Molecules in Medicine: Maintaining Blood Glucose Levels

ATP from Glucose

  • Major energy source for all cells.
  • Aerobic breakdown: yields 30-32 ATP.
  • Anaerobic breakdown: converts glucose to lactate, yielding 2 ATP.

Glucose – Tissue Requirements

  • Cells without mitochondria: absolute dependence on glucose for energy needs through anaerobic glycolysis (2 ATP).
  • Nervous System Cells: have mitochondria, consume ~20% of body’s oxygen and ~60% of glucose. Glucose is the sole energy source under normal conditions, requiring continuous supply from the bloodstream.
  • Muscle and Adipose Tissue: can utilize glucose or fatty acids depending on the level of glucose in the bloodstream.

Glucose in the Bloodstream

  • Normal levels: maintained between 4-10 mM.
  • Post-meal: high blood glucose levels trigger insulin release from the pancreas.

Glucose Uptake by Tissues

  • Transport: via glucose transporters (GLUTs) found on various cells (brain, RBC, liver, kidney).
  • GLUT-4: specific to muscle and adipose tissue, insulin-sensitive, takes up glucose when blood levels are high, promoting lower blood glucose.

Fed State – High Blood Glucose Effects

  • Insulin actions:
    • Stimulates glucose uptake by muscle/adipose tissue.
    • Increases glycolysis and glycogen synthesis in liver/skeletal muscle.
    • Excess glucose in liver: converted to glycogen and fatty acids.
    • In skeletal muscle: stored as glycogen.
    • In cardiac muscle: increased glucose used for ATP production.
    • In adipose tissue: converted to triglycerides for storage.

Regulation of Glycolysis & PFK-1 in Fed State

  • Fructose 2,6-Bisphosphate (F2,6BP): increases in response to insulin; overrides ATP and citrate effects, enhancing glycolysis.
  • Acetyl-CoA: produced for fat synthesis; relates to energy status and cell feedback mechanisms.

Glycogen Synthesis Stages

  1. Priming: energy is required.
  2. Elongation: glucose added to chains.
  3. Branching: formation of a1-6 branch points; enzymes located in glycogen granules.

Priming in Glycogen Synthesis

  • Activated glucose: UDP-glucose, derived from Glc-1-P and UTP, plays a role in biosynthesis.

Elongation & Branch Formation in Glycogen Synthesis

  • Glycogen Synthase: major regulatory enzyme, activated by Glc-6-P.
  • Branching enzyme: creates branches in glycogen structure for energy storage.

Effects of Insulin on Blood Glucose

  • Insulin facilitates the decrease of blood glucose levels, leading to a drop in levels.
  • In a fasting state, glucagon is released, enhancing glucose availability from the liver.

Fasting State – Low Blood Glucose Details

  • Insulin levels drop: results in cessation of glucose uptake in muscle/adipose tissues (GLUT-4 expression reduced).
  • Glucagon's role:
    • Decrease glycolysis in liver & promote glycogen breakdown.
    • Increase gluconeogenesis, releasing glucose into the bloodstream.

Glycogen Degradation

  • Regulatory enzyme: Glycogen phosphorylase.
  • Conversion of Glc-1-P to Glc-6-P by phosphoglucomutase.

Liver vs Muscle Glycogen Degradation

  • Muscle: Glycogen phosphorylase inhibited by Glc-6-P and ATP, activated by AMP (for muscle's own energy needs).
  • Liver: glycogen phosphorylase inhibited by glucose.
  • Glucose-6-Phosphatase: present in liver but not in muscle, allowing glucose release into bloodstream.

Hormonal Regulation of Glycogen Metabolism

  • Insulin & Glucagon: reciprocally regulate via phosphorylation of enzymes.
  • High blood glucose leads to insulin activation of glycogen synthesis; low blood glucose leads to glucagon stimulation of glycogen breakdown.
  • Catecholamines: enhance phosphorylation, blocking synthesis and promoting breakdown.

Genetic Deficiencies and Their Effects

  • Glycogen storage diseases: lead to enzyme deficiencies and complications including fasting hypoglycemia and muscle wasting.
  • Key diseases:
    • Von Gierke Disease: Glucose-6-phosphatase deficiency prevents Glc-6-P conversion to glucose in liver.
    • McArdle Disease: glycogen phosphorylase deficiency in muscle tissue prevents energy production.

Prolonged Fasting State Insights

  • Liver glycogen stores: deplete after approx 24 hours of fasting, relying on gluconeogenesis for blood glucose maintenance.

Gluconeogenesis vs Glycolysis

  • Shares 7 enzymes with glycolysis.
  • Bypass reactions: occur at the three major regulatory steps of glycolysis.
  • ATP Consumption: gluconeogenesis consumes 6 ATP to form glucose, necessitating energy input from non-carbohydrate sources (e.g., lactate, alanine, glycerol).