Liver does 90 percent of it. Kidney cortex handles the rest during prolonged fasting. Muscle and brain cannot make glucose: they lack glucose-6-phosphatase, the final exit enzyme.
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When It Runs
Fasting state, starvation, intense exercise. Glucagon switches it on. Insulin shuts it off. The body needs about 200 grams of glucose per day just for brain and red blood cells.
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The Substrates
Lactate from red blood cells and muscle, alanine from muscle protein breakdown, glycerol from adipose triglycerides. Acetyl-CoA from fat cannot be used: there is no net path from fat to glucose in humans.
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The Key Rule
Gluconeogenesis is glycolysis run backward, except at three irreversible steps. Those three steps need four bypass enzymes (one step needs two enzymes working together). Learn the bypasses and you own the pathway.
Clinical anchor: Every clinical scenario involving gluconeogenesis failure produces the same core symptom: fasting hypoglycemia. The organ fails to export glucose when the blood sugar drops. The specific enzyme that fails determines the other features: Von Gierke (G6Pase) adds hepatomegaly + hyperlipidemia + hyperuricemia. Biotin deficiency (PC) is isolated fasting hypoglycemia. PEPCK deficiency is early severe hypoglycemia with normal liver glycogen. Always ask: which step in gluconeogenesis is broken, and what backs up upstream of it?
Energy cost: Making one molecule of glucose from scratch costs 6 ATP equivalents (4 ATP + 2 GTP). Glycolysis only yields 2 ATP per glucose. The liver "invests" 6 ATP (consuming its own stored energy) to make glucose it will export for free to the brain and RBCs. During prolonged starvation, the liver is essentially subsidizing the rest of the body from its own reserves.
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The 4 Bypass Enzyme Bosses
Tap each card to flip and learn why it matters
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Pyruvate Carboxylase
Mitochondrial Starter
LocationMitochondria
CofactorBiotin + ATP
SubstratePyruvate
ProductOxaloacetate
BIOTIN-DEPENDENT
tap to flip
Why It Matters
What It Does
Converts pyruvate to oxaloacetate (OAA). This is the first step into gluconeogenesis from pyruvate. The enzyme fixes a CO2 molecule onto pyruvate, and biotin acts as the CO2 carrier. ATP is consumed in the process.
Activation
Activated by acetyl-CoA. Why does that make sense? High acetyl-CoA means lots of fat breakdown is occurring, which signals the body is fasting and needs to make glucose. So acetyl-CoA tells pyruvate carboxylase to start the gluconeogenesis chain.
Clinical Note
Biotin deficiency knocks this enzyme out. Patients develop hypoglycemia during fasting because pyruvate cannot enter gluconeogenesis. Biotin deficiency can come from prolonged raw egg white ingestion (avidin binds biotin) or rare genetic biotinidase deficiency.
Lock It
Activated by acetyl-CoA. Inhibited by ADP. Requires biotin. If a question mentions a vitamin-dependent carboxylase causing hypoglycemia, this is your enzyme.
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PEPCK
PEP Carboxykinase
LocationCyto + Mito
CofactorGTP
SubstrateOAA
ProductPEP
RATE CONTROL PARTNER
tap to flip
Why It Matters
What It Does
Converts oxaloacetate to PEP and releases CO2. Uses GTP, not ATP. This works in partnership with pyruvate carboxylase to bypass the pyruvate kinase step of glycolysis, which is irreversible.
Two Steps For One Bypass
Pyruvate carboxylase makes OAA inside the mitochondria. PEPCK then converts OAA to PEP. OAA can also be shuttled to the cytoplasm first as malate, where cytoplasmic PEPCK takes over. Either way, you need two enzymes to bypass one glycolysis step.
Regulation
Induced by glucagon through the cAMP and CREB transcription factor pathway. Suppressed by insulin at the gene level. Cortisol also induces PEPCK during prolonged stress and starvation.
Lock It
Regulation of PEPCK is transcriptional, not allosteric. Hormones change how much PEPCK is made, not how active existing enzyme is. This is slower regulation than the on-off switches at FBPase-1.
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FBPase-1
Rate-Limiting Gate
LocationCytoplasm
SubstrateF1,6BP
ProductF6P
BypassesPFK-1
RATE-LIMITING STEP
tap to flip
Why It Matters
What It Does
Converts fructose-1,6-bisphosphate to fructose-6-phosphate by removing the 1-phosphate. This bypasses the irreversible PFK-1 step of glycolysis. It is the rate-limiting step of gluconeogenesis.
Inhibitors
AMP inhibits FBPase-1: low energy state means no glucose synthesis (cell needs to make ATP, not spend it). Fructose-2,6-bisphosphate (F2,6BP) is the most potent inhibitor: it is made by PFK-2 when insulin is high.
Activated By
Glucagon raises cAMP, which activates PKA. PKA phosphorylates PFK-2, flipping it into phosphatase mode. F2,6BP levels then fall, releasing FBPase-1 from inhibition. FBPase-1 turns on and gluconeogenesis runs. Citrate is also a positive allosteric activator (high Krebs output signals enough energy to make glucose).
Lock It
F2,6BP controls both pathways at once: it activates PFK-1 (glycolysis on) and inhibits FBPase-1 (gluconeogenesis off). One molecule decides which way carbon flows in the liver.
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G6Pase
The Exit Gate
LocationER membrane
SubstrateG6P
ProductFree Glucose
Found InLiver + Kidney
MUSCLE CANNOT EXPORT
tap to flip
Why It Matters
What It Does
Converts glucose-6-phosphate to free glucose plus inorganic phosphate. This is the final step that releases free glucose into the bloodstream. Glucose without the 6-phosphate can leave the cell through GLUT2.
Why Muscle Cannot
Muscle lacks glucose-6-phosphatase. So G6P in muscle stays trapped: it either gets stored as glycogen or fed through glycolysis. Muscle can never release free glucose into the blood. This is why muscle glycogen is selfish: it serves muscle only.
Von Gierke Connection
G6Pase deficiency causes Von Gierke disease (Type Ia glycogen storage disease). Glucose gets trapped as G6P, causing massive hepatomegaly, severe fasting hypoglycemia, lactic acidosis, and hyperuricemia. It is the classic example of why this enzyme matters.
Lock It
Absent in muscle and brain. That is exactly why only liver and kidney raise blood glucose. If a question asks which tissue contributes to blood glucose during fasting, the answer is liver first, kidney second, never muscle.
Quick pattern for the 4 bypasses: Two bypass the pyruvate kinase step together (PC first, PEPCK second, both needed in sequence). One bypasses PFK-1 (F1,6BPase, rate-limiting). One bypasses hexokinase (G6Pase, liver/kidney only). If a board question mentions biotin deficiency, PC is the target. If it mentions GTP, PEPCK is the enzyme. If it mentions metformin or AMP, F1,6BPase is the enzyme. If it mentions Von Gierke or why muscle can't export glucose, G6Pase is the enzyme.
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The Gluconeogenesis Pathway
3 irreversible glycolysis steps, 4 bypass enzymes, one direction: up toward glucose
Steps 2 through 9 run shared enzymes with glycolysis, but in reverse. Only the bypass points differ.
Subcellular compartment logistics: Gluconeogenesis spans two compartments. Pyruvate carboxylase makes OAA inside the mitochondria. OAA cannot cross the inner mitochondrial membrane directly. It is first converted to malate (by mitochondrial malate dehydrogenase) or to aspartate (by mitochondrial aspartate aminotransferase) to be shuttled into the cytoplasm. Once in the cytoplasm, it is reconverted to OAA, then PEPCK acts on it to make PEP. All remaining steps of gluconeogenesis occur in the cytoplasm. This compartmental separation is why gluconeogenesis requires two separate enzyme steps to bypass the single pyruvate kinase reaction.
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The Cori Cycle
Muscle sends waste to liver, liver converts it back to glucose, sends it back. Repeat.
Muscle
Glucose enters muscle through GLUT4. During exercise, especially anaerobic, muscle runs glycolysis hard: glucose breaks down to pyruvate, which is then converted to lactate by lactate dehydrogenase. ATP is produced rapidly but inefficiently (only 2 ATP per glucose). Lactate builds up.
→Lactate
←Glucose
Liver
Liver picks up lactate from blood. Lactate dehydrogenase converts lactate back to pyruvate. Pyruvate carboxylase plus PEPCK then begin gluconeogenesis. Glucose is built back up, costing 6 ATP per molecule. Glucose then leaves the liver through GLUT2.
Net cost: 6 ATP consumed in liver per glucose made; muscle gets back only 2 ATP worth of work from that glucose. Net energy loss of 4 ATP per cycle. Worth it: muscle keeps working without oxygen, and the liver burns fat to power the gluconeogenesis.
Alanine-Glucose Cycle: Alanine from muscle protein breakdown also travels to the liver. After transamination, alanine donates its amino group and becomes pyruvate, which then feeds gluconeogenesis. The nitrogen gets disposed through the urea cycle. This is how muscle protein indirectly becomes blood glucose during starvation.
When the Cori cycle breaks down: If the liver cannot process lactate (liver failure, metformin overdose, hypoxia), lactate accumulates in blood. pH drops. Anion gap rises. The result is type B lactic acidosis. Metformin inhibits mitochondrial complex I in the liver, reducing the liver's ability to run gluconeogenesis and clear lactate. In normal therapeutic doses this is clinically irrelevant, but in overdose or renal failure (metformin accumulates), lactic acidosis can become severe. Board: metformin-associated lactic acidosis = contraindicated in renal failure (reduced clearance) and in hypoxic states (compounded inhibition of lactate clearance).
Why RBCs are always Cori cycle participants: Red blood cells have no mitochondria. They can only run anaerobic glycolysis. Every glucose entering an RBC exits as two lactate. 24 hours a day, 7 days a week, RBCs are producing lactate that the liver must convert back to glucose. This basal Cori cycle activity accounts for approximately 25 grams of glucose recycled per day, requiring constant low-level hepatic gluconeogenesis even in the fed state.
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The Master Switch: F2,6BP
One metabolite controls both pathways simultaneously. The liver cannot run both at once.
Fasting State: Gluconeogenesis ON
Glucagon rises in response to low blood glucose.
cAMP rises inside hepatocytes, activating PKA.
PKA phosphorylates PFK-2, flipping it to its phosphatase activity (so it now degrades F2,6BP instead of making it).
F2,6BP levels fall toward zero.
FBPase-1 is released from inhibition: gluconeogenesis turns ON.
PFK-1 loses its main allosteric activator: glycolysis turns OFF.
Result: liver makes glucose and exports it to the blood.
Fed State: Glycolysis ON
Insulin rises after a meal in response to high blood glucose.
Insulin signaling dephosphorylates PFK-2, putting it back into its kinase mode.
PFK-2 kinase makes F2,6BP from F6P: F2,6BP levels rise.
F2,6BP activates PFK-1 allosterically: glycolysis turns ON.
Insulin also suppresses PEPCK gene transcription, so even the slower regulation is shut down.
Result: liver stores glucose as glycogen and lipogenesis.
METFORMIN: activates AMPK in hepatocytes, which raises AMP levels. AMP directly inhibits FBPase-1, independent of the F2,6BP axis. Lower FBPase-1 activity equals less gluconeogenesis, which equals lower fasting glucose. This is the primary mechanism of metformin in type 2 diabetes, and it explains why metformin lowers fasting glucose more than postprandial glucose.
Cortisol Connection
Cortisol upregulates PEPCK and PC gene expression. Prolonged high cortisol (Cushing syndrome, exogenous steroids) = chronic gluconeogenesis stimulation = steroid-induced hyperglycemia. Board: "started on prednisone, blood glucose is now elevated" = cortisol stimulating PEPCK transcription.
Biotin Dependency
Pyruvate carboxylase requires biotin as a CO2 carrier. Raw egg whites contain avidin, which binds biotin and prevents absorption. Biotin deficiency knocks out PC, the very first step of gluconeogenesis. Patient eating only raw egg whites + fasting hypoglycemia = blocked PC, substrate starved gluconeogenesis.
Acetyl-CoA Paradox
Acetyl-CoA activates pyruvate carboxylase allosterically (signals "fat is being burned, make OAA to absorb it"). But acetyl-CoA itself cannot become net glucose: it enters the TCA cycle but 2 carbons leave as CO2 before OAA is regenerated. Activating the pathway is not the same as being a gluconeogenic substrate.
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When the Exit Gate Fails: Von Gierke Disease
Type Ia Glycogen Storage Disease. G6Pase deficiency. Everything piles up at G6P.
A 3-month-old presents with failure to thrive, massive hepatomegaly, and a fasting blood glucose of 32 mg/dL. Labs show hyperlipidemia, lactic acidosis, and hyperuricemia. On exam, the abdomen is markedly distended. The infant becomes lethargic when feedings are delayed by more than 2 to 3 hours.
Enzyme missing: Glucose-6-Phosphatase (G6Pase). Without it, the liver cannot release free glucose. Everything piles up upstream at G6P.
Hypoglycemia
Cannot release free glucose from the liver. G6P accumulates instead of becoming free glucose. Blood glucose crashes with any fast longer than 2 to 3 hours. Brain and red blood cells suffer first.
Lactic Acidosis
G6P backs up, glycolysis backs up, pyruvate accumulates and gets converted to lactate. The Cori cycle breaks down: muscle keeps sending lactate to the liver, but the liver cannot convert it back to glucose, so lactate climbs.
Hepatomegaly
G6P is shunted to glycogen synthesis (glycogen piles up inside hepatocytes) and to fatty acid synthesis (triglycerides build up too). The liver swells massively and becomes a doughy mass that fills the abdomen.
Hyperuricemia
Excess G6P is shunted into the pentose phosphate pathway, producing ribose-5-phosphate. Purine synthesis ramps up, and purine catabolism follows, generating uric acid. Children with Von Gierke can develop gout-like joint disease and kidney stones.
BOARD MOVE: Think Von Gierke whenever you see the tetrad: fasting hypoglycemia + hepatomegaly + lactic acidosis + hyperuricemia. The deficient enzyme is glucose-6-phosphatase. It is Type Ia glycogen storage disease. Treatment: frequent feedings, cornstarch at night (slow-release glucose) to prevent overnight hypoglycemia.
The glucagon stimulation test (most testable fact): In a healthy person, glucagon injection raises blood glucose within 30 minutes by stimulating hepatic glycogenolysis and gluconeogenesis. In Von Gierke disease, both of those processes run normally, producing G6P. But G6P hits the broken G6Pase step and cannot become free glucose. Blood glucose does not rise. Lactate rises instead (glycogenolysis produces G6P, G6P backs up into glycolysis, pyruvate converts to lactate). The test result: glucagon given, lactate rises, blood glucose stays flat. This paradox is pathognomonic for Von Gierke.
Type Ib variant: Same phenotype as Type Ia but caused by deficiency of the G6P translocase (the protein that moves G6P into the ER so G6Pase can access it). Since G6Pase sits on the ER membrane and needs G6P transported in, broken translocase = same functional outcome as broken G6Pase. Additionally, Type Ib causes neutropenia and recurrent infections (mechanism separate from the glucose defect). Board: Type Ia = G6Pase deficiency; Type Ib = translocase deficiency + neutropenia.
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Glycogen Storage Disease Lineup
Von Gierke is Type Ia but the boards test the whole family. Each type has one broken enzyme. Hepatic vs. myopathic = the sorting key.
Type
Name
Broken Enzyme
Classic Board Finding
Location
Ia
Von Gierke
Glucose-6-phosphatase
Hepatomegaly, severe fasting hypoglycemia, no glucose on glucagon test, high TG + uric acid + lactic acidosis
Liver / kidney
II
Pompe
Lysosomal acid maltase (alpha-1,4-glucosidase)
Cardiomegaly, hypotonia, "floppy infant." No hypoglycemia. Lysosomal accumulation.
All tissues
III
Cori / Forbes
Debranching enzyme (alpha-1,6-glucosidase)
Milder hepatomegaly, milder hypoglycemia. Short-chain glycogen. Can affect muscle too.
Liver + muscle
IV
Andersen
Branching enzyme
Cirrhosis, liver failure in childhood. Long unbranched glycogen chains. Rare.
Liver
V
McArdle
Muscle glycogen phosphorylase
Exercise cramps, myoglobinuria. No rise in venous lactate with forearm exercise test. No hypoglycemia.
Sorting key: Hepatic GSDs (I, III, IV, VI) cause hepatomegaly + hypoglycemia. Myopathic GSD (V) causes cramps + myoglobinuria, no hypoglycemia. Pompe (II) is unique: lysosomal storage disorder with cardiomegaly as the killer finding. GSD Ia (Von Gierke) is the only one that fails the glucagon stimulation test.
Gluconeogenesis: Board Style
Before you start: Every question follows the clinical vignette format. Read the last sentence first (the actual question being asked), then read the stem for clues. For gluconeogenesis questions, identify: (1) which enzyme is implicated, (2) what is the substrate/product blocked, (3) what accumulates upstream. Those three answers unlock every explanation below.
5 vignettes. Tap to answer. Each explanation covers the clue, the chain, and the high-yield point.
Target: 4/5 or better. Gluconeogenesis shows up reliably on both Step-level and board exams.
Question 1 of 5
A researcher feeds isotope-labeled substrates to isolated hepatocytes from a fasted rat and tracks how much labeled carbon ends up in newly synthesized glucose. Which of the following substrates will produce NO net incorporation into glucose?
Answer: C. Acetyl-CoA cannot produce net glucose in humans. The pyruvate dehydrogenase reaction (pyruvate to acetyl-CoA) is irreversible, so acetyl-CoA cannot go back to pyruvate. While acetyl-CoA enters the Krebs cycle and condenses with OAA to make citrate, every turn of the cycle releases 2 CO2 molecules, so no net carbon is added to OAA for gluconeogenesis. Lactate (via LDH to pyruvate), alanine (via transamination to pyruvate), and glycerol (via glycerol kinase to DHAP) all feed into gluconeogenesis with net glucose production. BOARD MOVE: fat cannot become glucose in humans because acetyl-CoA cannot bypass pyruvate dehydrogenase.
Question 2 of 5
A researcher applies a selective inhibitor of PFK-2 kinase activity to hepatocytes isolated from a 24-hour fasted animal. Which of the following is the most likely effect on gluconeogenesis?
Answer: A. CLUE: fasted state means glucagon is high, PKA is active, and PFK-2 is already phosphorylated into phosphatase mode (so F2,6BP is already low). CHAIN: inhibiting PFK-2 kinase activity further reduces any residual F2,6BP synthesis. Lower F2,6BP releases FBPase-1 from its main inhibitor, so FBPase-1 activity rises. BOARD MOVE: F2,6BP simultaneously inhibits FBPase-1 (gluconeogenesis) and activates PFK-1 (glycolysis). Anything that lowers F2,6BP pushes the liver toward gluconeogenesis.
Question 3 of 5
A patient on long-term parenteral nutrition lacking biotin supplementation develops fasting hypoglycemia, lactic acidosis, and a generalized seborrheic rash. Which of the following enzymes is most directly affected by the underlying vitamin deficiency to produce the hypoglycemia?
Answer: A. CLUE: biotin is the cofactor for carboxylase enzymes (pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase). CHAIN: without biotin, pyruvate carboxylase cannot convert pyruvate to oxaloacetate, blocking the entry step of gluconeogenesis from pyruvate. Lactate piles up because the Cori cycle cannot regenerate glucose. Hypoglycemia develops during fasting. PEPCK uses GTP (no biotin), and FBPase-1 plus G6Pase are hydrolases (no cofactor). BOARD MOVE: any time you see fasting hypoglycemia plus a carboxylase mentioned, think biotin.
Question 4 of 5
A 6-month-old presents with massive hepatomegaly, fasting hypoglycemia (28 mg/dL), lactic acidosis, hyperuricemia, and hyperlipidemia. Hepatic biopsy shows glycogen-laden hepatocytes with normal glycogen structure. Which of the following enzyme deficiencies is most likely?
Answer: A. CLUE: the classic Von Gierke tetrad: hepatomegaly + fasting hypoglycemia + lactic acidosis + hyperuricemia, in an infant under 1 year old. CHAIN: G6Pase deficiency traps glucose as G6P in the liver. G6P backs up into glycogen (hepatomegaly with normal-structure glycogen) and into glycolysis (lactic acidosis as Cori cycle fails). Excess G6P enters PPP, driving purine catabolism (hyperuricemia). FBPase-1 deficiency causes similar fasting hypoglycemia and lactic acidosis but no hepatomegaly. G6PD deficiency causes hemolytic anemia, not GSD. PEPCK deficiency is exceedingly rare and presents differently. BOARD MOVE: Von Gierke is Type Ia GSD; the enzyme is G6Pase.
Question 5 of 5
An investigational drug selectively blocks the hepatic glucagon receptor with high potency. Which of the following is the most likely direct downstream effect on the regulation of gluconeogenesis within hepatocytes?
Answer: A. CLUE: glucagon receptor blockade means no Gs activation, no adenylyl cyclase, no cAMP rise, no PKA activation. CHAIN: PFK-2 is normally phosphorylated by PKA into its phosphatase mode (which lowers F2,6BP). Without PKA activity, PFK-2 stays dephosphorylated and active as a kinase, so it keeps making F2,6BP. Rising F2,6BP activates PFK-1 (more glycolysis) and inhibits FBPase-1 (less gluconeogenesis). PEPCK transcription would DECREASE without glucagon (CREB is downstream of PKA). Pyruvate carboxylase activation depends on acetyl-CoA, not glucagon signaling directly. G6Pase expression is induced by glucagon, so blocking glucagon decreases G6Pase. BOARD MOVE: this is exactly how investigational glucagon receptor antagonists lower fasting glucose in type 2 diabetes trials.
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Board-Style Walkthrough
Board-Style Walkthrough
Original board-style vignettes. Shuffled, never-repeat, full Chicago explanations.