biochem 6-6

METABOLISM of MONOSACCHARIDES

A. Glucose - most common monosaccharide consumed by humans

B. Other Sugars - major sources of cellular energy

1. Fructose - significant amounts in the diet - important contributions to energy metabolism

2. Galactose - significant amounts in the diet - important contributions to energy metabolism - important component of cell structural carbohydrates

3. Mannose

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

FRUCTOSE METABOLISM

D. Phosphorylation of Fructose 1. Enzymes a. Hexokinase - phosphorylates glucose in all cells - several other hexoses can serve as substrates - low affinity (high Km) for fructose  low amount of fructose is converted to fructose 6-phospohate by this enzyme - can directly phosphorylate fructose to F6P in muscle b. Fructokinase (Ketohexokinase) - primary mechanism for fructose phosphorylation - converts fructose  fructose 1-phosphate (ATP as phosphate donor) i. Found in - liver (processes most of dietary fructose) - kidney - small intestinal mucosa ii. Activity is Not Affected by - feeding-fasting cycle - insulin levels

FRUCTOSE METABOLISM

E. Fructose 1-Phosphate Cleavage 1. Aldolase B (Phosphofructaldolase/Fructose 1-Phosphate Aldolase) - isoenzyme of aldolase in the glycolytic pathway - cleaves (aldol cleavage) fructose 1-phosphate into - DHAP  glycolysis or gluconeogenesis - D-glyceraldehyde  metabolized by a number of pathways Aldolase A - cleaves fructose 1,6-biphosphate  DHAP + glyceraldehyde 3-phosphate

F. Interconversion of DHAP and Glyceraldehyde 1. Enzyme - triose phosphate isomerase 2. Fate of Glyceraldehyde 1. Phosphorylated to glyceraldehyde 3-phosphate  glycolysis, gluconeogenesis - by glyceraldehyde kinase 2. Oxidized to glycerate  serine 3. Reduced to glycerol  gluconeogenesis, triglyceride biosynthesis

FRUCTOSE METABOLISM

G. Kinetics of Fructose Metabolism - fructose 1-phosphate metabolism  trioses  bypass PFK (major rate-limiting glycolytic step)  rate of fructose is more rapid than glucose metabolism - elevated dietary fructose  rapid acetyl CoA production  elevates rate of liver lipogenesis

FRUCTOSE METABOLISM

H. Genetic Diseases of Fructose Metabolism 1. Fructokinase Deficiency (Essential Fructosuria) - benign condition - accumulated fructose  urine 2. Aldolase B Deficiency (Hereditary Fructose Intolerance)  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

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)

I. Conversion of Mannose to Fructose 1. Mannose - carbon 2 epimer of glucose - little mannose in the dietary carbohydrate - most intracellular mannose is synthesized from - fructose - salvaging of preexisting mannose by hexokinase - important component of glycoproteins - hexokinase phosphorylates mannose  mannose 6-phosphate  isomerized to fructose 6-phosphate by phosphomannose isomerase

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 reduction of an aldehyde group  polyol formation  increased hydrophilic nature  increased water shell  inability to cross membranes 1. D-Sorbitol Synthesis a. Aldose Reductase - interconversion of glucose and sorbitol i. Found in - lens - retina - Schwann cells of peripheral nerves - kidney - placenta - RBCs - cells of the ovaries and seminal vesicles

b. Sorbitol Dehydrogenase - interconversion of sorbitol and fructose i. Found in - liver - ovaries - sperm - seminal vesicle ii. Function - provides a mechanism by which sorbitol is converted into a substrate that can enter glycolysis or gluconeogenesis c. Fructose - preferred carbohydrate energy source for sperm cells

d. Sperm Mitochondria - only such organelle to contain LDH - lactate formed from fructolysis  oxidized to CO2 and H2O

2. Effect of Hyperglycemia on Sorbitol Metabolism - insulin is not required for the entry of glucose into the cells listed above  hyperglycemia  increased intracellular glucose entry  increased intracellular glucose concentration + adequate NADPH supply  aldose reductase to produce increased amount of sorbitol (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)

GALACTOSE METABOLISM 1. Lactose (Galactosyl -1,4-Glucose) - major dietary source of galactose - disaccharide - milk sugar - from - milk - milk products - lysosomal degradation of complex carbohydrates (glycoproteins, glycolipids) - normal cell turnover - digested by -galactosidase 2. Galactose - differs from glucose in the configuration of the OH- group at C4 - galactose entry into cells is not insulin-dependent

GALACTOSE

A. Function - converts dietary galactose (from lactose) to a form that can enter glycolysis

B. Location - kidney - liver - brain

C. Phosphorylation of Galactose - catalyzed by galactokinase - ATP as phosphate donor - produces galactose 1-phosphate

GALACTOSE METABOLISM

D. UDP-Galactose Formation 1. Enzymes a. Galactose 1-Phosphate Uridyltransferase - deficient in patients with classic galactosemia - transfers the UMP group of UDP-glucose to produce UDP-galactose and glucose-1- phosphate i. Phosphoglucomutase - converts glucose-1-phosphate to glucose-6-phosphate 2. Enzyme Deficiency  accumulation in cells and tissues - galactose 1-phosphate - in neural tissues  mental retardation - in liver  liver cirrhosis - galactose  cataracts - physiologic consequences are similar to essential fructose intolerance but a wider spectrum of tissues is affected

GALACTOSE METABOLISM

E. Use of UDP-Galactose as a Carbon Source for Glycolysis or Gluconeogenesis 1. UDP-Hexose 4-Epimerase - convert UDP-galactose to its carbon 4 epimer UDP-glucose

F. Role of UDP-Galactose in Biosynthetic Reactions 1. UDP-Galactose - donor of galactose units in a number of synthetic pathways a. Synthesis of - lactose - glycoproteins - glycolipids - glycosaminoglycans

2. -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 2. Galactokinase Deficiency - accumulation of galactose in blood and tissues - in lens of the eye  galactose  reduced to galactitol by aldose reductase  osmotic effect  cataracts 3. Effects of Aldose Reductase (nsa picture)

H. Case: Galactosemia 1. Presentation A male infant, although normal at birth, was difficult to feed and, when fed he exhibited vomiting and diarrhea and an overall failure to thrive. At 5 days of age, he began to exhibit jaundice. 2. Diagnosis a. Urinalysis - (+) test for reducing sugars - no glucose present by glucose oxidase assay b. Galactose assay - urine and serum  high levels c. Assay for Galactose 1-Phosphate Uridyl Transferase in RBCs - no activity  enzyme deficiency  galactosemia 3. Discussion a. Mild Jaundice - indicative of the onset of damage to the liver b. High Reducing Sugar Levels with No Glucose  galactosemia 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

LACTOSE METABOLISM

A. Lactose (-D-Galactosyl-(1,4)--D-Glucose) Synthesis - milk sugar 1. Enzyme - lactose synthase (UDP-galactose : glucose galactosyltransferase) - in the endoplasmic reticulum - transfers galactose from UDP-galactose to glucose releasing UDP a. 2 Proteins i. Protein A (-D-Galactosyltransferase) - in tissues other than lactating mammary glands, it transfers galactose from UDP-galactose to N-acetyl-D-glucosamine  same -1,4 linkage  N-acetyllactosamine formation (component of structurally important glycoproteins) ii. Protein B - found only in lactating mammary glands - it is -lactalbumin found in large quantities in milk - forms a complex with protein A  specificity change of protein A  lactose formation

LACTOSE METABOLISM

B. Hormonal Control of Lactose Synthesis 1. Prior to and During Pregnancy - mammary glands synthesize N-acetyllactosamine 2. During Pregnancy - progesterone inhibits protein B synthesis 3. After Birth - progesterone levels drop significantly  stimulating synthesis of prolactin (peptide hormone)  stimulates synthesis of galactosyl transferase and -lactalbumin (protein B) 4. Protein B - forms a complex with protein A (enzyme)  changed specificity of that transferase (instead of N-acetyllactosamine)  lactose production