Carbohydrate Metabolism I - PDF

Comenius University in Bratislava Jessenius Faculty of Medicine in Martin Department of Medical Biochemistry Carbohydrate- saccharide metabolism I. Carbohydrate digestion and absorption. Glycogenolysis and glycogenogenesis, (principles and regulation) Glycaemia- blood...

Comenius University in Bratislava Jessenius Faculty of Medicine in Martin Department of Medical Biochemistry Carbohydrate- saccharide metabolism I. Carbohydrate digestion and absorption. Glycogenolysis and glycogenogenesis, (principles and regulation) Glycaemia- blood level Glc- 3,3- 5,8 mmol/l results of tisssue metabolism and hormonal regulation Determination of glycosylated Hb- if prolonged level of Glc is higher than physiol. level  Normal Glc level in blood Time Glucose (mg/dL) Glucose (mmol/L) fasting 80-100 4,4-5,6 12 h 80 4,4 3d 70 3,9 5-6 weeks 65 3,6  Sources of blood Glc G mmol/l l u Dietary Glc c o s e in Glycogenolysis gluconeogenesis b 5 l o d hours days Meal fasting prolonged fasting  1. Carbohydrates in the food - primary energy source for organisms, essential part of the food - developed countries app. 40 % (250 – 800 g) per day, average app. 300 g (could be 80 – 90 %) - origin: plants (starch) and animals (minority importance(glycogen)), except: lactose  Carbohydrates – clinical notes !!!! - de novo synthesis-gluconeogenesis from: 1. AA (glucogenic AA) „proteins saving mechanism“ - liver limitation in the nitrogen processing 2. lipids – only glycerol; ↑ lipolysis leads to ketone bodies = ketoacidosis Atkinson-keto diet - therapeutic CAVE: Glycemic index Substitution of Glc- isoglucose (Fru+Glc), Fru- caution-ATS!!!!!  Saccharides in the human diet Polysaccharides: Starch: - amylose (non-branched, α-1,4 bonds, 15 – 35 %) - amylopectin (branched, α-1,6 bonds, 65 – 85 %) Glycogen- animal, alpha bonds - non-starch polysaccharides: - cellulose - pectines Free saccharides: - monosaccharides: glucose, fructose, mannose, ribose, deoxyribose - disaccharides: succrose, lactose, mannose - oligosaccharides: rafinose, fructanes - saccharidic alcohols: sorbitol, manitol, gulcitol, inositol  INTAKE OF saccharides DIGESTION, ABSORPTION AND TRANSPORT  Intestinal absorption: only monosacharides 1. To the liver and 2. extra hepatic perifery Central metabolic role of glucose Glucose substrate for metabolic pathways: 1.) To release energy: a) glycolysis to pyruvate, resp. AcCoA– synthesis of FA and TAG – VLDL in liver b) synthesis of glycogen (storage form of energy) 2. conversion of Glc to other saccharides 3) As intermediates for synthesis of non-sacharidic compounds - lipids, AA 4) Production of glycoproteins, proteoglycans and glycolipids Metabolism starts by Glc phosphorylation glucose -6-P!!.  Saccharides digestion Enzymes: i) salivary glands, ii) pancreas and iii) intestine Salivary glands: Salivary -amylase (ptyalin): - Only hydrolysis of α –1,4 glycosidic bonds Pancreas- hormon CHOLECYSTOKININE Pancreatic -amylase: - continues starch digestion - products of cleavage: amylose – maltose and maltotriose, amylopectin and from glycogen – α - maltodextrines Intestinal mucosal cells- hormon SECRETIN = brush border membranes - cleavage of disaccharides into monosaccharides by enzymes - specific enzymes for single types of disaccharides: – saccharase → glucose + fructose – lactase → glucose + galactose – maltase → glucose + glucose (from starch)  CLINICAL NOTE !!! Intake of maltodextrins- as additive to food High glycemic index, high insulin release, GIT disorders!!!!  Absorption of monosaccharides Duodenum and jejunum - brush border membrane of enterocytes - secondary active transport - symport with Na+ (ratio 1:2) from the intestinal lumen into enterocyte - apical part - facilitate diffusion – from enterocyte → blood on the basolateral membrane - basolateral membrane – Na+, K+-ATPase (antiport 3 Na+ out and 2 K+ in) - simple diffusion – pentose transport - monosaccharides transport velocity is different (Gal → Glu → Fru → Man → pentoses)  Non digested forms of saccharides Cellulose, hemicelllulose, inulin, pektin are resistent to human digestive enzymes- BALLAST bodies- FIBERS Partially hydrolyzed and anaerobically metabolized by intestinal bacteria – probiotics and substrates prebiotics Bacterial fermentation- hydrogen, methane, CO2, sulfane, acetate, propionate, butyrate, lactate- as cancer prevention Effect on cholesterol absorption Physiological role of MICROBIOME – preventative and therapeutical Hormonal regulation of blood glucose level 1. High – insulin, incretins 2. Low- Glu, Epi, GluC, GH INCRETINS 1a. High Glucose in intestine- stimulation of INCRETINES secretion High level of Glc in blood- beta cell of pancreas - INSULIN  INCRETINES – hormones of GIT -support Glc metabolism by stimulation of Insulin secretion GIP-gastric inhibition peptide -endocrinne cells of duodenum,jejunum Effect: high glucose dependent stimulation of Insulin secretion proliferation of ß-cells of pancreas regulation of lipid metabolism in adipocytes stimulation of lipoproteine lipase, regulation of FA synthesis GLP 1 – glucagon like peptid Effect: inhibition of glucagon secretion expression of insuline gene lowers meal intake stimulation of insulin secretion stimulation of glycogenesis and lipogenesis  Clinical note: Incretine therapy, incretine effect „incretine effect“ : difference in insuline response to oral and intravenous glc Support in reaching euglycemia in diabetics Peroral intake Glc Intravenous intake Minutes 2. Hypoglycemia- life threatening situation!!!  Pathologies of saccharide digestion and metabolism Lactose intolerance: - lactose (dimer of Glu and Gal, β-1,4 glycosidic bond) - lactase (β-galactosidase) - bacterial fermentation of lactose - symptoms: abdominalgia, diarrhoea Fructose intolerance: - aldolase B absence – accumulation of fructose-1-P - symptoms: hypoglycaemia, vomitus, icterus, hemorrhage Glycogenogenesis and glycogenolysis Glycogen - storage form in the animals - liver (the highest concentration) - Release of Glc into the blood !!!!! - skeletal muscle (the highest content) - Muscles – fuel reserve for ATP synthesis for musc. contraction - No release into the blood !!!! - branched chain homopolysaccharide – GLUCAN, (α-D-Glucose) - linear chain - primary α-1,4 glycosidic bond - branch - containing α-1,6 glycosidic bond - chains app. 8 – 14 Glu residues - glycogen molecule – app. 4000 chains - discrete cytoplasmic granules Glycogenogenesis (synthesis of glycogen) (Liver, sk. muscles, also kidneys, brain...) - endergonic reaction (energy from ATP and UTP) - Localisation of the synthesis: cytosol - main substrate: Glucose 6-P to be converted to Glc -1P 1. synthesis of UDP-glucose – Glu-1-phosphate + UTP → UDP-glucose + PPi - enzymes: - phosphoglucomutase (Glu 6-P conversion) and - UDP-glucosepyrophosphorylase 2. Elongation of glycogen chains an formation of branches - enzymes: a) glycogen synthase – key enzyme of glycogen synth. b) branching enzyme Glycogen synthase: - reaction of UDP-Glu and glycogen fragment containing n- Glu residues - formation of α -1,4 glycosidic bonds - endergonic reaction –1 macroergic bond - synthesis app. 12 – 16 Glu residues - primer (GLYCOGENIN- protein) - in the absence of glycogen serves as acceptor of UDP-Glu – autoglycosylation - glycogen initiator synthase (first Glu residue) - autocatalytic formation of the glycosylated protein (glycogenin + Glu) Branching enzyme: - amylo-α 1, 4 → α-1,6 transglucosidase (glucosyl α-4:6 transferase) - no energy requirements Glycogenolysis (degradation of glycogen) - The primary product is Glu 1-P 1. Shortening of chains - exergonic reaction - Key enzyme: glycogen phosphorylase - key enzyme Glycogen phosphorylase (coenzyme PyridoxalP, vitamine B6) - cleaves the α –1,4 glyc. bonds at the nonreducing ends by phosphorolysis - reaction between inorganic phosphate and glycogen - Glu-1-P and glycogen (n-1) units - breakdown until Glu 4 from the branching site 2. Removal of branches Debranching enzyme - bifunctional protein, two enzymatic activities - transglycosylase activity – - transfer of 3-Glu remnant and formation of α -1,4 glycosidic bond - α -1,6 hydrolase activity (amylo α -1,6 glucosidase activity) – Glu release !!! Glu – Glu-1-P release → phosphoglucomutase → Glu-6-P (90 %) - free Glu (10 %) Glucose 6-P phosphatase - Present only in liver - cleaves Glu 6-P to phosphoric acid and free glucose, which can pass through the cell membrane into the blood to maintain Glc level General overview of glycogen metabolism Regulation of glycogen synthesis and degradation Regulation is based on the blood glucose concentration High level of glucose in blood – prefer glycogen synthesis (mostly liver) activated by insulin, in parallel inhibition of the glycogen cleavage Lower level of glucose in blood (deficiency of Glu in tissues) – cleavage of glycogen, stimulated by glucagone and adrenaline ( alpha and beta) in LIVER,muscles Mechanism of the action of glycogen enzymes: phosphorylation or dephosphorylation of the molecules (covalent modification) Glycogen phosphorylase (glycogen cleavage) – after phosphorylation is converted from inactive to active forrm Glycogen synthase – after de- phosphorylation, from inactive phosphorylated form is changed to active dephosphorylated P P P active form inactive form High glucose: High INSULIN: 1. Phosphatases- activation 2. Glykogen Synthase kinase 3- inhibtion glycogenosynthesis  1. Phosphatases activation by insulin: - Stimulation Glyc. synthase - Inhibition of Glyc. phosphorylase - Increase of Glycogen synthesis Insulin Insulin Insulin  2. Inhibition of Glycogen synthase kinase 3 (GSK3) role also in other insulin functions 2. GSK3  Low plasma glucose- release of contraregulatory hormons 1. Glucagon via cAMP dep. protein kinases- PK-A 2. Adrenalin/noradrenalin via beta Adrenergic R: 3. Adrenalin - Alpha Adr.R- CaCaM dep. Protein kinases activation ------- glycogenolysis  LIVER 1. GLUCAGON- Protein Kinase-A 2. Epinephrin- beta adrenergic R-  LIVER 3. Alpha Adrenergic R--------PL-C------PKC - Epinephrine  MUSCLES 1. AMP 2. Ca-CaM 3. Beta AdrenergicR POSITIVE REGULATION of glycogenolysis in muscles- :1. AMP- allosterically – muscle work- high AMP 2. Ca2+-CaM (endoplasmic reticulum )- part of contraction 3. beta AR- cAMP-adrenalin Nerve impulse Muscle contraction 1. Allosteric regulation- mainly in skeletal muscles allosteric effectors: a) 5´ AMP – activation of phosphorylase kinase → activation of glycogen phosphorylase MUSCLE - signal – low energetic charge, ATP level is low b) ATP, Glu-6-P – allosteric inhibitors of phosphorylase kinase→ inhibition of of glycogen phosphorylase c) Glu 6-P – allosteric activator of glycogen synthase –LIVER, MUSCLE- glycogenosynthesis Phosphorylation – activated by glucagone and adrenaline Dephosphorylation – activated by insulin These processes are catalyzed by several different protein kinases that are regulated by cAMP or other signalling mechanisms. Insulin – leads to decrease of cAMP concentration in the cells, and causes dephosphorylation of particular enzymes.  Glycogen storage diseases - Rare inherited diseases - defect of the particular enzyme (in the synthesis or degradation of the glycogen) - formation of the abnormal structure of glycogen or accumulation of the normal structure of glycogen - glycogen accumulation (tesaurismoses) - If affected liver – main symptoms: hepatomegaly and hypoglycaemia - If affected muscle – main symptoms: weakness and muscle pain Clinical notes 1/ m. von Gierke 5/ m McArdle - Glu-6-phosphatase deficiency - glycogen phosphorylase deficiency - affected organs: liver, kidney and intestine - affected organ: muscle 2/ m. Pompe 6/ m. Hers - α-1,4 glukosidase deficiency - glycogen phosphorylase deficiency - generalized affected organs: heart, muscle, liver - affected organ: liver 3/ m. Cori 7/ m. Tarui - amylo-1,6 glucosidase deficiency - phosphofructo kinase deficiency - affected organs: liver, muscle - affected organ: muscle 4/ m. Anderson - branching enzyme deficiency - affected organs: liver, kidney