1. Metabolism of saccharides Pavla Balínová

2. Sources of glucose (Glc) ● from food (4 hours after meal) ● from glycogen (from 4 to 24 hours after meal) ● from gluconeogenesis (days after meal, during starvation) Figure was assumed from Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley ‑ Liss, Inc., New York, 1997

3. Glycemia glucose concentration in the blood physiological range of fasting glycemia 3,3 – 5,6 mmol/L is regulated by hormones (insulin, glucagon, epinephrine, kortisol, …)

4. Glucose can enter into cells: a) by facilitative diffusion (GLUT 1 – 7) GLUT 1 – blood-brain barrier, erythrocytes GLUT 2 – liver, β-cells in pancreas GLUT 3 – neurons GLUT 4 – skeletal muscles, heart muscle, adipose tissue b) by cotransport with Na + ion (SGLT-1 and 2) Figure was assumed from textbook: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley ‑ Liss, Inc., New York, 1997. small intestine, kidneys

5. An effect of insulin on insulin-sensitive cells Transport of Glc into cells is dependent on insulin effect (GLUT-4) in the following tissues: skeletal and heart muscle and adipose tissue Figure is found on http://www.mfi.ku.dk/ppaulev/chapter27/Chapter%2027.htm

6. Metabolic pathways included in utilization of Glc – glycolysis, pentose cycle, glycogen synthesis Phosphorylation of glucose  after enter into cell Glc is always phosphorylated to form Glc-6-P  enzyme hexokinase catalyzes esterification of Glc  ATP is a donor of phosphate group!  enzyme is inhibited by excess of Glc-6-P  2 isoenzymes of hexokinase exist: hexokinase and glucokinase  hexokinase has a higher affinity to glucose than glucokinase

7. Hexokinase vs. glucokinase Figure is found on http://web.indstate.edu/thcme/mwking/glycolysis.html K M hexokinase = 0,1 mM K M glucokinase = 10 mM

8. Glycolysis substrate: Glc-6-P product: pyruvate function: source of ATP subcellular location: cytosol organ location: all tissues regulatory enzymes: hexokinase/glucokinase, 6-phosphofructokinase-1 (main regulatory enzyme), pyruvatekinase Regulatory enzymes are activated by hormone insulin!

9. Glycolysis Figure is found on http://web.indstate.edu/thcme/mwking/glycolysis.html

10. Production of ATP in glycolysis  conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate  conversion of phosphoenolpyruvate (PEP) to pyruvate These reactions are examples of substrate level phosphorylation!

11. Regulation of glycolysis Regulatory enzymes ● Hexokinase – inhibited by Glc-6-P ● Glucokinase – activated by insulin – inhibited by Fru-6-P ● 6-phosphofructokinase-1 (PFK-1) – activated by insulin, ↑ AMP / ATP - inhibited by ↑ ATP /AMP, citrate ● Pyruvatekinase – activated by insulin, Fru-1,6-bisP - inhibited by glucagon, ↑ ATP /AMP, acetyl-CoA

12. Pentose phosphate pathway substrate: Glc-6-P product: CO 2, NADPH + H + function: gain of NADPH + H +, production of rib-5-P for nucleotide synthesis, mutual conversions of monosacharides subcellular location: cytosol organ location: all tissues regulatory enzyme: glucose 6-phosphate dehydrogenase

13. Pentose phosphate pathway – oxidative stage produces rub-5-P Figure is found on http://web.indstate.edu/thcme/mwking/pentose-phosphate-pathway.htmlhttp://web.indstate.edu/thcme/mwking/pentose-phosphate-pathway.html

14. Pentose phosphate pathway – non-oxidative stage includes interconversions of monosaccharides Figure is found on http://web.indstate.edu/thcme/mwking/pentose-phosphate-pathway.htmlhttp://web.indstate.edu/thcme/mwking/pentose-phosphate-pathway.html

15. Glycogen synthesis (glycogenesis) substrate: Glc-6-P product: glycogen function: glucose storage in the form of glycogen cellular location: cytosol organ location: especially in the liver and skeletal muscles, other tissues have lower glycogen storage regulatory enzyme: glycogen synthase Enzyme glycogen synthase is inhibited by phosphorylation (glucagon in liver and epinephrine in muscles)!

16. Glycogen synthesis Glc-6-P → Glc-1-P Glc-1-P + UTP → UDP-Glc + PP i Glycogen synthase catalyzes the formation of  (1 → 4) glycosidic bonds. Branching (formation of  (1 → 6) glycosidic bonds) is performed by enzyme amylo-(1,4 – 1,6)- transglycosylase („branching enzyme“). Figure is found on http://en.wikipedia.org/wiki/Glycogenhttp://en.wikipedia.org/wiki/Glycogen

17. Metabolic pathways serving to supplementation of Glc into the bloodstream – glycogen degradation and gluconeogenesis Glycogen degradation (glycogenolysis) ● substrate: glycogen product: Glc-6-P function: releasing of Glc from glycogen subcellular location: cytosol organ location: liver, skeletal muscles, but also other tissues regulatory enzyme: glycogen phosphorylase Enzyme glycogen phosphorylase is activated by phosphorylation which is induced by hormones glucagon and epinephrine. Insulin inhibits enzyme phosphorylation.

18. Glycogen degradation Glycogen (n Glc) + P i → Glc-1-P + glycogen (n - 1 Glc) Enzyme glycogen phosphorylase catalyzes the cleavage of  1 → 4 bonds. Enzyme amylo-  1 → 6-glucosidase („debranching enzyme“) cleaves  1 → 6 bonds. Glc-1-P ↔ Glc-6-P phosphoglucomutase Glc-6-P glucose-6-phophatase (liver, kidneys, enterocytes) Glc

19. Gluconeogenesis substrates: lactate, pyruvate, glycerol, amino acids – Ala, Asp, Gln etc. product: glucose function: synthesis of Glc from non-sugar precursors subcellular location: mitochondrial matrix + cytosol organ location: liver + kidneys regulatory enzymes: pyruvate carboxylase and PEP carboxykinase Regulatory enzymes are activated by hormones glucagon and cortisol. Insulin inhibits them.

20. Scheme of gluconeogenesis Figure is found on http://web.indstate.edu/thcme/mwking/gluconeogenesis.htmlhttp://web.indstate.edu/thcme/mwking/gluconeogenesis.html

21. Gluconeogenesis Synthesis of PEP is divided into 2 steps: Pyr → matrix of mitochondria → Pyr is carboxylated to oxaloacetate (OA) by pyruvate carboxylase CH 3 -CO-COO - + CO 2 + ATP → - OOC-CH 2 -CO-COO - + ADP + P i OA is transported to the cytosol and decarboxylated to PEP by PEP carboxykinase - OOC-CH 2 -CO-COO - + GTP → PEP + CO 2 + GDP Synthesis of 1 mol Glc consumes 4 mol ATP and 2 mol GTP!

22. Figure was assumed from http://www.biochem.arizona.edu/classes/bioc462/462b/glycolysis.htmlhttp://www.biochem.arizona.edu/classes/bioc462/462b/glycolysis.html

23. Regulation of gluconeogenesis Hormones: activation: cortisol, glucagon, epinephrine inhibition: insulin Enzyme pyruvate carboxylase activation: acetyl-CoA from β-oxidation of FA → source of ATP Enzyme fructose-1,6-bisphosphatase activation: citrate, starvation inhibition: AMP, Fru-2,6-bisP Enzyme glucose-6-phosphatase (in ER of liver, kidneys and enterocytes !)

24. Cori cycle Figure was assumed from http://web.indstate.edu/thcme/mwking/gluconeogenesis.htmlhttp://web.indstate.edu/thcme/mwking/gluconeogenesis.html

25. Glucose-alanine cycle Figure is found on http://web.indstate.edu/thcme/mwking/gluconeogenesis.htmlhttp://web.indstate.edu/thcme/mwking/gluconeogenesis.html

26. Fructose metabolism Fru is a component of sucrose (Glc + Fru) part of Fru in converted to Glc in enterocytes: Fru-6-P → Glc-6-P → Glc part of Fru is absorbed and it is transferred via blood into liver: Fru + ATP → Fru-1-P + ADP by enzyme fructokinase Fru-1-P is broken down to glyceraldehyde (GA) and dihydroxyacetonephosphate (DHAP) by aldolase DHAP enters glycolysis and GA → glyceraldehyde-3-P → glycolysis

27. Galactose metabolism Gal is a component of lactose (Glc + Gal) Gal is absorbed by the same mechanism in enterocytes like Glc → liver Gal is phosphorylated in liver to form Gal-1-P: Gal + ATP → Gal-1-P + ADP by enzyme galactokinase Gal-1-P is converted to UDP-Gal: Gal-1-P + UTP → UDP-Gal + PP i by uridyltransferase UDP-Gal is used to lactose synthesis in mammary gland during lactation epimerization of UDP-Gal to UDP-Glc → glycogen synthesis / synthesis of glucuronic acid / glycoprotein synthesis