Carbohydrate Metabolism I

Carbohydrate Metabolism I - Carbohydrate Metabolism 1...

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1 Carbohydrate Metabolism 1 Reading: Gropper, 4 th ed, Ch 4, pp. 79–87; Ch 3, pp. 57–62 Gropper, 5 th ed, Ch 3, pp. 71–82; Ch 1, pp. 20–27
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2 Objectives for today: Review monosaccharide absorption into tissues and maintenance of blood glucose levels Overview of glucose metabolism Glycogenesis and glycogenolysis
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3 Overview Why is glucose so special? Constituent in starches and all 3 dissacharides Many cell types dependent upon a continuous supply Strict hormonal regulation Intracellular fate of glucose: Stored in form of glycogen Most glycogen derived from gluconeogenic precursors Used for energy production (ATP) Stored as fat (fatty acids) Used to make amino acids or DNA/RNA
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4 Glucose absorption Active Transport via the s odium- gl ucose t ransporter 1 (SGLT1) Enterocyte and renal tubules Against a concentration gradient driven by Na + /K + ATPase symport system Requires ATP Carrier must be preloaded with Na + Gene mutations associated with glucose-galactose malabsorption Facilitated transport via Glu cose T ransporters (GLUT)…more in a minute… Nearly all other cells admit glucose without energy
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5 Microvillus Na + -driven glucose symport Transport protein mediating facilitated diffusion of glucose Lumen of gut Intestinal epithelium Na + /K + ATPase Pump Blood ATP ADP + P Glucose Na + Na + Na + K + Fig. 1-17, p. 16 Glucose Glucose SGLT1
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6 Glucose transport from enterocyte Glucose leaves the enterocyte by 3 routes: 15% leaks back into the intestinal lumen 25% diffuses through the basolateral membrane into the circulation ~60% is transported into circulation via facilitated diffusion using GLUT2 Enterocyte maintains very little for its own use
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7 Microvillus Na + -driven glucose symport Transport protein mediating facilitated diffusion of glucose Lumen of gut Intestinal epithelium Na + /K + ATPase Pump Blood ATP ADP + P Glucose Na + Na + Na + K + Fig. 1-17, p. 16 Glucose Glucose GLUT2 SGLT1
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8 Now about those GLUTs… GLUTs are a family of protein carriers (homology) Integral proteins that span lipid bilayer multiple times (hydrophobic and hydrophilic regions) Facilitated diffusion All cells express at least one GLUT isoform
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GLUT Family Isoforms 9
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10 A) Insulin-insensitive glucose transport (GLUT1, 2 or 3) GLUCOSE RBC Brain Kidney/Liver B) Insulin-responsive glucose transport (GLUT4) GLUCOSE INSULIN + + Adipocyte Muscle or Heart
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11 hyperglycemia – too much glucose, too little insulin
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