carbohydrate metabolism

Lecture1: covers the fundamentals of carbohydrate digestion, absorption, and initial metabolic fates. It describes how dietary carbohydrates (plant starch/cellulose, animal glycogen, and disaccharides sucrose/lactose) are hydrolyzed by salivary and pancreatic α-amylase in the mouth and small intestine, followed by brush-border disaccharidase activity to generate glucose, galactose, and fructose. It also explains mono-sugar uptake mechanisms across intestinal epithelial cells, including Na⁺-dependent transport and facilitated diffusion via GLUT transporters (GLUT1–GLUT5), and how these sugars are transported to liver and peripheral tissues. The document then details key galactose metabolism steps (galactokinase and the UDP-galactose pathway) and fructose metabolism routes in the liver (hexokinase vs fructokinase, and downstream reactions via aldolase B/DHAP). Clinical correlations include galactosaemia, hereditary fructose intolerance (aldolase B deficiency), cataract formation, and lactose intolerance, plus related absorption/digestion defects such as lactase deficiency and glucose/galactose malabsorption Lecture 2 — Glycolysis (Embden–Meyerhof) Glycolysis (overview): Cytosolic breakdown of glucose to pyruvate, functioning under both aerobic and anaerobic conditions. Cytosolic location: Main reactions occur in the cytosol, separating glycolysis from mitochondrial steps. Two phases (preparatory & payoff): Glycolysis is organized into an energy-investment phase followed by an energy-yielding phase. Aerobic vs anaerobic operation: Pyruvate handling differs depending on oxygen availability, while glycolysis itself remains central. Energy capture (ATP investment & gain): Uses ATP early and generates ATP later, producing a net energy outcome for the pathway. NADH generation: Produces reducing power (NADH) during glucose-to-pyruvate conversion, linking metabolism to redox needs. Key regulation concept (irreversible steps): Control points prevent wasteful cycling and commit carbon through the pathway. PFK-1 control point: The rate-controlling “commitment step” in glycolysis that strongly shapes pathway flux. Pyruvate kinase control point: A major terminal control step that helps determine the final glycolytic throughput. Lecture 3 — Citric Acid / TCA (Krebs) Cycle TCA cycle (overview): Mitochondrial oxidation of acetyl-CoA to CO₂, generating energy carriers for oxidative metabolism. Mitochondrial location: Occurs in the mitochondria, coupling carbohydrate oxidation to the ETC. Acetyl-CoA entry: The pathway is fed by acetyl-CoA produced from earlier steps (e.g., pyruvate breakdown). CO₂ release: Progressive decarboxylation drives carbon loss as CO₂ and maintains thermodynamic direction. Energy capture (NADH/FADH₂): Produces reduced cofactors that supply electrons to drive oxidative phosphorylation. Connection to ETC: Serves primarily as an “energy and reducing equivalents” generator for the electron transport chain. Reaction types (big-picture): Includes condensation, dehydration, redox reactions, and substrate-level phosphorylation at a key point. Substrate-level phosphorylation (overview): Generates ATP directly at a defined step, adding to energy output beyond NADH/FADH₂. Regulation importance: Enzymatic control helps match flux to cellular energy state and mitochondrial demand. Biosynthetic intermediate role: Intermediates support anabolic pathways, not just energy production. Disease link concept: Defects in key mitochondrial pyruvate-to-mitochondria processing can impair energy generation (e.g., congenital lactic acidosis themes). Lecture 4 — Gluconeogenesis Gluconeogenesis (overview): Synthesis of glucose (primarily from non-carbohydrate sources) to maintain blood glucose during fasting. Primary site (liver & kidneys): Mainly occurs in liver and kidneys, with contributions from other tissues such as GI epithelium. Fasting function: Ensures continued glucose availability when dietary carbohydrate is limited. Glucogenic substrates (overview): Uses glucogenic amino acids, lactate, pyruvate, and glycerol as carbon sources. Pathway direction opposite glycolysis (concept): Reverses glycolysis’s overall output, but uses distinct bypass strategies to overcome irreversible glycolytic steps. Bypasses (conceptual barriers): Uses alternative reactions to get around “locked” glycolytic control points. Key bypass enzyme themes: Major irreversible bypass steps include regulation concepts around conversion steps such as pyruvate carboxylation and PEP formation. Fructose-1,6-bisphosphatase control concept: A key regulatory step influenced by cellular signaling through fructose-2,6-bisphosphate. F-2,6-BP regulation logic: Fructose-2,6-bisphosphate acts as a control signal that shifts gluconeogenesis vs glycolysis tendency. **Hormonal regulation (glucagon/ins Lecture 5 — Glycogen storage & mobilization (keyword tag + brief description) Glycogen role (storage of glucose) — Comprises glucose units stored as glycogen (mainly in liver and muscle) to buffer energy and maintain blood glucose. Glycogen synthase vs glycogen phosphorylase (opposing functions) — Liver/muscle balance glycogenesis (build) vs glycogenolysis (breakdown) to match nutrient and energy status. Liver glycogen: blood glucose support — In hepatocytes, glycogenolysis helps maintain systemic glucose availability between meals. Muscle glycogen: local fuel for exercise — In skeletal muscle, glycogen breakdown supplies ATP/CHO intermediates for contraction and local energy needs. Insulin signaling: promotes glycogenesis — Insulin favors glycogen storage by promoting pathways leading to glycogen synthase activation and reducing glucose output from liver. Glucagon/epinephrine signaling: promotes glycogenolysis — In liver (and also muscle under stress for catecholamine effects), glucagon/epinephrine promotes glycogen breakdown to raise glucose availability. Glycogenolysis overview (phosphorylase-driven cleavage) — Breaks glycogen into glucose units (high-level concept: phosphorylation-enabled release) to generate free glucose-1-phosphate for metabolism. Glycogen branching structure & mobilization — Branching increases glycogen solubility and creates many non-reducing ends for faster mobilization of glucose units. Terminal glucose handling (free glucose vs phosphate) — Liver converts glycogen-derived units toward free glucose export, whereas muscle primarily channels glycogen-derived carbon to local metabolism. Allosteric control concept (energy/redox status) — Glycogen mobilization is tuned to energy state (e.g., demand of ATP vs availability of energy equivalents), shifting net direction toward fueling when needed. Cyclic AMP (cAMP) control concept (hormone relay) — Hormones (notably glucagon/epinephrine) use cAMP-linked signaling to drive glycogen phosphorylase toward an “on” state in liver. Ca²⁺ link in muscle (exercise activation concept) — In muscle, Ca²⁺ during contraction supports activation of glycogen breakdown to match immediate energy demand. Net outcome: match storage to feeding vs fasting — Feeding favors storage (glycogenesis), fasting/exercise favors mobilization (glycogenolysis) to maintain energy and glucose homeostasis. Clinical relevance (dysregulation impacts glucose/energy) — Abnormal control of glycogen storage/mobilization can impair blood glucose regulation (liver) or exercise tolerance/energy supply (muscle).