ESSENTIALS OF GLYCOBIOLOGY LECTURE 4 MONOSACCHARIDE METABOLISM AND SUGAR NUCLEOTIDE TRANSPORTERS Hud Freeze

INTRODUCTION Traditional Approach--Study the enzymes and substrates in cells Holistic Approach--Add homeostasis and disease considerations Glucose is the central player, but not the only player Monosaccharides = sugars First Principles Meet the players Source of sugars: Transport into cells salvage them make them in cells Activate the sugars Deliver activated sugars to the proper location

Glc Man Gal GalNAcGlcNAc Fuc GlcA Xyl Sia THE SUGAR LEAGUE Fru-6-P

THREE WAYS TO ACTIVATE A SUGAR

Activated Sugar Donors

MONOSACCHARIDE TRANSPORTERS

G P NH 2 N GRK GR E RG w PESPRYL ERVGRR PETKG E Y Y COOH G G Facilitated Diffusion Transporter Topology and Signature Motifs

GLUT1 GLUT2 GLUT4 GLUT3 GLUT14 GLUT8 GLUT6 GLUT10 GLUT12 GLUT5 GLUT9 GLUT11 HMIT fructose glucose/fructose H + -myo-inositol erythrocytes, brain liver, islets brain, testis, ovary muscle, fat, heart Intestine, testis, kidney spleen, leukocytes, brain blastocysts, brain, testis liver, kidney Liver, pancreas Heart, muscle Heart, prostate brain testis Human Hexose Facilitated Diffusion Transporters SPECIFICITY NAME LOCALIZATION

OVERVIEW OF SUGAR METABOLISM IN CELLS Remember: 1. Glucose is central 2. Pathways not robust in all cells 3. Map is 2-D, not 3-D

Galactosemia is caused by a Failure in Gal-1-P:UDP-Glc Uridyl transferase

GALACTOSE ACTIVATION USES ALL THE OPTIONS

GALACTOSE METABOLISM

UDP-Glc + Gal-1-P Glc-1-P + UDP-Gal X Galactosemia Inability to metabolize Galactose--potentially lethal Treatment: Galactose restricted diet Even with proper diet: patients have dyspraxic speech, ovarian failure, poor growth, neurological impairment Knockout mouse does not have disease phenotype! Other genes important

Mannose supplements can treat a human disorder: Fru-6-P-->Man-6-P

Oral fucose supplements were used to treat a human genetic disorder

UDP-GlcNAc epimerase/kinase Defective in two different human diseases

Fru-6-P is the only freely soluble glycolytic intermediate Are the enzymes and substrates in the cytosol really “ soluble ” ?

If all the sugars can be inter-converted, how can you follow just one?

Remains as [ 3 H]Mannose Remains as [4- 3 H]Galactose TRICKS OF THE TRADE

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All of the precursors are made in the cytoplasm or nucleus-- BUT, most glycoconjugates are made in the Golgi or ER

TRANSPORTERS IN THE GOLGI

TRANSPORTERS IN THE ENDOPLASMIC RETICULUM

HOW IT WORKS

TRANSPORTERS: A FAMILY IN LOVE WITH ITS MEMBRANES

SUMMARY AND TAKE HOME MESSAGES Glucose can be made into all sugars (monosaccharides) All sugars require activation and most require transport of the activated forms Transporters deliver the sugars to cells Salvaged and imported sugars can contribute to glycoconjugate synthesis The relative contributions of each source may be cell/tissue specific Some disorders in sugar metabolism can be treated with dietary modifications Specificity of sugar nucleotide and monosaccharide transporters is hard to predict Sugar is good for you---mostly