Carbohydrate Metabolism I

• Glucose entry driven by concentration gradient, independent of Na⁺
• GLUT1–4 transporters

GLUT2

• Low-affinity transporter in hepatocytes and pancreatic cells
• Captures excess glucose for storage
• $K_m = 15$ (high)
  • When $[glucose]$ drops below $K_m$, glucose bypasses liver → goes into peripheral circulation
• Liver preferentially picks up excess glucose after a meal and stores it
• In pancreatic β-islet cells: GLUT2 + glucokinase = glucose sensor for insulin

GLUT4

• In adipose tissue and muscle
• Responds to $[glucose]$ in peripheral blood
• Transport rate increased by insulin (only one that responds)
  • Insulin stimulates movement of GLUT4 to membrane
• $K_m$ similar to normal levels (~5 mM)
  • Transporter saturated when blood glucose > normal
  • Can increase intake by increasing number of transporters on surface
• High insulin → increased glucose transport in adipose + muscle
  • Muscle stores excess as glycogen
  • Adipose stores excess as DHAP → glycerol phosphate
    • Stores incoming fatty acids as triacylglycerols

• Energy-yielding pathway in cytoplasm
• Produces NADH for ETC

Hexokinase and glucokinase

• Phosphorylate glucose → traps it (prevents leaving via transporter)
• Produces glucose-6-phosphate (G6P)
• Hexokinase:
  • Widely distributed
  • Inhibited by G6P
  • Low $K_m$
• Glucokinase:
  • Liver + pancreatic β-islet cells only
  • Induced by insulin
  • High $K_m$

Phosphofructokinases (PFK1/2)

• PFK1 = rate-limiting enzyme (main control point)
• Reaction: F6P → F1,6BP (uses ATP)
• PFK1 regulation:
  • Inhibited by ATP and citrate
  • Activated by AMP
  • Insulin stimulates, glucagon inhibits
• Insulin activates PFK2 → converts F6P → F2,6BP
• Glucagon inhibits PFK2 → lowers F2,6BP
• F2,6BP activates PFK1 → overrides ATP inhibition → glycolysis continues

Glyceraldehyde-3-phosphate dehydrogenase

• Oxidation + adds Pi to G3P
• Produces 1,3-bisphosphoglycerate (1,3BPG)
• Reduces NAD⁺ → NADH
• Oxidation = ↑ bonds to O
• Reduction = ↑ bonds to H
• Aerobic glycolysis: NADH oxidized by ETC

3-Phosphoglycerate kinase

• Transfers high-energy phosphate from 1,3BPG to ADP → ATP + 3-phosphoglycerate
• Substrate-level phosphorylation
• Only ATP source in anaerobic tissue

Pyruvate kinase

• Substrate-level phosphorylation using PEP to make ATP
• Activated by F1,6BP (feed-forward activation)
  • Product of earlier step stimulates later step

Fermentation

• Lactate dehydrogenase oxidizes NADH → NAD⁺
  • Replenishes coenzyme for G3P dehydrogenase
• Produces lactate (no carbon loss)
• Low O₂ (exercise) → lactate production starts
• Fermentation (general): pyruvate → ethanol + CO₂

Important intermediates

• DHAP:
  • Used in hepatic + adipose tissue for triacylglycerol synthesis
  • Formed from F1,6BP
  • Isomerized to G3P → converted to glycerol
• 1,3BPG and PEP:
  • High-energy intermediates used for substrate-level phosphorylation
  • Only ATP gained in anaerobic respiration

Irreversible enzymes

• Keep pathway one-direction:
  • Glucokinase
  • PFK1
  • Pyruvate kinase

Glycolysis in erythrocytes

• Anaerobic glucose metabolism → net 2 ATP per glucose
• Bisphosphoglycerate mutase makes 2,3BPG from 1,3BPG
  • Mutases move functional group (here phosphate)
• 2,3BPG binds allosterically to β chains of HbA → decreases affinity
  • Right shift → unloading in tissues while still allowing 100% saturation
• Doesn’t bind well to fetal Hb → HbF higher affinity
• High 2,3BPG, low pH, high pCO₂ → right shift
• Exercise is the RIGHT thing to do

Galactose metabolism

• Lactose hydrolyzed to glucose + galactose by lactase
• Galactose goes to liver via hepatic portal vein
• Phosphorylated by galactokinase
• Galactose-1P → glucose-1P via:
  • Galactose-1P uridyl transferase
  • Epimerase
• Epimerase converts sugar epimers into another

Fructose metabolism

• Found in honey + fruit
• Part of sucrose (with glucose)
• Liver uses fructokinase → fructose-1P
• Aldolase B cleaves fructose-1P → glyceraldehyde + DHAP
• Then enters glycolysis
• Skips rate-limiting enzymes fructokinase and PFK1

• Pyruvate dehydrogenase complex (PDH) is irreversible
• Converts pyruvate → acetyl-CoA
• Activated by insulin (signals well-fed state)
  • Shifts toward storage rather than oxidation
• Multi-enzyme complex (reactions in succession)
• 1 of 3 fates of pyruvate:
  • Lactate
  • OAA
  • Acetyl-CoA (PDH)
• Inhibited by product acetyl-CoA
• Buildup of acetyl-CoA → less pyruvate → acetyl-CoA, more pyruvate → OAA

• Glycogen = branched polymer, storage form of glucose
• Stored in cytoplasm as granules:
  • Protein core + polyglucose chains radiating → sphere
• Linear chains would pack glucose near core (high density)
• Branching → highest density at periphery
• Liver glycogen:
  • Broken down to maintain blood glucose
  • Mobilized between meals to prevent hypoglycemia
• Muscle glycogen:
  • Broken down to fuel muscle during activity
• Plants store starch (long α-linked glucose chains)

Glycogenesis

• Starts with glycogenic protein
• G6P → G1P
• Activated by coupling to UDP → allows integration by glycogen synthase
• Reaction: G1P + UTP → UDP-glucose + PPi
• Glycogen synthase:
  • Rate-limiting enzyme
  • Forms α-1,4 glycosidic bonds (linear chains)
  • Stimulated by G6P and insulin
  • Inhibited by epinephrine and glucagon
• Branching enzyme (glycosyl α-1,4; α-1,6 transferase):
  • Introduces α-1,6 branches
  • Glycogen synthase then extends branch

Glycogenolysis

• Glycogen phosphorylase:
  • Rate-limiting enzyme of breakdown
  • Uses Pi (not water) to cleave → produces G1P → G6P
  • Breaks α-1,4 bonds
  • Stops at branch points (can’t break α-1,6)
  • Regulation:
    • Activated by glucagon in liver (glucose for body)
    • Activated by AMP and epinephrine in muscle
    • Inhibited by ATP
• Debranching enzyme (glucosyl α-1,4; α-1,4 transferase + α-1,6 glucosidase):
  • Two-enzyme complex
  • Breaks 1,4 bond near branch → forms new 1,4 bond
  • Breaks 1,6 bond
  • Moves most of branch to end of chain
  • Releases single glucose from old branch

Glycogen storage diseases

• Isoforms = different versions of same protein
• Diseases cause accumulation or lack of glycogen in tissues
• Most common: von Gierke’s (defect in glucose-6-phosphatase)
  • Affects gluconeogenesis
  • Low blood sugar between meals
  • Requires continuous feeding

• Liver maintains blood glucose; kidney can also do it
• Promoted by glucagon and epinephrine
• Inhibited by insulin
• Sole source of glucose after 24 hours

Substrates

• Glycerol-3-phosphate (from stored fats in adipose)
• Lactate (anaerobic glycolysis)
• Glucogenic amino acids:
  • All except leucine and lysine
  • Converted to intermediates feeding gluconeogenesis
• Ketogenic amino acids:
  • Converted into ketone bodies (alternative fuel)

Carbon source limits

• Dietary fructose and galactose can be converted
• Can’t convert acetyl-CoA → glucose (so fatty acids can’t produce glucose)
  • Exception: odd-chain fatty acids → propionyl-CoA

Key conversions

• Lactate → pyruvate via lactate dehydrogenase
• Alanine via alanine aminotransferase
• G3P → DHAP via glycerol-3P dehydrogenase

Pyruvate carboxylase

• Mitochondrial enzyme
• Activated by acetyl-CoA
• Pyruvate → OAA
• OAA reduced to malate → exits via malate-aspartate shuttle
• In cytoplasm, malate oxidized back to OAA
• Acetyl-CoA activation means “don’t need more acetyl-CoA, make OAA instead”
  • Acetyl-CoA comes from fatty acid oxidation → fatty acids must be burned

PEP carboxykinase (PEPCK)

• Cytoplasmic
• Induced by glucagon and cortisol → raises blood sugar
• OAA → PEP (requires GTP)
• PEP → F1,6BP
• Pyruvate carboxylase + PEPCK bypass pyruvate kinase

Fructose-1,6-bisphosphatase

• Key control point, rate-limiting step of gluconeogenesis
• Reverses PFK1:
  • Removes phosphate from F1,6BP → F6P
• Activated by ATP
• Inhibited by AMP and F2,6BP
• F2,6BP is marker for “satisfactory energy levels” in cell

Glucose-6-phosphatase

• Located in ER lumen of liver cells
• Not in muscle → muscle glycogen can’t provide blood glucose
• Glucose from hepatic gluconeogenesis is not energy source for liver

Ketones

• Acetyl-CoA from fatty acids can’t become glucose
• Can become ketone bodies for brain fuel

Cori cycle

• RBCs deliver lactate to liver
• Lactate → pyruvate → glucose

• PPP = hexose monophosphate shunt (HMP)
• Produces NADPH and ribose-5-phosphate (R5P)
• Starts with G6P, ends with ribulose-5P (irreversible)

Key enzyme

• Rate-limiting: G6P dehydrogenase (G6PD)
• G6PD induced by insulin (abundant sugar shunted)
• Shunt inhibited by NADPH
• Activated by NADP⁺

Second part

• Produces R5P and other stuff
• Can feed back into glycolysis
• Uses transketolase and transaldolase

Functions of NADPH

• Electron donor, strong reducing agent
• Lipid + cholesterol biosynthesis
• Helps cellular bleach production (antibacterial)
• Maintains glutathione to protect vs reactive species
• NOT used as energy carrier
• Protects cells from free radical damage

G6PD deficiency

• Higher risk from oxidative stress
• Free radicals attack membrane lipids → cell lysis
• Also damage DNA → cancer