ESSENTIALS OF GLYCOBIOLOGY

LECTURE 4MONOSACCHARIDE 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

GlcGlc

Man

Gal

GalNAcGlcNAc

Fuc

GlcA

Xyl

Sia

THE SUGAR LEAGUE

Fru-6-P

THREE WAYS TO ACTIVATE A SUGAR

Activated Sugar Donors

MONOSACCHARIDE TRANSPORTERS

GP

NH2

N

GRK

GR

E RG

w

PESPRYL ERVGRRPETKG

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 central2. Pathways not robust in all cells3. Map is 2-D, not 3-D

Galactosemia is caused by aFailure in Gal-1-P:UDP-GlcUridyl transferase

GALACTOSE ACTIVATION USES ALL THE OPTIONS

GALACTOSE METABOLISM

UDP-Glc + Gal-1-P Glc-1-P + UDP-GalX

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 wereused to treat a human genetic disorder

UDP-GlcNAc epimerase/kinase Defective in two different humandiseases

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 [3H]Mannose

Remains as [4-3H]Galactose

TRICKS OF THE TRADE

?’s

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