Metabolic Integration

Metabolic Integration

Metabolic integration refers to the coordinated regulation of all metabolic pathways to maintain cellular and whole-body homeostasis. Different organs specialize in different metabolic roles and exchange substrates via the blood.

Key Crossroad Metabolites

  • Glucose-6-Phosphate: Junction of glycolysis, glycogen synthesis, HMP shunt, and gluconeogenesis
  • Pyruvate: Links glycolysis to: TCA (via PDC), gluconeogenesis (via Pyruvate Carboxylase), alanine synthesis, lactate production
  • Acetyl-CoA: Central to: TCA cycle (energy production), FA synthesis, ketogenesis, cholesterol synthesis, acetylcholine synthesis
  • OAA: Links TCA to gluconeogenesis (via PEPCK); connects amino acid catabolism to glucose synthesis

Organ Specialization

  • Liver: Metabolic hub. Glycogen storage and release; gluconeogenesis; FA synthesis and oxidation; ketogenesis; cholesterol synthesis; urea cycle; protein synthesis (albumin, clotting factors)
  • Muscle (Skeletal): Uses glucose, FA, ketones, amino acids for energy. Makes lactate (Cori cycle) and alanine (alanine cycle). Cannot release glucose (no G6Pase).
  • Brain: Obligate glucose user (normally). Uses ~120g glucose/day. In starvation, adapts to use ketone bodies after 3–4 weeks (↓glucose needs by 60%). Does NOT use FA (cannot cross BBB). Has no glycogen stores.
  • Adipose Tissue: Stores TG; releases FFA + glycerol during fasting under glucagon/epinephrine. Glucose needed for DHAP (glycerol backbone). Cannot release glycerol for gluconeogenesis (lacks glycerol kinase to reuse it).
  • RBCs: No mitochondria → entirely dependent on anaerobic glycolysis → produce lactate exclusively. Use 2,3-BPG to regulate Hb-O₂ affinity.
  • Kidney: Major gluconeogenic organ during prolonged starvation (up to 40% of glucose). Also ammoniagenesis (NaHCO₃ conservation in acidosis).

Fed State Pathways

High Insulin/Glucagon ratio: ↑Glycolysis, ↑Glycogenesis (liver + muscle), ↑FA synthesis (liver), ↑Protein synthesis, ↑TG synthesis (adipose). Fuel stored as glycogen and fat.

Fasted/Starvation State (24h–weeks)

  • 0–24h: Glycogen depleted → gluconeogenesis from amino acids (muscle protein catabolism), lactate, glycerol
  • 24h–2 weeks: ↑FA oxidation → ↑Ketogenesis (liver) → Ketone bodies feed muscle and brain
  • Weeks: Brain adapts to ketones → spares glucose → spares muscle protein; glucose mostly for RBCs and renal medulla

Fuel Hierarchy

During fasting: Glucose (1st) → Fat (FFA, ketones 2nd) → Protein (3rd — last resort). Glucose used preferentially by brain and RBCs (others switch to FA).

AMPK — Master Energy Sensor

AMP-activated protein kinase. Activated when AMP:ATP ratio rises (low energy). Actions: ↑FA oxidation (phosphorylates/inhibits ACC → ↓Malonyl-CoA → ↑CPT-I), ↑Glucose uptake (↑GLUT4), ↑Glycolysis, ↓FA synthesis, ↓Cholesterol synthesis, ↓Gluconeogenesis. Metformin activates AMPK (indirectly via Complex I inhibition).

mTOR — Anabolic Master Regulator

Mammalian Target of Rapamycin. Activated by: Insulin, amino acids (especially Leu), growth factors. Actions: ↑Protein synthesis, ↑Ribosome biogenesis, ↑Lipid synthesis; ↓Autophagy. Inhibited by AMPK (antagonism of anabolic/catabolic states). Rapamycin inhibits mTOR → immunosuppressant, anticancer.