Glucose Metabolism

BCH 100 — Introductory Biochemistry · Dr. Radi

build Jul 17 · 19:00 · CC BY-NC-SA 4.0 · owned figures (RDKit / matplotlib / PyMOL)
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By the end of this unit, you can…

  • Walk all ten steps of glycolysis — the enzymes, the reactions, and the running ATP/NADH ledger (net 2 ATP + 2 NADH per glucose)
  • Identify the regulated steps (hexokinase, PFK-1, pyruvate kinase) and the hormonal + fructose-2,6-bisphosphate control of glycolysis
  • Trace the fates of pyruvate — aerobic oxidation, lactate fermentation, and ethanol fermentation — and the Cori cycle
  • Explain how galactose and fructose feed into glycolysis, and the diseases when they can't (galactosemia, fructose→fat)
  • Describe gluconeogenesis and its four bypass enzymes, and how glycogen is built and broken down under hormonal control
Dr. Radi

Today's route 🗺️

  1. Glycolysis — The Ten Steps
  2. Glycolysis — Energy Yield & Regulation
  3. Other Sugars — Galactose & Fructose
  4. Fermentation & the Cori Cycle
  5. Gluconeogenesis — Making Glucose
  6. Glycogen — Storing & Mobilizing Glucose
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1 · Glycolysis — The Ten Steps

"SUGAAAARRRR!!! Walk glucose to pyruvate — all ten enzymes, two ATP invested, four made, two NADH banked."

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SUGAAAARRRR!!! 🍬

Glucose is a freaking fantastic fuel — the universal one, burned by almost every cell on Earth. Glycolysis ("sugar-splitting") is where it starts: ten enzymes in the cytosol chop one 6-carbon glucose into two 3-carbon pyruvate, banking a little ATP and NADH along the way.

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The plan: spend a little to make a lot

Glycolysis runs in two halves. The investment phase (steps 1–5) spends 2 ATP to prime and split the sugar. The payoff phase (steps 6–10) earns it back with interest — 4 ATP + 2 NADH. You have to spend before you earn.

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Step 1: Hexokinase

Glucose + ATP → glucose-6-phosphate + ADP. The very first thing we do is phosphorylate C6 — and it costs us. That charged phosphate traps glucose inside the cell (it can't slip back across the membrane). Irreversible and regulated. Needs Mg²⁺.

ATP −1 · running total: ATP −1
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Why Step 1 only goes one way

Phosphorylating glucose by itself is uphill. Hexokinase couples it to ATP hydrolysis — and that downhill drop more than pays for it, so the whole step tips strongly downhill. A one-way valve like this is exactly where the cell puts a control point.

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Step 2: Phosphoglucose isomerase

Glucose-6-P → fructose-6-P. A quick isomerization: an aldose (six-membered ring) becomes a ketose (five-membered ring). Why bother? It sets up a symmetric sugar we can cleanly split in half two steps from now. Freely reversible.

no ATP change · running total: ATP −1
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Step 3: PFK-1 — the committed step

Fructose-6-P + ATP → fructose-1,6-bisphosphate + ADP. Phosphorylate the other end (C1) — spending our second ATP. This is THE commitment: past here, the sugar is going through glycolysis. PFK-1 is the most regulated enzyme in the pathway. Irreversible.

ATP −1 · running total: ATP −2
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Why PFK-1 is the master switch

Same trick as hexokinase — coupled to ATP hydrolysis, so it's a strongly downhill, irreversible valve. But this one sits at the branch point where glucose is committed. Put your thermostat on the one-way door, and PFK-1 becomes the pathway's main control point.

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Step 4: Aldolase — the split!

Fructose-1,6-bisphosphate → DHAP + glyceraldehyde-3-phosphate. Aldolase cleaves the C3–C4 bond, breaking one 6-carbon sugar into two 3-carbon pieces. This is the lysis in glyco-lysis. From here on, everything happens twice.

×2 begins · one glucose is now two 3-carbon molecules · running total: ATP −2
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Step 5: Triose phosphate isomerase

DHAP → glyceraldehyde-3-phosphate. Only G3P can continue — so TPI converts the "dead-end" DHAP into a second G3P. Now we have two identical G3P, and one of the fastest enzymes known makes sure neither carbon is wasted.

no ATP change · now ×2 · running total: ATP −2
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You are here — the payoff begins

We've spent our 2 ATP and split the sugar into two G3P. The books are in the red. Everything from here counts twice, and it's all downhill earnings — 4 ATP and 2 NADH on the way to pyruvate.

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Step 6: G3P dehydrogenase (GAPDH)

G3P + NAD⁺ + Pᵢ → 1,3-bisphosphoglycerate + NADH. The first energy capture! We oxidize the aldehyde (reducing NAD⁺ → NADH) and use that energy to attach a phosphate, making a high-energy acyl-phosphate. Reversible.

NADH +2 (×2) · running total: ATP −2, NADH +2
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Step 7: Phosphoglycerate kinase

1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP. That high-energy phosphate we just installed gets handed straight to ADP → ATP. This is substrate-level phosphorylation — and it's our first ATP made (×2, so we've now broken even).

ATP +2 (×2) · first ATP made! · running total: ATP 0, NADH +2
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Step 8: Phosphoglycerate mutase

3-phosphoglycerate → 2-phosphoglycerate. A little housekeeping: move the phosphate from C3 to C2. No energy in, no energy out — but this sets up the next enzyme to build a really high-energy phosphate. Reversible.

no ATP change · running total: ATP 0, NADH +2
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Step 9: Enolase

2-phosphoglycerate → phosphoenolpyruvate (PEP) + H₂O. Enolase pulls out a water, forming a double bond — and that turns the phosphate into a super high-energy enol phosphate (PEP), one of the most energetic little molecules in the cell. Reversible.

no ATP change · running total: ATP 0, NADH +2
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Step 10: Pyruvate kinase

PEP + ADP → pyruvate + ATP. PEP is so high-energy it powers a second substrate-level phosphorylation — our last ATP. The enol snaps to the stable keto form of pyruvate, locking the step irreversible and regulated. We made it!

ATP +2 (×2) · running total: ATP +2, NADH +2 → NET
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Why Step 10 is a one-way payoff

PEP is so high-energy that even after paying to build an ATP, the step still drops strongly downhill — that's why it's irreversible and a control point. This enormous drop is what makes glycolysis able to make ATP with no oxygen at all.

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So what happens to pyruvate?

We netted 2 ATP + 2 NADH — but the 2 pyruvate still hold most of glucose's energy. What happens next comes down to oxygen: with O₂, pyruvate is fully burned in the mitochondria; without it, we ferment to keep glycolysis running.

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Cells that live on glycolysis alone

Some cells run almost entirely on glycolysis. Your red blood cells have no mitochondria at all — glycolysis is their only ATP source. Fast-twitch muscle leans on it during a sprint, and many tumors crank it up (the Warburg effect). Two ATP per glucose, no oxygen required — humble, but life-saving.

Red blood cells: Arek Socha, CC0 (Wikimedia Commons)
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2 · Glycolysis — Energy Yield & Regulation

"Add up the ATP, then meet the control panel — the three valves, fructose-2,6-bisphosphate, and the hormones that flip the switch."

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Was it worth it? Do the accounting

Let's settle the books. We invested 2 ATP (hexokinase, PFK-1) and earned 4 ATP + 2 NADH — remember, everything past the split counts ×2. Net: 2 ATP + 2 NADH per glucose. And those 2 NADH cash in for ~5 more ATP later in the electron transport chain!

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Where do you put the controls?

You regulate a pathway at its irreversible steps — the one-way valves, where there's no going back. Glycolysis has exactly three: hexokinase, PFK-1, and pyruvate kinase. The other seven run near equilibrium and just follow along. Control the valves, control the flow.

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PFK-1: the master switch

PFK-1 is where the cell reads its own energy level and decides. Plenty of ATP or citrate? Turn glycolysis down — we don't need more fuel. Lots of AMP (energy's running low)? Turn it up. It's a beautiful piece of feedback — the pathway listens to the cell.

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The secret weapon: fructose-2,6-bisphosphate

The most powerful activator of PFK-1 isn't ATP or AMP — it's fructose-2,6-bisphosphate. It's made by a wild bifunctional enzyme (PFK-2) that's both a kinase and a phosphatase. Which half wins? Hormones decide — by phosphorylating it.

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Hormones flip the whole switch

Zoom out: the hormones set the tone for the whole body. Insulin (fed) says burn the glucose — glycolysis ON. Glucagon (fasting) says save it for the brain — glycolysis OFF. Epinephrine (stress) mobilizes fuel fast. They act largely through fructose-2,6-bisphosphate.

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The switch, in real life — and in diabetes

This is why your blood sugar stays remarkably steady: after a meal, insulin flips glycolysis on and stashes the glucose; while you fast, glucagon flips it off. In diabetes, that hormonal switch is broken — glucose can't get the "burn me" signal, so it stays dangerously high.

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3 · Other Sugars — Galactose & Fructose

"Glucose isn't the only sugar you eat — see how galactose and fructose sneak into glycolysis, and what goes wrong when they can't."

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Glucose isn't the only sugar you eat

You eat plenty of galactose (from milk) and fructose (from fruit and — a lot — from soda). Neither can run glycolysis directly. The trick: the cell converts each one into a glycolysis intermediate, then lets the main pathway take over.

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Galactose: the Leloir pathway

Galactose gets a phosphate, then swaps places with a glucose using the enzyme GALT (and a UDP-glucose helper), ending up as glucose-6-phosphate — right in the middle of glycolysis. Three little steps and it's just glucose again.

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When galactose can't get through: galactosemia

Babies born without GALT can't process the galactose in milk — so it backs up and becomes toxic. The result: jaundice, an enlarged liver, and cataracts. This is why every newborn is screened; the treatment is simply a galactose-free (lactose-free) diet.

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Fructose: a shortcut past the guard

In the liver, fructose takes a different door: fructokinase and aldolase B chop it straight into trioses — dropping it into glycolysis below PFK-1. Remember, PFK-1 is the main control valve — so fructose skips the regulation entirely. (Muscle plays it safe: fructose → fructose-6-P.)

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Why HFCS hits different

Here's the catch. Glucose that piles up trips PFK-1's "we have enough — slow down" signal. Fructose never sees that brake, so a big dose of high-fructose corn syrup pours straight through to acetyl-CoA and gets stashed as fat. Same calories, very different metabolic ride.

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One more: lactose intolerance

Lactose (milk sugar) is glucose + galactose — but you need the enzyme lactase to split it. Most adults lose lactase after childhood, so lactose sails on to the colon, where gut bacteria ferment it into gas (CH₄ + H₂) — cue the bloating.

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4 · Fermentation & the Cori Cycle

"No oxygen? No problem. See how fermentation keeps glycolysis alive by recycling NAD⁺ — and how the liver bails out your sprinting muscles."

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Pyruvate is a crossroads — and there's a catch

Pyruvate can go three ways: with oxygen, into the mitochondria as acetyl-CoA to feed the citric acid cycle; without it, to lactate or ethanol. Why the detour? Glycolysis spends NAD⁺ at step 6 (GAPDH) — and there's barely any. If we don't get NAD⁺ back, glycolysis grinds to a halt.

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Lactate fermentation

When your muscles outrun their oxygen supply, they take the shortcut: pyruvate + NADH → lactate, run by lactate dehydrogenase. The whole point isn't the lactate — it's regenerating NAD⁺ so glycolysis (and that quick 2 ATP) can keep going. Your red blood cells do this all the time.

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Ethanol fermentation

Yeast solves the same NAD⁺ problem differently — in two steps. Pyruvate decarboxylase knocks a CO₂ off pyruvate to make acetaldehyde (those are the bubbles!), then alcohol dehydrogenase reduces it to ethanol, regenerating NAD⁺ on that second step. Humanity has been exploiting this for beer, wine, and bread for millennia.

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Lactate vs ethanol: same goal, different exit

Both fermentations exist for one reason: regenerate NAD⁺. They just dump the electrons onto different molecules. We (and lactic-acid bacteria) make lactate; yeast makes ethanol + CO₂. Neither one makes extra ATP — the payoff is simply keeping glycolysis running.

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The Cori cycle: the liver has your back

That lactate from your muscles doesn't just sit there. It rides the blood to the liver, which spends 6 ATP to turn it back into glucose (gluconeogenesis) and ships it out again. Your muscle got a quick 2 ATP; the liver eats the cost. The liver really is the most giving organ!

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Fermentation, in a glass

That "reduce pyruvate to regenerate NAD⁺" trick is doing real work every time yeast turns grape sugar into wine — the alcohol and the fizz are literally ethanol and CO₂ from glycolysis running without oxygen. Same chemistry as the burn in your legs, just a tastier product.

Red wine: André Karwath, CC BY-SA 3.0 (Wikimedia Commons)
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5 · Gluconeogenesis — Making Glucose

"Run glycolysis backward to make new glucose — but three steps won't reverse, so four special enzymes bypass them."

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Making glucose from scratch

Your brain and red blood cells run on glucose — so when you're fasting, the liver makes fresh glucose from pyruvate, lactate, and amino acids. It's mostly glycolysis run in reverse… but three steps are irreversible and simply won't back up. Gluconeogenesis needs four special enzymes to go around them.

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Bypass 1, part 1: Pyruvate carboxylase

Pyruvate + CO₂ → oxaloacetate. We can't push pyruvate straight back to PEP, so we take a detour through oxaloacetate. This happens in the mitochondrion, costs an ATP, and needs the vitamin cofactor biotin to carry the CO₂.

−1 ATP · + CO₂ · needs biotin · in the mitochondrion
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Bypass 1, part 2: PEPCK

Oxaloacetate → phosphoenolpyruvate + CO₂. Now PEPCK pops that CO₂ back off and adds a phosphate (using GTP) to make PEP — finally past the pyruvate-kinase roadblock. Two enzymes, one bypass.

−1 GTP · − CO₂ · makes PEP
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Bypass 2: Fructose-1,6-bisphosphatase

Fructose-1,6-bisphosphate → fructose-6-P + Pᵢ. PFK-1 was irreversible, so we don't run it backward — instead we just hydrolyze the phosphate off with a phosphatase. No ATP made, no ATP spent — the phosphate simply leaves as Pᵢ.

releases Pᵢ · reciprocal with PFK-1 (never both at once)
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Bypass 3: Glucose-6-phosphatase

Glucose-6-P → glucose + Pᵢ. The last hurdle: hexokinase had trapped glucose by phosphorylating it, so to release free glucose we hydrolyze that phosphate. This enzyme lives in the ER, mostly in liver and kidney — the body's glucose exporters.

releases Pᵢ · mainly liver & kidney · glucose → blood
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One on, the other off

Glycolysis and gluconeogenesis are opposite directions — running both at once would just burn ATP in a circle. So they're reciprocally regulated: glucagon and cortisol (fasting) turn gluconeogenesis on, insulin (fed) turns it off, and fructose-2,6-bisphosphate flips the two in opposite directions at PFK-1 / FBPase-1.

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The cost — and why the liver pays it

Gluconeogenesis is expensive: 6 high-energy phosphates (ATP + GTP) to build one glucose, versus the 2 ATP glycolysis got from splitting one. That's why it's a fasting program, run when blood sugar dips — and why the liver's ability to do it keeps your brain fueled through the night.

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6 · Glycogen — Storing & Mobilizing Glucose

"Meet glycogen — your glucose battery. Build it, break it, and see how one phosphate flips the whole thing between store and spend."

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Glycogen: your glucose battery

When glucose is plentiful, you don't leave it floating around — you store it as glycogen, a big branched polymer of glucose. Straight chains use α-1,4 bonds; every ~10 residues an α-1,6 bond starts a branch. Why so branched? Many free ends means you can release glucose fast when you need it.

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Breaking it down

Glycogen phosphorylase chews glucose off the ends one at a time — and cleverly uses phosphate (not water) so the product comes out already primed as glucose-1-phosphate → glucose-6-P (muscle burns it in glycolysis; the liver frees it as blood glucose). It stops near a branch, where the debranching enzyme clears the α-1,6 knot.

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Building it up

Synthesis needs a primer: the protein glycogenin starts the chain by adding glucose to itself. Then glycogen synthase extends the α-1,4 chain — adding glucose delivered as UDP-glucose, the activated donor — and a branching enzyme relocates a block to make the α-1,6 branches.

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One phosphate, opposite effects

Here's the elegant part: a single phosphorylation controls both enzymes — but in opposite directions. Adding a phosphate turns phosphorylase ON (break down) and synthase OFF (stop building). So you never waste energy doing both at once. AMP (low energy) also flips phosphorylase on; ATP and glucose-6-P (plenty) turn it off.

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Hormones: store or mobilize

The hormones drive that phosphorylation. Glucagon (fasting) and epinephrine (stress) fire off a cAMP → PKA cascade that phosphorylates everything → breakdown ON. Insulin (fed) activates a phosphatase that strips the phosphates off → synthesis ON. Store when fed, mobilize when stressed.

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When storage goes wrong

If an enzyme in this cycle is missing, glycogen piles up where it shouldn't — the glycogen storage diseases. In von Gierke disease (no glucose-6-phosphatase), the liver swells with glycogen and blood sugar crashes between meals. You can see it here: hepatocytes so stuffed with glycogen they look pale and ballooned.

Liver histology (glycogen storage disease): Nephron, CC BY-SA 3.0 (Wikimedia Commons)
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Can you…?

  • ☐ walk all ten steps of glycolysis — the enzymes, the reactions, and the running ATP/NADH ledger (net 2 ATP + 2 NADH per glucose)?
  • ☐ identify the regulated steps (hexokinase, PFK-1, pyruvate kinase) and the hormonal + fructose-2,6-bisphosphate control of glycolysis?
  • ☐ trace the fates of pyruvate — aerobic oxidation, lactate fermentation, and ethanol fermentation — and the Cori cycle?
  • ☐ explain how galactose and fructose feed into glycolysis, and the diseases when they can't (galactosemia, fructose→fat)?
  • ☐ describe gluconeogenesis and its four bypass enzymes, and how glycogen is built and broken down under hormonal control?

If any box stays empty, the practice site has a drill for it. 🧪

Dr. Radi