Metabolism of saccharides Pavla Balínová

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

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, …)

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, small intestine, kidneys

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

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

Hexokinase vs. glucokinase Figure is found on K M hexokinase = 0,1 mM K M glucokinase = 10 mM

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!

Glycolysis Figure is found on

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!

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

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

Pentose phosphate pathway – oxidative stage produces rub-5-P Figure is found on

Pentose phosphate pathway – non-oxidative stage includes interconversions of monosaccharides Figure is found on

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)!

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

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.

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

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.

Scheme of gluconeogenesis Figure is found on

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!

Figure was assumed from

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 !)

Cori cycle Figure was assumed from

Glucose-alanine cycle Figure is found on

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

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