Step 1 · Biochemistry

Glucose Transporters & Hexokinase vs Glucokinase

Name: Glucose Transporters & Hexokinase vs Glucokinase
Creator: Veli Bakalov, MD

Where glucose gets in, and which enzyme decides to keep it

Biochemistry·Carbohydrate Metabolism·4 min read·8 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

Study materialsNo account needed

Card deckActive-recall cards · quick checks · quiz

High-yield

GLUT1 (RBCs, blood-brain barrier, placenta) and GLUT3 (neurons) provide insulin-independent glucose uptake to the brain.
GLUT1 deficiency (SLC2A1) causes infantile refractory seizures with low CSF glucose and normal blood glucose; treat with a ketogenic diet.
GLUT2 is the high-Km, bidirectional transporter of hepatocytes, β cells and basolateral enterocytes and proximal tubule cells; its AR loss causes Fanconi-Bickel syndrome (hepatorenal glycogen, rickets, glucose dysregulation).
GLUT4 in muscle and fat moves to the membrane in response to insulin or exercise; GLUT5 is the fructose transporter of enterocytes and sperm.
SGLT1 couples Na+ to glucose and galactose in the gut and stays intact in cholera, the basis of oral rehydration solution.
SGLT2 in the early proximal tubule reclaims ~90% of filtered glucose; gliflozins cause glucosuria, genital yeast infections and euglycemic DKA.
Hexokinase has low Km, low Vmax and is inhibited by glucose-6-phosphate; glucokinase has high Km, high Vmax, is insulin-induced and is sequestered in the nucleus by GKRP when fasting.
Glucokinase is the β-cell glucose sensor: inactivating GCK mutations cause MODY2 (mild stable fasting hyperglycemia), activating ones cause hyperinsulinemic hypoglycemia.

Two ways to move glucose

Facilitated GLUTs. The GLUT family (SLC2A genes) moves sugars down their concentration gradient by facilitated diffusion, with no energy cost. Tissue distribution, affinity (Km) and insulin sensitivity differ between isoforms, and those differences explain which tissues need insulin to take up glucose.

Sodium-coupled SGLTs. SGLT1 and SGLT2 (SLC5A genes) are secondary active cotransporters: the Na+ gradient created by the basolateral Na+/K+-ATPase drives glucose into the cell against its gradient. Glucose then leaves the cell on the blood side through GLUT2.

Glucose and fructose transporters

Transporter	Main sites	Key features	Clinical link
GLUT1	RBCs, blood-brain barrier endothelium, placenta	Basal uptake; insulin-independent	GLUT1 deficiency syndrome
GLUT2	Hepatocytes, β cells, basolateral enterocytes and proximal tubule	High Km; bidirectional; also galactose and fructose	Fanconi-Bickel syndrome
GLUT3	Neurons (also placenta)	High affinity; insulin-independent	Keeps neurons supplied at low glucose
GLUT4	Skeletal and cardiac muscle, adipose	Insulin- and exercise-driven translocation	Insulin resistance; exercise benefit
GLUT5	Apical enterocyte, spermatozoa	Fructose only; no Na+ or insulin needed	Fructose absorption
SGLT1	Apical small intestine (also late proximal tubule)	2 Na+ per glucose or galactose	Oral rehydration; glucose-galactose malabsorption
SGLT2	Early proximal tubule	Low affinity, high capacity; ~90% of filtered glucose	Gliflozins

Brain supply. GLUT1 carries glucose across the blood-brain barrier and GLUT3 delivers it into neurons. Neither requires insulin, so the brain keeps taking up glucose during fasting. GLUT2 has a high Km, so flux rises and falls with blood glucose, and it runs in both directions: the liver imports glucose after meals and exports it when fasting. GLUT4 waits in intracellular vesicles until insulin (PI3K/Akt signaling) or muscle contraction (AMPK and Ca2+ signaling, independent of insulin) moves it to the membrane, which is why exercise lowers glucose even in insulin resistance. GLUT5 takes fructose into enterocytes; seminal fluid is rich in fructose, the main fuel for sperm.

Transporter disorders and drugs

GLUT1 deficiency syndrome. Usually a de novo heterozygous SLC2A1 mutation. Poor glucose transport across the blood-brain barrier causes infantile seizures resistant to antiseizure drugs, developmental delay, acquired microcephaly and movement disorders. The key finding is low CSF glucose with normal blood glucose, normal CSF cell count and low-to-normal CSF lactate. A ketogenic diet is the treatment: ketones enter the brain through monocarboxylate transporters, bypassing GLUT1.

Fanconi-Bickel syndrome. Autosomal recessive SLC2A2 (GLUT2) deficiency. Glycogen accumulates in liver and kidney, producing hepatomegaly and a proximal tubulopathy (glucosuria, phosphaturia, aminoaciduria) with hypophosphatemic rickets. Glucose handling is doubly disturbed: fasting hypoglycemia and postprandial hyperglycemia, with galactose intolerance.

SGLT1 and oral rehydration. Cholera toxin drives chloride secretion but spares SGLT1, so glucose-linked Na+ absorption continues and water follows. This is the basis of glucose-containing oral rehydration solution. SGLT1 mutations cause congenital glucose-galactose malabsorption: severe watery diarrhea on milk that resolves on a fructose-based formula.

SGLT2 and gliflozins. SGLT2 reclaims about 90% of filtered glucose and downstream SGLT1 takes the rest; glucosuria appears once plasma glucose exceeds roughly 180–200 mg/dL. Empagliflozin, dapagliflozin and canagliflozin block SGLT2, producing glucosuria, natriuresis and mild weight loss, with benefits in heart failure and chronic kidney disease.

SGLT2 inhibitors cause genital mycotic infections and UTIs, volume depletion, and euglycemic diabetic ketoacidosis.

Hexokinase vs glucokinase

Trapping glucose. The first step of glucose use is phosphorylation to glucose-6-phosphate, which cannot leave through GLUTs. Two enzyme types do this with very different kinetics.

Hexokinase vs glucokinase

Feature	Hexokinase	Glucokinase
Location	Most tissues	Liver, pancreatic β cells
Km (affinity)	Low (high affinity); half-saturated below 0.1 mM	High (low affinity); half-saturated near 8 mM
Vmax (capacity)	Low	High
Glucose-6-phosphate	Inhibits it	No inhibition
Main control	Product (glucose-6-phosphate) inhibition	Induced by insulin; held in nucleus by GKRP when fasting

Division of labor. Hexokinase traps glucose even when blood levels are low, protecting tissues such as brain and RBCs. Glucokinase works hardest when glucose is abundant, letting the liver store the post-meal surplus.

GKRP. In the fasting liver, glucokinase regulatory protein binds glucokinase (favored by fructose-6-phosphate) and holds it inactive in the nucleus. Rising glucose after a meal releases it to the cytosol. Fructose-1-phosphate from dietary fructose also frees glucokinase, boosting hepatic glucose uptake.

Glucokinase as the glucose sensor

Setting the threshold. Glucokinase's high Km lies within the physiologic glucose range, so its rate of phosphorylation, and hence glycolytic ATP output, rises in step with blood glucose.

Glucose enters the β cellVia GLUT2
Glucokinase phosphorylates itRate-limiting glucose sensor
ATP/ADP ratio risesATP-sensitive K+ channels close
Membrane depolarizesVoltage-gated Ca2+ channels open
Insulin is releasedCa2+-triggered exocytosis

β-cell glucose sensing

MODY2 (GCK-MODY). Heterozygous inactivating GCK mutations (autosomal dominant) raise the set point for insulin release. Fasting glucose is mildly high from birth (about 100–150 mg/dL) and stable over life, HbA1c rarely exceeds about 7.5%, complications are rare, and treatment is usually unnecessary. By contrast, HNF1A-MODY is progressive. Loss of both GCK alleles causes permanent neonatal diabetes.

Activating GCK mutations lower the set point, so insulin is released even at low glucose levels, causing congenital hyperinsulinemic hypoglycemia.