STEPTRACKERBIOCHEMISTRY + ENDOCRINOLOGY | CARBOHYDRATE METABOLISM | HIGH-YIELD REVIEW

Gluconeogenesis

Making new glucose in liver (and kidney) from lactate, amino acids and glycerol — four enzymes bypass glycolysis's one-way steps.

FOCUS ON
The four bypasses: pyruvate carboxylase (mitochondria, biotin, + acetyl-CoA), PEP carboxykinase (GTP), fructose-1,6-bisphosphatase (rate-limiting; − F-2,6-BP, − AMP), glucose-6-phosphatase (ER; liver and kidney only — not muscle). Deficiency of a bypass enzyme → fasting hypoglycemia (von Gierke = G6Pase). Even-chain fatty acids can't make glucose; odd-chain (propionyl-CoA) and glycerol can.
ENZYME · TOPIC
REACTION · LOCATION
REGULATION
CLUE · DEFICIENCY
Pyruvate carboxylase
Pyruvate → oxaloacetate · mitochondria
Biotin, ATP; activated by acetyl-CoA
Deficiency: lactic acidosis, hypoglycemia, developmental delay
Malate shuttle
OAA can't cross the inner membrane → leaves as malate (or aspartate)
—
Rebuilt to OAA in cytosol (makes NADH)
PEP carboxykinase
OAA → phosphoenolpyruvate · cytosol (and mitochondria)
GTP; induced by glucagon, cortisol
—
Fructose-1,6-bisphosphatase
rate-limiting
F-1,6-BP → F-6-P · cytosol
+ ATP, citrate · − AMP, F-2,6-BP
Deficiency: fasting hypoglycemia + lactic acidosis
Glucose-6-phosphatase
G6P → glucose · ER lumen
Liver, kidney (not muscle)
Von Gierke: severe fasting hypoglycemia, lactic acidosis, hepatomegaly (8.07)
Substrates
Lactate (Cori), alanine and other glucogenic amino acids, glycerol (from triglyceride), propionyl-CoA (odd-chain FA, some amino acids)
—
Leucine and lysine are purely ketogenic → no glucose
Organs
Liver (~90%), kidney (more in prolonged fasting, acidosis), small intestine
—
Muscle can't release glucose
Hormones
Glucagon, cortisol, epinephrine stimulate; insulin inhibits
Via PKA (↓ F-2,6-BP) and gene induction (PEPCK)
Cortisol excess → hyperglycemia
Alcohol effect
↑ NADH diverts pyruvate → lactate, OAA → malate
Blocks substrate supply
Hypoglycemia in fasting alcoholics
Energy cost
2 pyruvate → 1 glucose uses 4 ATP + 2 GTP + 2 NADH
—
Paid for by β-oxidation in fasting

Mechanism — bypassing the one-way steps

Pyruvate
→
Pyruvate carboxylase→ OAA
→
PEPCK→ PEP
→
Reverse glycolysis→ F-1,6-BP
→
F-1,6-BPase→ F-6-P → G6P
→
G6Pase→ glucose

Glycolysis vs gluconeogenesis at each control point

STEP
GLYCOLYSIS
GLUCONEOGENESIS
Glucose ↔ G6P
Hexokinase / glucokinase
Glucose-6-phosphatase
F-6-P ↔ F-1,6-BP
PFK-1
Fructose-1,6-bisphosphatase
PEP ↔ pyruvate
Pyruvate kinase
Pyruvate carboxylase + PEPCK

How it's tested

COMMON CONFUSIONGlycerol enters as DHAP and can make glucose; acetyl-CoA from even-chain fatty acids cannot (PDH is irreversible).
THE TRAPA child who tolerates feeding but becomes hypoglycemic with lactic acidosis after a few hours of fasting → think a gluconeogenic enzyme (G6Pase, F-1,6-BPase).
YOUR APPROACHFasting hypoglycemia → which substrate fails (lactate, alanine, glycerol) or which bypass enzyme → associated findings (lactate, liver size, ketones).

If you see… think…

Rate-limiting enzyme of gluconeogenesis → fructose-1,6-bisphosphatase
Gluconeogenic enzyme needing biotin → pyruvate carboxylase
Activator of pyruvate carboxylase → acetyl-CoA
Enzyme missing in muscle so it can't release glucose → glucose-6-phosphatase
Fatty acids that contribute to glucose → odd-chain (propionyl-CoA)
Amino acids that can't make glucose → leucine, lysine
Fasting hypoglycemia after binge drinking → ↑ NADH blocks gluconeogenesis
Hormone inducing PEPCK → cortisol, glucagon
OAA leaves mitochondria as → malate
Infant with hypoglycemia, lactic acidosis, big liver → von Gierke