ACTIONS OF INSULIN

Molecular Structure and Clearance

• Insulin = 56-amino acid polypeptide with two peptide chains (α & β) joined by two disulfide bridges.
• Secreted into portal vein → ~80% cleared by hepatocyte insulin receptors on first pass.
• Overall Action: Anabolic – promotes synthesis of carbohydrates, fats, proteins.

Insulin Receptor

1. Structure
   • Heterotetrameric glycoprotein (two α- and two β-subunits, joined by disulfide bonds).
   • α-subunits: extracellular, bind insulin.
   • β-subunits: transmembrane + intracellular, have intrinsic tyrosine kinase activity.
2. Signal Transduction
   • Insulin binding → autophosphorylation of β-subunit tyrosine residues.
   • Phosphorylation of insulin receptor substrates (IRS-1, 2, 3, 4) → activation of PI3K & MAPK pathways.
     • PI3K pathway → metabolic (glucose transport, glycogen/protein synthesis, anti-apoptosis).
     • MAPK pathway → proliferative, differentiation effects.
3. Receptor Modulation
   • Downregulated by obesity, hyperinsulinemia.
   • Upregulated by exercise, starvation.

Glucose Transport and Effects

1. Transporters
   • GLUT 1: all tissues, high-affinity; basal uptake.
   • GLUT 2: low-affinity, in liver/pancreatic β-cells (handles postprandial hyperglycemia).
   • GLUT 3: high-affinity for neurons.
   • GLUT 4: muscle & adipose; insulin-responsive.
2. Muscle
   • Insulin → PI3K → GLUT 4 translocation to plasma membrane → ↑ glucose uptake.
   • Promotes glycogen synthesis (↑ glycogen synthase, ↓ glycogen phosphorylase).
   • Enhances protein synthesis (↑ AA transport, kinase-mediated anabolic signals).
3. Adipose Tissue
   • Inhibits lipolysis (dephosphorylation of hormone-sensitive lipase).
   • ↓ breakdown of triglycerides → less FFA/glycerol → less substrate for ketogenesis.
   • Induces lipoprotein lipase → frees FFA from chylomicrons/VLDL → FFA uptake → re-esterification into TGs.
   • Stimulates lipogenesis: activates acetyl-CoA carboxylase, ↑ α-glycerol phosphate (from ↑ glucose uptake) → TG synthesis.
4. Liver
   • Increases enzymes for glucose utilization (pyruvate kinase, glucokinase).
   • Decreases gluconeogenic enzymes (glucose-6-phosphatase, PEP carboxykinase).
   • Enhances glycogen storage (dephosphorylation of glycogen synthase & phosphorylase).
   • Promotes triglyceride, VLDL, and protein synthesis.

GLYCOLYSIS

1. Definition
   • Main pathway for glucose metabolism in cytosol of all cells.
   • Converts glucose (6C) → pyruvate (3C), can be aerobic or anaerobic.
2. Overall Reactionmathematica Glucose + 2 ADP + 2 NAD+ + 2 Pi → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
3. Key Steps
   • Hexokinase/Glucokinase phosphorylates glucose to glucose-6-phosphate.
   • Phosphofructokinase forms fructose-1,6-bisphosphate.
   • Aldolase cleaves to glyceraldehyde 3-phosphate + dihydroxyacetone phosphate (DHAP).
   • Glyceraldehyde 3-phosphate → 1,3-bisphosphoglycerate (NAD+ → NADH).
   • Substrate-level phosphorylation → 2 ATP net from the 1,3-bisphosphoglycerate and phosphoenolpyruvate (PEP) steps.
4. Fate of Pyruvate
   • Aerobic: enters mitochondria, converted to acetyl-CoA, TCA cycle.
   • Anaerobic: reduced to lactate (LDH) for regenerating NAD+ (2 ATP net per glucose).
5. Regulation
   • Major regulated enzymes = hexokinase, phosphofructokinase, pyruvate kinase.
   • Aerobic glycolysis yields ~30 ATP/glucose; anaerobic yields 2 ATP/glucose.

TRICARBOXYLIC ACID (TCA) CYCLE

1. Overview
   • Also called citric acid or Krebs cycle.
   • Final common pathway for oxidation of carbs, fats, proteins → Acetyl-CoA enters TCA.
   • Produces intermediates for gluconeogenesis, fatty acid synthesis, protein catabolism.
2. Steps
   • Pyruvate dehydrogenase: pyruvate → Acetyl-CoA + CO2 + NADH.
   • Citrate synthase: Acetyl-CoA + oxaloacetate → citrate.
   • Aconitase: citrate → isocitrate.
   • Isocitrate dehydrogenase: isocitrate → α-ketoglutarate + CO2 + NADH.
   • α-Ketoglutarate dehydrogenase: α-ketoglutarate → succinyl-CoA + CO2 + NADH.
   • Succinyl-CoA synthetase: succinyl-CoA → succinate + GTP/ATP.
   • Succinate dehydrogenase: succinate → fumarate + FADH2.
   • Fumarase: fumarate + H2O → malate.
   • Malate dehydrogenase: malate → oxaloacetate + NADH.
3. Energy Yield
   • From 1 turn (Acetyl-CoA): 3 NADH, 1 FADH2, 1 ATP (via GTP).
   • Plus PDH step: 1 extra NADH.
   • Each NADH ~2.5 ATP, FADH2 ~1.5 ATP → total ~12.
4. Vitamin Cofactors
   • Riboflavin (B2, FAD), Niacin (B3, NAD), Pantothenic acid (B5, CoA), Thiamine (B1, decarboxylation).
5. Regulation
   • Intermediates modulate enzymes, e.g. citrate, succinyl-CoA, NADH, ATP.
   • Links to HIF regulation, paraganglioma, pheochromocytoma in SDH or VHL gene mutations.

GLYCOGEN METABOLISM

1. Glycogen Structure
   • Branched α-D-glucose polymer. Storage form in liver, muscle.
2. Glycogenesis
   • Hexokinase/Glucokinase: Glc → Glc-6-P.
   • Phosphoglucomutase: Glc-6-P → Glc-1-P.
   • UDPGlc pyrophosphorylase: Glc-1-P + UTP → UDP-Glc.
   • Glycogen synthase: extends α(1→4) chain.
   • Branching enzyme: creates α(1→6) branch points.
3. Glycogenolysis
   • Glycogen phosphorylase: cleaves α(1→4) → Glc-1-P until 4 residues from branch.
   • Debranching enzyme: moves trisaccharide + hydrolyzes α(1→6) link.
   • Glc-6-P can go to:
     • Glycolysis
     • Pentose phosphate pathway
     • Glycogenesis (recycle)
     • Glucose (in liver/kidney via Glc-6-phosphatase).
4. Regulation
   • Key enzymes: glycogen synthase (anabolic) + glycogen phosphorylase (catabolic).
   • Insulin → promotes glycogen synthesis.
   • Glucagon, epinephrine → promote glycogenolysis.

CONSEQUENCES OF INSULIN DEPRIVATION

1. Causes:
   • Pancreatectomy, autoimmune β-cell destruction (type 1 diabetes mellitus), etc.
   • Lack of insulin → insulin-sensitive tissues deprived of glucose uptake + anabolic regulation.
2. Impaired Glucose Utilization
   • ↓ GLUT4-mediated uptake in muscle/adipose.
   • Glycogenesis slowed; hepatic glucose production ↑ (gluconeogenesis, glycogenolysis).
   • Blood glucose rises, exceeding renal threshold → glucosuria & osmotic diuresis → polyuria, polydipsia, dehydration.
3. Protein Catabolism
   • Muscle breakdown → ↑ amino acid release → negative nitrogen balance, weight loss, possible cachexia.
4. Enhanced Lipolysis
   • ↑ free fatty acids to liver → ketone bodies (acetoacetate, β-hydroxybutyrate) → ketoacidosis.
   • Sodium lost with ketones → bicarbonate buffering leads to metabolic acidosis.
5. Electrolyte Depletion
   • K+ deficit from osmotic diuresis & excretion with ketones.
   • Phosphate deficit from diuresis + lack of insulin.
6. Severe Metabolic Consequences
   • Diabetic ketoacidosis (DKA) → acidosis, dehydration, hypotension, shock, coma, death (if untreated).