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Carbohydrate

metabolism IV


Outline

• Metabolism of amino sugars

GAG, proteoglycans, glycoproteins and

glycolipids

• Metabolism of complex saccharide

• Disorders of complex carbohydrate

metabolism


Amino sugars

• An amino group replaces a monosaccharide

group -OH

• Amino group is sometimes acetylated

a-D-glucosamine

a-D-N-acetylglucosamine

GlcNac


Others Amino sugars

• N-acetylneuraminic acid - sialic acid

(is often found as a terminal residue of

oligosaccharide chains of glycoproteins)

Sialic acid imparts negative charge to glycoproteins,

because its carboxyl group tends to dissociate a proton at

physiological pH.


Biosynthese of aminosaccharides

Fructose-6-P

Gln

Glu

Ac-CoA

Glucosamine-6-P

Co-A

N-AcetylGlucosamine-6-P

mutase

N-AcetylGlucosamine-1-P

UTP

P-Pi

UDP-N-AcetylGlucosamine

epimerase

epimerase

ADP

UDP-N-Acetyl-Galactosamine

ADP

Glucosamine-1-P UTP

phosphoglucomutase

ATP

Glucosamine

ATP

N-Acetylglucosamine

N-AcetylManosamine-6-P

Mucopolysaccharides

(GAG) – hyaluronic acid

Glycoprotheins

P-Pi

UDP-glucosamine

GAG - heparine

Sialic acids, gangliozids,

glykoprotenes

N-AcetylNeuramine-9-P

phosphoenolpyruvate

Glycoprotheins, GAG

(chondroitins)


Complex saccharides

Glycoconjugates

Proteoglycans

Glycosaminoglycans

Proteins conjugated

to polysaccharides

with serial repeat

units

Repeat unit

HexN and HexUA

Carbohydrate >> protein

Glycoproteins

Proteins conjugated

to saccharides

lacking a serial

repeat unit

Protein >> carbohydrate


Glycosaminoglycans (GAG)

GAGs = mucopolysaccharides

• Long heteropolysaccharides

• Typically unbranched (i.e., they are linear molecules)

• Strongly negatively charged; therefore, they are

extended, highly hydrated, compressible, and slippery

• Constitute the body's ground substance and extracellular

matrix; interact with fibrous proteins

• Abundant in skin, tendons, cartilage, ligaments, basement

membranes, matrix of bone, etc.

GAGs consist of repeating disaccharide units (acidic

sugar + amino sugar)

– acidic sugar: glucuronic acid (GIcUA) or iduronic acid (IdUA – an

epimer), often with sulfated -OH groups (general term: uronic

acid)

– amino sugars: glucosamine (GIcN) or galactosamine (GaIN),

usually acetylated (GIcNAc and GalNAc)


Glycosaminoglycans

• Hyaluronic acid - lubricant and cushioning

substance in joints

• Chondroitin sulfate - most abundant

glycosaminoglycan in teeth and cartilage

• Keratan sulfate - important component of

cartilage

• Heparin - blood coagulation

• Heparin sulfate - important in adhesion

between cells of the retina


Glycopeptide bonds

CH 2 OH

O

NH 2

H

O

HN

C CH 2 CH

COOH

NH-Asn

Glc

H

OH

H

OH

O

H

Type I

H

HN C CH 3

NAc

N-Glycosyl linkage to Asn

CH 2 OH

NH 2

CH 2 OH

NH 2

Glc

H

OH

H

OH

O

H

O

O

H

CH 2 CH COOH

Ser

Glc

H

OH

H

OH

O

H

O

H

CH

CH 2

CH 2

CH 2

HO-Lys

H

HN C CH 3

Type II

NAc

O-Glycosyl linkage to HO-Ser (Thr)

H

OH H 2 N CH COOH

Type III

O-Glycosyl linkage to 5-HO-Lys


Glycosaminoglycans

COO -

b-1,3

CH 2 OH

b-1,4

H

H

OH

O

H

O

H

H

O

H

O

H

OH

O

H

H

OH

H

HN

C CH 3

GlcUA

GlcNAc

Hyaluronate

No protein link

No sulfate

b-1,3 glycosidic linkage


Glycosaminoglycans

H

b-1,3

CH 2 OH

b-1,4

H

COO -

OH

O

H

-

O OSO 3

H

O

H

O

H

H

O

H

H

OH

H

HN

C CH 3

IdUA

GalNAc

Dermatan sulfate

IdUA with some GlcUA

Sulfate at 4 or 6 C of GalNAc

b-1,3 glycosidic linkage


Glycosaminoglycans

a-1,4

COO -

CH 2 OSO 3

-

H

H

OH

O

H

H

H

H

OH

O

H

H

a-1,4

O

O

H

GlcUA

OSO 3

-

H

GlcNAc

NHSO 3

-

Heparin

Heparan sulfate

GlcN and GlcUA or IdUA

N and O sulfate (C2,3,6)

a-1,4 glycosidic linkage

> NAc

< N and O sulfate


Glycosaminoglycans

CH 2 OH

b-1,4

CH 2 OSO 3

-

b-1,3

OH

H

O

H

O

H

H

OH

O

H

O

H

H

O

H

H

OH

GlcUA

H

HN

GlcNAc

C CH 3

Keratan sulfate I

Keratan sulfate II

GlcNAc and Gal (no UA)

Sulfate on C6 of Gal or HexN

b-1,4 glycosidic linkage

GalNAc-O-Ser or Thr


Synthesis of GAG

• Activated UDP-sugars are used by

specific transferases (recall glycogen)

• Reactions occur in the ER and Golgi

because GAGs and proteoglycans are

exported from cells


Synthesis of complex saccharide


Degradation of GAG

• Glycosaminoglycans turn over (half-lives: 3-120

days).

• Phagocytosis lysosomes; digested by acid

hydrolases (pH ~5)

• Degradation involves many enzymes, starting

with endoglycosidases, followed by sequential

degradation of oligosaccharides, by removal of

sulfate groups and individual monosaccharides

from non-reducing end.


Proteoglycans

complexes of proteins with GAGs

• core protein has covalently

attached GAGs (added to

Ser -OH residues through

Xyl-Gal-Gal trisaccharide)

• extended proteoglycan

monomers are noncovalently

bound to

hyaluronic acid via link

proteins.


Sialic acids

(e.g., N-acetylneuraminic acid, NANA)

• a 9+2 carbon derivative of N-

acetylmannosamine-6-P and

phosphoenolpyruvate

• found in gangliosides and

glycoproteins

• target for some pathogenic

organisms

• for biosynthesis, sialic acid is

activated by CTP to CMP-

Nacetylneuraminic acid plus

PPi.


Glycoproteins

• Have covalently attached carbohydrate chains (2-

15 residues each; making up 2 – 60 % of total

molecular weight of protein).

• Oligosaccharide structures vary from protein to

protein and are usually branched.

• Integral membrane glycoproteins have various

functions, including: cell surface recognition

(receptors); antigenicity; adhesive proteins;

receptors for viruses.

• In circulating glycoproteins, carbohydrate may

provide protection from proteolysis.

• Lysomsomal acid hydrolases are also

glycoproteins.


Glykoproteins


In glycoproteins:

two types of linkages for oligosaccharides

• O-linked: attached to Ser or Thr (or

hydroxylysine in collagen)

– O-linked oligosaccharides (short) are found in

many membranebound glycoproteins, including

ABO blood group antigens

• N-linked: attached to Asn


• N-linked oligosaccharides are present in

membrane glycoproteins and circulating

glycoproteins

• Two classes of N-linked oligosaccharides:

– complex oligosaccharides

– high mannose oligosaccharides

• Both contain a core pentasaccharide:

– 2 GlcNAc, Man, 2 Man branches


Metabolism of glycoproteins

• Precursors are activated sugar nucleotides:

• UDP-sugars (UDP-glucose, UDP-galactose,

UDP-N-acetylglucosamine, UDP-Nacetylgalactosamine)

• GDP-sugars (GDP-mannose, GDP-L-fucose)

• CMP-N-acetylneuraminic acid


Synthesis of O-Linked

Oligosaccharides

• As with GAGs, O-linked glycosides are

added one sugar at a time in the ER and

Golgi to protein by specific

glycosyltransferases.


Steps in N-linked oligosaccharide

addition to proteins

• N-Linked glycoside synthesis involves a

lipid intermediate (dolichol phosphate).

Glycosyltransferases add N-

acetylglucosamine, mannose,and glucose

to the dolichol phosphate.

• The oligosaccharide is then transferred to

the protein asparagine group by a proteinoligosaccharide

transferase, in the ER.


Steps in N-linked oligosaccharide addition

to proteins


Dolichol Phosphate

• Long hydrocarbon consisting of isoprene

units

• Carrier for oligosaccharides to be attached to

proteins (for N-linked carbohydrate chains)

• Oligosaccharide contains GlcNAc, Man and

Glc (attached via pyrophosphate linkage).


Steps in N-linked oligosaccharide

addition to proteins:

• The N-linked oligosaccharide is further

processed by removal of specific glucose

and mannose residues and addition of

other specific sugars, usually capped by

N-acetylneuraminic acid (sialic acid)

residues.

• Fate of N-linked glycoproteins is secretion,

exposure on cell surfaces; also delivery to

lysosomes.


Steps in N-linked oligosaccharide addition

to proteins:


How are acid hydrolases targeted

to lysosomes?

• Phosphorylation of a specific mannose

residue is the signal for lysosomal

delivery, via binding of proteins to a

specific mannose-6-phosphate receptor

and sorting vesicles.


How are acid hydrolases targeted to

lysosomes?


• I-cell disease is a defect in this specific phosphotransferase.

This results in deficiency of all

lysosomal enzymes (acid hydrolases) in the

lysosomes.

• This disease also results in the secretion of

lysosomal hydrolases from the cell.


Degradation of glykoproteins

• Degradation of glycoproteins is similar to

that of proteoglycans.

• It occurs in lysosomes by acid

glycosidases. As with GAGs, degradation

is generally the reverse of synthesis.

• Circulating glycoproteins trimmed of sialic

acid residues are recognized and removed

by a sialoglycoprotein receptors in the

liver.


Inborne Metabolic Diseases of Sugar Metabolism

At the level of monomers

galactosemia

fructose intolerance

fructosuria

pentosuria….

Glycogenoses

1.Girke, 2.Pompe,

3.Fabri, 4.Andersen

5.Mak Ardle, 6.Hears

7.Thomson, 8.Taren

9.Hage...

At the level of:

Glycoproteines: Glycolipids: Proteoglycanes:

a) disturbed synthesis a) disturbed synthesis a) disturbed synthesis

b) disturbed breakdown b) disturbed breakdown b) disturbed breakdown

glycoproteidoses glycolipidoses mucopolysacharidoses

(mannosidoses, fucosidoses…) (gangliosidoses -Tey-Sax etc.) (Hurler disease etc.)


Sources of Blood Glucose

Five Phases of Glucose Homeostasis

Phase I: well-fed state

Phase II: Post-absorptive state

Phase III: Fasting

Phases IV: Starved

Phase V: Prolonged Starvation


Glycosylation of Hb

• Glycosylated hemoglobin (GHb) is formed

nonenzymatically (< 6 %) by the two-step

reaction shown below.

• The first reaction is rapid, reversible, dependent

on the ambient glucose concentration, and

produces a labile aldimine or Schiff base.

• Over time, the aldimine slowly undergoes

Amadori rearrangement and is converted to a

stable ketoamine, glycosylated hemoglobin.

• Most HbA1c assays measure this stable

ketoamine, not the labile product which is more

prone to be influenced by recent dietary intake.


Glycosylation of Hb

• This process is reversible and % glycosylation is

proportional blood glucose level in last month

• For compensation of diabetes

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