Glycolysis & Gluconeogenesis Deep Dive

Catabolic State: Breaking Down for Energy

Catabolic = breaking down for energy = starvation state

The Nervous System Takes Over

Sympathetic nervous system controls catabolic pathways. Enzymes work hardest when PHOSPHORYLATED.

The Catabolic Hormones

Starvation signals these hormones to wake up:

Epinephrine (adrenaline) → fight or flight
Glucagon → "I need glucose"
Cortisol → stress hormone, enables muscle breakdown
Growth hormone → frees up FFAs

Second Messenger: cAMP

These hormones activate adenylyl cyclase → cAMP increases → Phosphorylates catabolic enzymes to turn them ON.

Location Matters

Mitochondria is catabolic HQ (TCA, oxidative phosphorylation, β-oxidation). Exception: Glycolysis happens in cytoplasm.

Where Your Energy Comes From

GLUCOSE

40% of diet

4 kcal/g

FATS

30% of diet

9 kcal/g

PROTEINS

30% of diet

4 kcal/g

KETONES

Last resort

> 36 hours starvation

Which Organ Prefers What Fuel?

Click a stress level to see what changes:

Organ	Normal: Preferred Fuel	Why This Matters (Board Alert!)
Brain	Glucose	Hypoglycemia destroys the brain first. Seizures, coma, death.
Heart	FFAs (60% of ATP)	In stress, heart DEMANDS glucose. Explains why acute MI + cardiogenic shock = give dextrose IV.
Muscles	Glucose + FFAs	During long fasts, muscles break down → FFAs, but also catabolized for gluconeogenesis (loss of mass).
RBCs	ALWAYS Glucose	RBCs have NO mitochondria. Only pathway = glycolysis (+ pentose phosphate for NADPH). G6PD deficiency → NADPH loss → oxidative hemolysis when triggered by oxidant stressors.

The Starvation Timeline

When glucose runs out, your body runs this playbook:

Plasma Glucose: 2-4 hours

Fed state → normal blood glucose

Liver Glycogen: 24-28 hours

Glycogenolysis maxed out, then depleted

Lipolysis: > 36 hours

Break down fat → FFAs → gluconeogenesis, ketogenesis

Ketogenesis: > 36 hours

Brain shifts to ketones, muscle finally spares glucose

⚠️ The RBC Connection: Why Hypoglycemia Kills Fast

RBCs only eat glucose via glycolysis. They have NO mitochondria → can't use FFAs, ketones, amino acids.

The Pentose Phosphate Pathway (The Other RBC Need)

RBCs need NADPH to protect themselves from oxidative damage. Only source: pentose phosphate pathway (happens in cytoplasm, requires glucose as substrate).

Board Alert: G6PD Deficiency & RBC Vulnerability

G6PD deficiency → RBCs can't make NADPH → no glutathione → oxidative stressors (primaquine, dapsone, fava beans, infections) trigger hemolytic anemia. Simple hypoglycemia alone does NOT cause hemolytic anemia in normal individuals.

Glycolysis: The Assembly Line

Click each step to reveal the enzyme, substrate, product, and ATP cost/gain.

ATP Counter

+4 created → −2 used = +2 net

The Net Yield

4 ATP created − 2 ATP invested = +2 ATP net

⚠️ Also produces 2 NADH per glucose (crucial for energy in the electron transport chain).

NADH: The Niacin (B3) Connection

Every NADH contains Niacin (Vitamin B3). Glycolysis makes 2 NADH per glucose.

Board Alert: Pellagra

Niacin deficiency → Pellagra → The 4 Ds:

Dermatitis (photosensitive rash on sun-exposed skin)
Diarrhea
Dementia
Death

Hartnup's Syndrome

Defective renal transport of tryptophan (can't reabsorb from urine) → tryptophan not available to make niacin → presents like pellagra.

2,3-DPG: The RBC Oxygen Delivery Driver

A branch off glycolysis (at G1,3DP) makes an RBC secret weapon:

What 2,3-DPG Does

2,3-DPG binds hemoglobin → lowers its O₂ affinity → hemoglobin more likely to DROP oxygen in tissues → tissues get MORE oxygen when they need it most.

The Stored Blood Problem

Stored blood loses its 2,3-DPG over time → hemoglobin grips O₂ too tightly → tissues get less O₂ → why transfusions can paradoxically worsen hypoxia if you give old blood.

The Fix: Inosine

Additive inosine to stored blood → helps RBCs regenerate 2,3-DPG → shelf life extends from 21 days → 45 days.

🔑 Sticky Mnemonics (Tap to Reveal)

🚪 The Bouncer (PFK-1)

PFK-1 is the RATE-LIMITING ENZYME → the bouncer at glycolysis' nightclub. Citrate tells it "we're full, slow down." F2,6DP says "let them in!" During exercise, PFK-1 gets too slow → F6P builds up → alternative pathway activates (F6P → F2,6DP via PFK-2) → F2,6DP activates PFK-1. Genius feedback loop.

⚗️ Mercury Loves Sulfur

Mercury blocks G3P Dehydrogenase (step 6) because Hg binds sulfur in the active site. Remember: Mad Hatter disease (mercury poisoning). In adults: tuna contamination. In kids: thermometer biting. Either way, step 6 dies → glycolysis stops → no ATP → cellular death.

🦷 Fluoride's White Teeth Clue

Fluoride blocks Enolase (step 9). Clue: fluoride = beautiful white teeth + strong bones (too much of a good thing → toxic). Enolase blocked → PEP can't form → pyruvate never forms → glycolysis stops at step 9.

🚚 2,3-DPG: The Delivery Driver

2,3-DPG is hemoglobin's delivery driver → makes it DROP oxygen off in tissues. Stored blood loses its driver (2,3-DPG degrades). Inosine = roadside assistance → helps RBCs regenerate the driver. Without it, tissues get O₂-starved from transfusion of old blood.

👮 Gluconeogenesis: 4 Bouncers

Gluconeogenesis needs 4 special bouncers to reverse glycolysis: Pyruvate Carboxylase → PEP Carboxykinase → F1,6DPase → G6Pase. Only liver (90%) and adrenal cortex (10%) have all 4 bouncers. Muscle can store glycogen but CAN'T do gluconeogenesis. Boom: that's why during starvation, muscle eats itself to feed the brain.

🔒 G6P: Glucose Trap

Glucose → G6P (step 1) is the FIRST REGULATORY STEP. G6P is charged (phosphorylated) → can't cross the cell membrane → glucose is TRAPPED inside the cell. This is how cells "capture" glucose. In the liver, G6Pase REMOVES the phosphate → glucose can escape → bloodstream gets fed.

Gluconeogenesis: Reverse Glycolysis (With Detours)

Making glucose from non-glucose sources during starvation. Only liver (90%) and adrenal cortex (10%) can do this.

Control & Signals

Triggered by epinephrine + glucagon → second messenger cAMP ↑ → activates Pyruvate Carboxylase (first committed step; the rate-limiting enzyme is Fructose-1,6-bisphosphatase).

The Pathway (7 Steps, 4 Bypasses)

Pyruvate → OAA (Oxaloacetate)

Pyruvate Carboxylase*

↓

⚠️ OAA can't cross membrane from mitochondria to cytoplasm.

↓ Solution: AST shuttle (OAA → Aspartate → shuttle out → back to OAA)

↓

OAA → PEP

PEP Carboxykinase*

↓

PEP → ... (reversible glycolysis steps)

↓

F1,6DP → F6P

F1,6DPase*

↓

F6P → G6P

↓

G6P → GLUCOSE

G6Phosphatase* (only liver!)

* Bypass enzymes → unique to gluconeogenesis, not found in glycolysis.

Enzyme Villains

Each enzyme has a story. Tap any card to flip it and get the full clinical breakdown.

🔑

Hexokinase vs Glucokinase

Step 1 Gatekeepers

Tap to reveal the war between them

The Two-Faced Gatekeeper

Hexokinase: everywhere. High affinity (low Km). Inhibited by G6P. Shuts off when the cell is full.
Glucokinase: liver only. Low affinity (high Km). Never inhibited by G6P. Keeps capturing glucose during high-glucose meals.
Clinical: hexokinase protects muscle ATP in starvation. Glucokinase buffers postprandial glucose floods in liver.

Board hit: Km difference on enzyme kinetics questions

🚪

PFK-1

Rate-Limiting Bouncer

Tap to see who controls this bouncer

The Gatekeeper of Glycolysis

Step 3: F6P to F1,6-bisphosphate. COMMITTED step.
ACTIVATED by: AMP (energy alarm), F2,6BP (master key during exercise and fed state).
INHIBITED by: ATP (full tank), citrate (Krebs backed up).
Insulin raises F2,6BP via PFK-2 activation. Glucagon lowers it.

Board hit: citrate inhibition = Randle cycle = heart in prolonged fasting

⚡

Pyruvate Kinase

Final ATP Generator

Tap to see why it kills RBCs

The Last ATP Machine

Step 10: PEP to pyruvate. Produces 2 ATP per glucose.
ACTIVATED by F1,6BP (feedforward), AMP. INHIBITED by ATP, alanine.
PK Deficiency: RBCs have no mitochondria. 100% ATP from glycolysis. No PK = no ATP = membrane failure = hemolytic anemia. Echinocytes, splenomegaly.

Board hit: chronic hemolytic anemia, negative Coombs, elevated 2,3-DPG

✂️

Aldolase

The Carbon Splitter

Tap to see what it splits and why it matters

The 6-Carbon Cleaver

Step 4: splits F1,6BP into DHAP + G3P (each 3-carbon).
DHAP converts to G3P via triose phosphate isomerase. Net: 1 glucose = 2 G3P molecules downstream.
Aldolase B deficiency: hereditary fructose intolerance. Fructose-1-P sequesters phosphate, crashes ATP and glucose.

Board hit: vomiting + hypoglycemia after fruit = Aldolase B deficiency

⚗️

GAPDH

Mercury's Favorite Target

Tap to see how mercury kills step 6

Glyceraldehyde-3-Phosphate DH

Step 6: G3P to 1,3-BPG. Produces first NADH. Requires NAD+.
Mercury (Hg) binds sulfhydryl group in active site. Irreversible. Step 6 dies.
Source of 2,3-DPG in RBCs via the Luebering-Rapaport shunt.
Niacin (B3) deficiency: NAD+ runs out and glycolysis stalls here.

Board hit: tuna history + neurological decline = mercury blocks GAPDH

🩸

PK Deficiency

The RBC Killer

Tap to see the full clinical picture

Pyruvate Kinase Deficiency

Most common glycolytic enzyme deficiency.
No PK = no ATP at step 10 = membrane pump failure = echinocytes.
Labs: hemolytic anemia, elevated 2,3-DPG, negative Coombs, splenomegaly.
Contrast with G6PD deficiency: oxidative trigger needed. PK deficiency is chronic, not episodic.

Board hit: chronic hemolytic anemia in newborn, elevated 2,3-DPG

Clinical Decision Tree

Walk through substrate, product, ATP yield, and clinical implication step by step.

A patient's RBCs are failing to produce ATP despite normal blood glucose. Which substrate enters glycolysis first?

Glucose

Fructose-6-Phosphate

Pyruvate

Glucose. Step 1 traps it as G6P. It cannot leave once phosphorylated. This trapping step is why hypoglycemia starves cells before glycolysis even starts.

F6P is step 2, not the entry. Glucose must be phosphorylated to G6P first, then isomerized to F6P.

Pyruvate is the exit product, not the entry. Glucose in, pyruvate out.

Glucose reaches PFK-1, the rate-limiting step. What is the direct product of PFK-1 action?

Fructose-1,6-bisphosphate

DHAP + G3P

Fructose-2,6-bisphosphate

Fructose-1,6-bisphosphate. PFK-1 adds a second phosphate at carbon 1. Aldolase (step 4) then cleaves it into two 3-carbon pieces.

DHAP and G3P are products of Aldolase (step 4), not PFK-1. PFK-1 makes F1,6BP, which Aldolase then splits.

F2,6BP is the regulator of PFK-1, not its product. It is made by PFK-2, a separate enzyme.

One glucose yields how much net ATP from glycolysis alone?

2 net ATP

4 ATP

6 net ATP

2 net ATP. Investment phase: 2 ATP spent (steps 1 and 3). Return phase: 4 ATP made (steps 7 and 10, x2 from carbon split). 4 minus 2 = 2 net. Plus 2 NADH for downstream ETC use.

4 ATP is the gross production. You spent 2 ATP at steps 1 and 3. Net = 2. Gross vs net is a classic board trap.

Too high. Net glycolysis ATP is always 2. The bulk of ATP (30 total) comes from Krebs and ETC downstream.

A newborn has chronic hemolytic anemia. ATP in RBCs is critically low despite intact G6PD and normal blood glucose. Which enzyme deficiency explains this?

Pyruvate Kinase deficiency

Hexokinase deficiency

Phosphoglucose Isomerase deficiency

Pyruvate Kinase deficiency is the most common glycolytic enzyme defect causing hemolytic anemia. Step 10 fails, no ATP at the final yield step, RBC membrane pump fails. Echinocytes, hemolysis, splenomegaly. Elevated 2,3-DPG because substrate backs up upstream.

Hexokinase deficiency is the second most common glycolytic enzyme defect but far rarer. PK deficiency is the board answer for this vignette.

PGI deficiency is very rare and presents with milder hemolysis. Not the first-line answer. PK deficiency is the classic board answer here.

Sticky Memory Hooks

Tap any highlighted term for the memory hook.

Regulatory Enzymes

HexokinaseHigh-affinity trapper in muscle and brain. Activates at low blood sugar. Gets product-inhibited by G6P accumulation. Shuts off when the cell is already full of phosphorylated glucose. catalyzes step 1 in most tissues, while the liver uses GlucokinaseThe liver's glutton. High Km means it only activates when glucose is flooding in after a meal. Never inhibited by G6P. Keeps eating glucose until blood sugar drops back to normal. The liver buffers postprandial glucose spikes through this enzyme. instead. The rate-limiting step is catalyzed by PFK-1The bouncer. Citrate says slow down (Krebs is full). F2,6BP says let them in (fed state or exercise). AMP says run faster (energy is low). ATP says relax (we have plenty). PFK-1 weighs all four and sets the pace of glycolysis., which is activated by F2,6-bisphosphateThe master key. Made by PFK-2 when insulin is high (fed state) or AMP is high (exercise). Overrides ATP and citrate inhibition of PFK-1. The most potent PFK-1 activator. Without it, glycolysis idles even in high glucose. and inhibited by citrateKrebs cycle's messenger to glycolysis: stop sending fuel, we are backed up. When fat is burning and Krebs is loaded, citrate leaks into the cytoplasm and hits the brakes on PFK-1. This is the Randle cycle. The heart is especially vulnerable during prolonged fasting..

Enzyme Poisons

Mercury (Hg)Mad Hatter disease. Mercury is a sulfur-lover. It finds the sulfhydryl group in GAPDH and latches on permanently. Step 6 dies. No NADH, no 1,3-BPG, glycolysis stops. Clinical clue: tuna or seafood history plus neurological symptoms. blocks GAPDH at step 6 by attacking the sulfhydryl group. FluorideToo much toothpaste. Fluoride inhibits Enolase (step 9) by chelating the Mg2+ cofactor. This is also why fluoride is added to blood collection tubes: it stops glycolysis in the sample by blocking Enolase, preserving blood glucose levels for lab analysis. blocks Enolase at step 9 by chelating Mg2+. ArsenicThe phosphate impostor. Arsenate mimics inorganic phosphate (Pi). It slips into the GAPDH reaction where Pi should bind, creating 1-arsenate-3-phosphoglycerate instead of 1,3-BPG. This intermediate spontaneously hydrolyzes. No ATP is made at step 7. The entire investment payoff of glycolysis is wiped out without a direct block on the enzyme itself. mimics phosphate in the GAPDH reaction, bypassing ATP production at step 7.

RBC-Specific Hooks

2,3-DPGThe delivery driver for hemoglobin. Binds beta-chains and forces hemoglobin to release oxygen in tissues. Stored blood loses 2,3-DPG over time. Inosine in storage media helps regenerate it. High altitude raises 2,3-DPG as an adaptation, shifting the O2-Hgb curve right. is generated via the Luebering-Rapaport shunt off step 6. PK DeficiencyThe RBC's ATP bank closes at step 10. No PK = no ATP = Na/K-ATPase fails = membrane crumples into echinocytes = spleen eats them. Chronic hemolytic anemia. Negative Coombs confirms no immune cause. Elevated 2,3-DPG from upstream backup shifts the O2-Hgb curve right, partially compensating. causes chronic hemolytic anemia because RBCs have no mitochondriaNo Krebs. No ETC. No fatty acid oxidation. Glycolysis is the only ATP source. That is why any glycolytic enzyme deficiency hits RBCs hardest. Every other cell can switch fuels. RBCs cannot., making glycolysis their only ATP source.

Gluconeogenesis Bypass Hooks

Gluconeogenesis requires Pyruvate CarboxylaseFirst reverse bounce. Pyruvate to OAA in mitochondria. Needs biotin. Activated by acetyl-CoA (fat burning signals: make OAA so Krebs can handle the acetyl-CoA load). Also the anaplerotic enzyme: keeps OAA stocked for Krebs when it is being consumed. in the mitochondria, then PEPCK in the cytoplasm via PEPCKRate-limiting enzyme of gluconeogenesis. Decarboxylates OAA to PEP. Induced by glucagon and cortisol. Inhibited by insulin. Its mRNA takes hours to increase, which is why gluconeogenesis cannot turn on instantly during acute hypoglycemia.. The irreversible glycolytic bypasses use F1,6-bisphosphataseReverses PFK-1's step. Removes phosphate from F1,6BP to make F6P. Inhibited by AMP (energy low, do not waste glucose). Activated by citrate. Only in liver and kidney. That is why only these organs can perform gluconeogenesis. and G6PhosphataseExit gate. Dephosphorylates G6P to free glucose for export into blood. Only in liver, kidney, and small intestine. Muscle lacks it. Muscle glycogen cannot raise blood glucose directly. Von Gierke disease: G6Pase missing. Glucose trapped in liver. Severe fasting hypoglycemia + hepatomegaly..

Clinical Photo Gallery

Tap any image to expand. Images via Wikimedia Commons (CC license).

ATP Structure Energy currency made at steps 7 and 10. Tap to expand.

Normal RBCs (SEM) No mitochondria = glycolysis only. Tap to expand.

Extra Board-Style Questions

Eight more questions. Pick the answer, then read the full breakdown.

A 4-year-old ingests a large amount of toothpaste. Elevated fluoride is confirmed. Which glycolytic step is most directly blocked?

Step 6 (GAPDH): blocked by arsenate mimicking phosphate

Step 3 (PFK-1): blocked by fluoride

Step 9 (Enolase): fluoride chelates Mg2+ cofactor

Step 10 (Pyruvate Kinase): blocked by fluoride

Glucokinase has a higher Km than Hexokinase. What is the physiological significance of this in the liver?

Glucokinase acts as a glucose sensor, activating only during postprandial hyperglycemia to buffer excess glucose

Glucokinase has higher affinity than Hexokinase and captures glucose even at low blood levels

Glucokinase is inhibited by G6P, preventing excess glucose trapping in hepatocytes

Glucokinase is the rate-limiting enzyme of glycolysis in hepatocytes

A patient presents with hemolytic anemia and elevated 2,3-DPG on RBC analysis. G6PD is intact. Which enzyme is most likely deficient?

G6PD

Pyruvate Kinase (PK)

Hexokinase

GAPDH

Insulin acts on the liver to increase glycolysis. Which molecular intermediate is elevated as a result of insulin signaling at PFK-2?

AMP

Citrate

Fructose-2,6-bisphosphate (F2,6BP)

ATP

Arsenate is added to a cell-free glycolysis system. Which step is sabotaged and what is the energy consequence?

Step 3 (PFK-1): arsenate displaces AMP, slowing catalysis

Step 9 (Enolase): arsenate chelates the active site metal

Step 6 (GAPDH): arsenate substitutes for Pi, forming unstable 1-arsenate-3-PG that hydrolyzes without producing ATP

Step 10 (PK): arsenate competes with ADP, blocking ATP synthesis

A long-distance cyclist at 3,500 m altitude shows a rightward shift of the O2-hemoglobin dissociation curve after 2 weeks. Which glycolytic metabolite is responsible for this chronic adaptation?

Lactate

CO2

2,3-DPG from the glycolytic Luebering-Rapaport shunt

Erythropoietin (EPO)

A patient with end-stage liver failure has hypoglycemia and lactic acidosis. Which gluconeogenic limitation best explains the hypoglycemia?

Loss of Glucokinase, preventing glucose entry into hepatocytes

Loss of all four gluconeogenic enzymes housed exclusively in the liver

Muscle glycogen depletion causing loss of systemic glucose supply

The brain switching to gluconeogenesis, consuming excess glucose

A patient on valproate develops fasting hypoglycemia. Which gluconeogenic enzyme is most sensitive to valproate-related inhibition?

Pyruvate Carboxylase

PEPCK (Phosphoenolpyruvate Carboxykinase)

Fructose-1,6-bisphosphatase

Glucose-6-Phosphatase

Board-Style Vignettes

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Glycolysis & Gluconeogenesis