Biochemistry - Metabolism of Monosaccharides & Disaccharides (1A) PDF

BIOCHEMISTRY Metabolism of Monosaccharides & Disaccharides BRENDO V. JANDOC, M.D. 1A...

BIOCHEMISTRY Metabolism of Monosaccharides & Disaccharides BRENDO V. JANDOC, M.D. 1A D. Phosphorylation of Fructose OUTLINE 1. Enzymes I. INTRODUCTION TO METABOLISM OF a. Hexokinase MONOSACCHARIDES AND DISACCHARIDES  phosphorylates glucose in all cells II. FRUCTOSE METABOLISM  several other hexoses can serve as substrates III. GALACTOSE METABOLISM  low affinity (high Km) for fructose > low amount of fructose IV. LACTOSE METABOLISM is converted to fructose 6-phospohate by this enzyme V. ETHANOL METABOLISM  can directly phosphorylate fructose to F6P in muscle b. Fructokinase (Ketohexokinase)  primary mechanism for fructose phosphorylation I. INTRODUCTION TO METABOLISM OF MONOSACCHARIDES AND  converts fructose > fructose 1-phosphate (ATP as DISACCHARIDES phosphate donor) METABOLISM of MONOSACCHARIDES i. Found in liver (processes most of dietary fructose), kidney, A. Glucose: most common monosaccharide consumed by humans small intestinal mucosa B. Other Sugars: major sources of cellular energy ii. Activity is Not Affected by feeding-fasting cycle and insulin levels 1. Fructose E. Fructose 1-Phosphate Cleavage  significant amounts in the diet (primarily in disaccharides) 1. Aldolase B (Phosphofructaldolase/Fructose 1-Phosphate  important contributions to energy metabolism Aldolase) 2. Galactose  isoenzyme of aldolase in the glycolytic pathway  significant amounts in the diet (primarily in disaccharides)  cleaves (aldol cleavage) fructose 1-phosphate into  important contributions to energy metabolism  DHAP > glycolysis or gluconeogenesis  important component of cell structural carbohydrates  D-glyceraldehyde > metabolized by a number of pathways 3. Mannose Aldolase A  cleaves fructose 1,6-biphosphate > DHAP + glyceraldehyde 3- phosphate II. FRUCTOSE METABOLISM A. Function: converts dietary fructose into a substrate for glycolysis B. Dietary Sources of Fructose 1. Western Diet: 10 % of the calories supplied by fructose (50 g/day) 2. Source of Fructose - sucrose (disaccharide of fructose and glucose), fruits, vegetables, honey 3. Fructose - cellular entry is not insulin dependent - extremely poor elicitor of insulin secretion C. Location: muscle, kidney, liver Trans | 1 of 9  BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides F. Interconversion of DHAP and Glyceraldehyde 1. Enzyme  triose phosphate isomerase 2. Fate of Glyceraldehyde  Phosphorylated to glyceraldehyde 3-phosphate > glycolysis, gluconeogenesis by glyceraldehyde kinase  Oxidized to glycerate > serine  Reduced to glycerol > gluconeogenesis, triglyceride biosynthesis G. Kinetics of Fructose Metabolism H. Genetic Diseases of Fructose Metabolism  fructose 1-phosphate metabolism > trioses > bypass PFK 1. Fructokinase Deficiency (Essential Fructosuria) (major rate-limiting glycolytic step) > rate of fructose is more  benign condition rapid than glucose metabolism  accumulated fructose > urine  elevated dietary fructose > rapid acetyl CoA production > 2. Aldolase B Deficiency (Hereditary Fructose Intolerance) elevates rate of liver lipogenesis  fructose 1-phosphate accumulation in the cells > liver and kidney damage  severe disturbance of liver and kidney metabolism  estimated to occur in 1:20,000 live births a. Manifestations  first symptoms appear when the baby is weaned and fed food containing sucrose or fructose  fructose 1-phosphate accumulate  significant fall of Pi levels (“sequestering of phosphate”) and therefore ATP > AMP degraded > hyperuricemia  decreased availability of hepatic ATP affects:  gluconeogenesis > hypoglycemia with vomiting  protein synthesis > decrease in blood clotting factors (> bleeding disorders) and other essential proteins S1T1 2 of 9  BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides b. Diagnosis of Hereditary Fructose Intolerance  fructose in the urine  restriction fragment length polymorphism test c. Treatment  limit the amount of dietary fructose (and sucrose) J. Conversion of Glucose to Fructose by Way of Sorbitol (Fructose Metabolism in Spermatozoa)  most sugars, upon intracellular entry, are rapidly phosphorylated > trapped within the cells  alternate metabolism of monosaccharide  provide an additional hydroxyl group by the Ingestion of Large Quantities of Fructose reduction of an aldehyde group > polyol has Profound Metabolic Consequences formation > increased hydrophilic nature > increased water shell > inability to cross  Diets high in sucrose or in high-fructose syrups (HFS) used membranes in manufactured foods and beverages lead to large 1. D-Sorbitol Synthesis amounts of fructose (and glucose) entering the hepatic a. Aldose Reductase: interconversion of glucose and sorbitol portal vein.  Found in lens, retina, Schwann cells of peripheral nerves,  Fructose > more rapid glycolysis in the liver than does kidney, placenta, RBCs, glucose because it bypasses the regulatory step catalysed cells of the ovaries and by phosphofructokinase > allows fructose to flood the seminal vesicles pathways in the liver > increased fatty acid synthesis, b. Sorbitol Dehydrogenase: esterification of fatty acids, and secretion of VLDL, which interconversion of sorbitol and may raise serum triacylglycerols and ultimately raise LDL fructose cholesterol concentrations  Found in liver, ovaries,  Fructokinase > does not act on glucose, and activity is not sperm, seminal vesicle affected by fasting or by insulin, which may explain why fructose is cleared from the blood of diabetic patients at a  Function: provides a normal rate mechanism by which sorbitol is converted I. Conversion of Mannose to Fructose into a substrate that can 1. Mannose enter glycolysis or gluconeogenesis  carbon 2 epimer of glucose c. Fructose: preferred  little mannose in the dietary carbohydrate carbohydrate energy source for  most intracellular mannose is synthesized from fructose sperm cells  salvaging of preexisting mannose by hexokinase d. Sperm Mitochondria  important component of glycoproteins  only such organelle to  hexokinase phosphorylates mannose > mannose 6- contain LDH phosphate > isomerized to fructose 6-phosphate by  lactate formed from phosphomannose isomerase fructolysis > oxidized to CO2 and H2O S1T1 3 of 9  BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides 2. Effect of Hyperglycemia on Sorbitol Metabolism 2. Galactose  insulin is not required for the entry of glucose into the cells  differs from glucose in listed above > hyperglycemia > increased intracellular the configuration of the glucose entry > increased intracellular glucose OH- group at C4 concentration + adequate NADPH supply > aldose  galactose entry into cells reductase to produce increased amount of sorbitol is not insulin-dependent (cannot pass freely across cell membranes (remains trapped in the cells)  increased intracellular sorbitol concentration is exacerbated by low or absent sorbitol dehydrogenase (retina, lens of the eye, kidney, nerve cells) > sorbitol accumulation > strong osmotic effects > water imbibition and retention > cell swelling > cataract formation, peripheral neuropathy, vascular problems > nephropathy, retinopathy (complications of diabetes mellitus) A. Function: converts dietary galactose (from lactose) to a form that can enter glycolysis B. Location: kidney, liver, brain C. Phosphorylation of Galactose Loading of the Liver with Fructose may Potentiate  catalyzed by galactokinase Hypertriacylglycerolemia, Hypercholesterolemia, & Hyperuricemia  ATP as phosphate donor  In the liver, fructose increases fatty acid and triacylglycerol  produces galactose 1-phosphate synthesis and VLDL secretion, leading to D. UDP-Galactose Formation hypertriacylglycerolemia —and increased LDL cholesterol— 1. Enzymes which can be regarded as potentially atherogenic. a. Galactose 1-Phosphate Uridyltransferase  In addition, acute loading of the liver with fructose, as can  deficient in patients with classic galactosemia occur with intravenous infusion or following very high  transfers the UMP group of UDP-glucose to produce UDP- fructose intakes, causes sequestration of inorganic galactose and glucose-1-phosphate phosphate in fructose-1-phosphate and diminished ATP  Phosphoglucomutase: converts glucose-1-phosphate to synthesis. As a result, there is less inhibition of de novo glucose-6-phosphate purine synthesis by ATP, and uric acid formation is 2. Enzyme Deficiency increased, causing hyperuricemia, which is the cause of  accumulation in cells and tissues gout.  galactose 1-phosphate  Since fructose is absorbed from the small intestine by  in neural tissues > mental retardation (passive) carrier-mediated diffusion, high oral doses may  in liver > liver cirrhosis lead to osmotic diarrhea.  galactose > cataracts  physiologic consequences are similar to essential fructose III. GALACTOSE METABOLISM intolerance but a wider spectrum of tissues is affected 1. Lactose (Galactosyl > β-1,4-Glucose) E. Use of UDP-Galactose as a Carbon Source for Glycolysis or  major dietary source of galactose: disaccharide, milk sugar Gluconeogenesis  from milk, milk products, lysosomal degradation of 1. UDP-Hexose 4-Epimerase: convert complex carbohydrates (glycoproteins, UDP-galactose to its carbon 4 epimer UDP-glucose glycolipids), normal cell turnover  digested by > β-galactosidase S1T1 4 of 9  BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides  Epimerase uses NAD+ to oxidize alcohol to ketone, then reduces ketone back to alcohol.  Glucose is converted to galactose for lactose synthesis, and galactose converted to glucose > part of the pathway for, metabolism of dietary galactose. F. Role of UDP-Galactose in Biosynthetic Reactions 1. UDP-Galactose: donor of galactose units  Synthesis of lactose, glycoproteins, glycolipids, glycosaminoglycans 2. β-Galactosidase Deficiency, Dietary Galactose Deficiency  glucose 1-phosphate > UDP-glucose > UDP-galactose by UDP-hexose 4- epimerase G. Disorders of Galactose Metabolism 1. Classic Galactosemia (Uridyltransferase deficiency )  Autosomal recessive disorder (1:30,000 births)  Causes galactosemia and galactosuria, vomiting, diarrhea, and jaundice  Accumulation of galactose 1-phosphate and galactitol in nerve, lens, liver, and kidney tissue causes liver damage, severe mental retardation, and cataracts  Antenatal diagnosis is possible by chronic villus sampling. Newborn screening is available.  Therapy: Rapid diagnosis and removal of galactose (and H. Case: Galactosemia therefore, lactose) from the diet 1. Presentation: A male infant, although normal at birth, was difficult to feed and, when fed he exhibited vomiting and diarrhea and an  Despite adequate treatment, at risk for developmental overall failure to thrive. At 5 days of age, he began to exhibit jaundice. delays and, in female, premature ovarian failure 2. Diagnosis 2. Galactokinase Deficiency a. Urinalysis  accumulation of galactose in blood and tissues  (+) test for reducing sugars  in lens of the eye > galactose > reduced to galactitol by  no glucose present by glucose oxidase assay aldose reductase > osmotic effect > cataracts b. Galactose assay  rare autosomal recessive disorder  urine and serum > high levels 3. Effects of Aldose Reductase c. Assay for Galactose 1-Phosphate Uridyl Transferase in RBCs  the enzyme is present in liver, kidney, retina, lens, nerve  no activity > enzyme deficiency > galactosemia tissues, seminal vesicles, and ovaries 3. Discussion  physiologically unimportant in galactose metabolism unless a. Mild Jaundice: indicative of the onset of damage to the liver galactose levels are high (as in galactosemia) b. High Reducing Sugar Levels with No Glucose: galactosemia  elevated galactitol can cause cataracts c. Confirmatory Test: assay for Galactose 1-Phosphate Uridyl Transferase in RBCs 4. Treatment  galactose-free diet  as infant grows older > diet excluding milk and milk products S1T1 5 of 9  BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides IV. LACTOSE METABOLISM B. Hormonal Control of Lactose Synthesis A. Lactose (β-D-Galactosyl-(1,4)-α-D-Glucose) Synthesis 1. Prior to and During Pregnancy  milk sugar  mammary glands synthesize N-acetyllactosamine 1. Enzyme 2. During Pregnancy  lactose synthase (UDP-  progesterone inhibits protein B synthesis galactose : glucose 3. After Birth galactosyltransferase)  progesterone levels drop significantly > stimulating  in the endoplasmic synthesis of prolactin (peptide hormone) > stimulates reticulum synthesis of galactosyl transferase and β-lactalbumin (protein B)  transfers galactose 4. Protein B from UDP-galactose to glucose releasing UDP  forms a complex with protein A (enzyme) > changed a. 2 Proteins specificity of that transferase (instead of N-  Protein A (β-D-Galactosyltransferase) acetyllactosamine) > lactose production  in tissues other than lactating mammary glands, it C. Lactose Utilization transfers galactose from UDP-galactose to N-acetyl- 1. Lactase D-glucosamine > same β-1,4 linkage  hydrolyzes lactose > galactose + glucose > N-acetyllactosamine formation (component of  in small intestines structurally important glycoproteins) a. 2 forms  Protein B  infants and young children and adults  found only in lactating mammary glands 2. Lactose Intolerance  it is α-lactalbumin found in large quantities in milk a. Cause: lactase deficiency  forms a complex with protein A > specificity change  infants unable to tolerate milk (their primary food) > serious of protein A > lactose formation consequences  treated by giving lactose-free formula  less serious in adults (treated by avoiding milk and milk products) b. 3 Types  premature infants (transient or congenital deficiency)  deficiency secondary to surgery where part of the small intestines is removed  deficiency due to mucosal cell damage S1T1 6 of 9  BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides V. ETHANOL METABOLISM METABOLISM of AMINO SUGARS  may replace carbohydrates as energy source when ingested A. Glucosamine 6-Phosphate in large amounts  precursor of all hexosamine residues in GAGs A. Oxidation to Acetate in the Liver Glucose Is the Precursor of Amino Sugars (Hexosamines) 1. Ethanol is oxidized by cytosolic alcohol dehydrogenase to  Amino sugars are important components of glycoproteins, acetaldehyde of certain glycosphingolipids (eg, gangliosides), and of 2. Acetaldehyde is further oxidized to acetate by glycosaminoglycans. mitochondrial aldehyde dehydrogenase  The major amino sugars are the hexosamines glucosamine, 3. Acetate leaves the liver > acetyl CoA > CO2 by other tissues galactosamine, and mannosamine, and the nine-carbon 4. Acetyl CoA may also be formed in the liver > lipid compound sialic acid. biosynthesis  The principal sialic acid found in human tissues is N-acetylneuraminic acid (NeuAc). URONIC ACID PATHWAY (GLUCURONIC ACID CYCLE) A. Reactions B. Functions 1. Source of Activated Glucuronate for a. Formation of Glucuronosides for Detoxifying the Body i. Glucuronoside: glucuronic acid connected to other compounds by glycosidic bond ii. Many substances and drugs are eliminated from the body by the formation of glucuronoside derivatives (conjugation) > become water-soluble > excreted in the bile or urine  steroid hormones  bilirubin  morphine  salicylic acid  menthol B. Ethanol may oxidized by microsomal cytochrome P450 oxidase (which is induced by ethanol) Formation of bilirubin diglucoronide  gylcosidic bond > the anomeric hydroxyl of glucronate and the carboxylate groups of bilirubin  addition of the hydrophilic carbohydrate group and the negatively charged carboxyl group of the glucoronide increases the water solubility of the conjugated bilirubin and allows the otherwise insoluble bilirubin to be excreted in the urine or bile S1T1 7 of 9  BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides iii. General Reaction  the –ide in the name glucoronide > compounds are glycosides  xenobiotics (drugs) > pharmacologically, endocrinologically, or toxicologically active substances that are not produced endogenously > foreign to an organism c. Chondroitin Sulfate (GAG) Biosynthesis 2. Integral to the Formation of Ascorbic Acid (Vitamin C) - in most animals except humans 3. Formation of Pentoses 4. Metabolism of Nonphosphorylated Sugar Derivatives 5. Source of UDP-Glucose for glycogen formation Formation of Glucuronate and Glucuronides  Glycosidic bond > anomeric hydroxyl of glucuronate (at carbon 1) and the hydroxyl group of a nonpolar compound Overview of UDP-glucose metabolism  Negatively charged carboxyl group of the glucuronate  Activated glucose moiety of UDP-glucose can be attached increases the water solubility and allows otherwise by a glycosidic bond to other sugars, as in glycogen or the nonpolar compiunds to be excreted in the urine or bile sugar oligosaccharide and polysaccharide side chains of b. Biosynthesis of Certain Polysaccharides proteoglycans, glycoproteins, and glycolipids UDP-Glucuronate  UDP-glucose also can be oxidized to UDP-glucuronate or  donor for the glucuronyl moiety in some polysaccharides epimerized to UDP-galactose, a precursor of lactose (heparin)  formed from UDP-Glucose Disruption of the Uronic Acid Pathway is caused by Enzyme Defects & Some Drugs Rare Benign Hereditary Condition- Essential Pentosuria  considerable quantities of xylulose appear in the urine > lack of xylulose reductase, the enzyme necessary to reduce xylulose to xylitol  xylulose is a reducing sugar > can give false positive results when urinary glucose is measured using alkaline copper reagents  Various drugs increase the rate at which glucose enters the uronic acid pathway > example, administration of barbital Metabolic Routes of UDP-GLucuronate or chlorobutanol to rats > significant increase in the conversion of glucose to glucuronate, l-gulonate,  glucuronate from UDP-glucuronate is incorporated into and ascorbate. glycosaminoglycans (GAGs), where certain of the  Aminopyrine and antipyrine increase the excretion glucuronate residues are converted to iduronate of xylulose in pentosuric subjects.  precursor of UDP-xylose, another sugar residue in  Pentosuria also occurs after consumption of relatively large incorporated into glycosaminoglycans amounts of fruits such as pears that are rich sources of  glucuronate is also transferred to the carboxyl groups of pentoses (alimentary pentosuria). bilirubin or the alcohol groups of steroids, drugs, and xenobiotics to form glucuronides S1T1 8 of 9 BIOCHEMISTRY Metabolism of Monosaccharides and Disaccharides REFERENCES 1. Harvey RA. Lippincott’s Illustrated Reviews: Biochemistry. 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2011. 2. Rodwell VW., et.al. Harper’s Illustrated Biochemistry. 13th ed. The McGraw-Hill Education, 2015. S1T1 9 of 9