylation using the current stoichiometry (2.5 ATP per NADH, 1.5 ATP per FADH2), the Krebs cycle contributes approximately 20 ATP per glucose — most of the total 30–32 ATP yield of aerobic respiration.
Regulation — what actually gets tested
The rate-limiting enzyme is isocitrate dehydrogenase. It's activated by ADP and Ca2+ and inhibited by ATP and NADH. α-Ketoglutarate dehydrogenase is also inhibited by ATP, NADH, and its own product succinyl-CoA. Both enzymes are stimulated by rising energy demand — a common Step 1 question stem.
Cofactor pearl: α-ketoglutarate dehydrogenase requires the same five cofactors as pyruvate dehydrogenase — thiamine (B1), lipoic acid, CoA (B5), FAD (B2), and NAD+ (B3). Thiamine deficiency (beriberi, Wernicke) impairs both enzymes.
Turning this into a spaced-repetition deck
Reading a mnemonic once is not enough — you need to see the intermediates in random order across weeks. Generate an Ellie deck from this page in one click and it produces cards for: intermediate ordering, enzyme matching, cofactor identification, ATP-yield calculations, and regulation. Review 15 minutes per day for two weeks and the Krebs cycle stops being a memorization exam.
Frequently asked questions
What is the easiest mnemonic for the Krebs cycle intermediates?
The most-cited mnemonic on r/medicalschool and r/premed is 'Citrate Is Krebs' Special Substrate For Making Oxaloacetate' — first letters give Citrate, Isocitrate, α-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate. If sentence mnemonics don't stick, some students prefer drawing the cycle five times from scratch on blank paper — it's rote but effective.
How many ATP does the Krebs cycle produce per glucose?
One turn of the Krebs cycle produces 3 NADH, 1 FADH2, and 1 GTP. Glucose produces 2 acetyl-CoA, so two full turns per glucose: 6 NADH, 2 FADH2, 2 GTP. Feeding those NADH and FADH2 into oxidative phosphorylation adds roughly 20 more ATP. Total contribution of the Krebs cycle: about 20 out of the ~30–32 ATP per glucose.
What is the rate-limiting enzyme of the Krebs cycle?
Isocitrate dehydrogenase. It's the third step (isocitrate → α-ketoglutarate), produces the first NADH, and releases CO2. It's activated by ADP and Ca2+ (energy demand) and inhibited by ATP and NADH (energy plenty). This is a very common Step 1 and MCAT question stem — memorize the activators and inhibitors, not just the enzyme name.
Which cofactors does α-ketoglutarate dehydrogenase need?
The same five as pyruvate dehydrogenase: thiamine pyrophosphate (B1), lipoic acid, coenzyme A (B5), FAD (B2), and NAD+ (B3). The classic exam pearl is that thiamine deficiency (Wernicke's encephalopathy, beriberi) inhibits both PDH and α-ketoglutarate dehydrogenase, which is why alcoholics develop lactic acidosis and neurologic symptoms.
Do I need to memorize the exact structures of the intermediates?
For the MCAT: know that citrate has a hydroxyl group and three carboxylates, and recognize succinate's symmetrical dicarboxylate structure. For USMLE Step 1: structures are almost never tested — the exam wants regulation, cofactors, and clinical correlations. For biochemistry courses that focus on organic mechanism, structures matter more.
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