CARBOHYDRATES IICARBOHYDRATES II

Lecture Contents…Lecture Contents…

the disaccharides sucrose, maltose, lactose, cellulose

the oligosaccharides N-linked, O-linked

the polysaccharides homopolysaccharides heteropolysaccharides

ObjectivesObjectives after you have studied this chapter, you should be able to

answer these questions: what are the general structural features of oligosaccharides

and polysaccharides? what structural properties of cellulose? what account for its

widespread occurrence in plants? what are the most common homopolysaccharides and

heteropolysaccharides and what are their functions? what structural properties distinguish proteoglycans from

glycoproteins? what are the functions of proteoglycans and glycoproteins?

The DisaccharideThe Disaccharide 2 monosaccharides joined together to form a glycoside bond

glycosidic bond: bond bet. one anomeric C of a monosaccharide with a

hydroxyl grp of another monosaccharide cat. by an enzyme glycosidase via a condensation

reaction various types: (12); (13); (14) (16) & , β

configurations

crystalline, water soluble, sweet tasting e.g.: sucrose, lactose, maltose, cellobiose

How is a glycosidic bond formed?

Different types of glycosidic bondDifferent types of glycosidic bond

-linkages

-linkage

Alpha, where the C1 OH that is formed ends up on the same side of the ring as the -OH of C2

Beta, where the C1 OH that is formed ends up

on the opposite side of the ring from the -OH of C2

(11)

(12)

(13)

(14)

(16)

Common disaccharides:Common disaccharides:

sucrose

lactose

maltose

cellobiose

also known as table sugar

extracted from sugar cane, sugar beet

made up of glucose and fructose with (12) glycosidic bond

a nonreducing sugar

common sweetener – less sweet than fructose

hydrolysis by sucrase/invertase

-D-glucopyranosyl (12) -D-fructofuranoside

also known as milk sugar -makes up around 2-8% of the solids in milk

made up of galactose and glucose with (14) glycosidic bond

a reducing sugar

hydrolysis by lactase – (lacking this enzyme can cause lactose intolerance)-D-galactopyranosyl (14) -D-

glucopyranose

also known as malt sugar

intermediate product of starch hydrolysis

present in germinating grain, in a small proportion in corn syrup

made up of 2 molecules of glucose with (14) glycosidic bond

a reducing sugar

hydrolysis by maltase

-D-glucopyranosyl (14) -D-glucopyranose

a degradation product of cellulose

made up of 2 molecules of glucose linked by (14) glycosidic bond

a reducing sugar

does not freely exists in nature-D-glucopyranosyl (14) -D-

glucopyranose

The OligosaccharideThe Oligosaccharide consists of between 2 - 20 monosaccharides can be found linked to polypeptide/lipid in glycoprotein &

glycolipid functions:

glycoprotein – immune cell recognition, improve stability of protein folding, sperm-egg interaction

glycolipid - provide energy, markers for cell recognition 2 types of glycans:

N-linked – after N-glycosylation at Asn O-linked – after O-glycosylation at hydroxylysine,

hydroxyproline, Ser or Thr

Oligosaccharides linked to polypeptides forming glycoproteins

polypeptide chain

SerAsn

oligosaccharides

N-linked O-linked

The diversity of glycoproteins

Lipid part

Carbohydrate part

The PolysaccharideThe Polysaccharide store energy or as structural components

homopolysaccharide formed from one type of monosaccharide

e.g.: starch, glycogen, cellulose, chitin

heteropolysaccharide formed from 2/more types of

monosaccharide e.g.: glycosaminoglycan, peptidoglycan

StarchStarch main food storage for plants consists of:

-amylose – linear glucose polymer linked by (14) bonds

amylopectin – glucose units are linked in a linear way with (1→4) bonds; branching takes place with (1→6) bonds occurring each 24 to 30 glucose units

degradation enzymes: amylases

-glucosidase starch phosphorylase debranching enzyme

Homopolysaccharide

Amylose of starch

(a) The D-glucose residues of amylose linked through (14) glycosidic bonds

(b) The amylose polymer forms a left-handed helix

GlycogenGlycogen main animal food storage basic structure similar to amylopectin

but more branches, every 8-12 glucose residues

more compact degradation:

amylases glycogen phosphorylase

glycogen debranching enzyme

Homopolysaccharide

CelluloseCellulose

main component of plant cell wall linear polymer of glucose – 15000 residues

linked by β(14) linkage H-bonded structure form parallel microfibril –

strong & water resistant cross-linked with matrix containing other

polysaccharides & lignin – resistant to pressure

degradation – celullase

Homopolysaccharide

ChitinChitin

main component of arthropod’s exoskeleton as well as algae/fungi cell wall

similar to cellulose except: every C2–OH grp replaced by acetamide polymer of N-acetyl-D-glucosamine

bonded by β(14) linkage 3 types of microfibril: , β, degradation: chitinase

Homopolysaccharide

Struture of chitin…

Comparison between struture of cellulose and chitin…

GlycosaminoglycanGlycosaminoglycan

or mucopolysaccharide major structural importance in vertebrate

animals – e.g. proteoglycan in connective tissues, skin

polymers of repeating disaccharide units (i.e. N-acetylgalactosamine or N-acetylglucosamine or one of their derivatives) with either sulfate or carboxylate grps

5 main classes: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin/heparan sulfate & keratan sulfate

Heteropolysaccharide

PeptidoglycanPeptidoglycan cross-linked complex of polysaccharides and peptides found in the cell

walls of bacteria protects bacteria from osmotic lysis framework: polysaccharide component – linear chains of alternating β(14)-linked

N-acetylglucosamine (NAG) & N-acetylmuramic acid (NAM) peptide component – D-amino acid containing tetrapeptide forms amide

linkage with NAM’s lactic acid residue neighbouring parallel peptidoglycan chains are covalently cross-linked

thru tetrapeptide side chains

Heteropolysaccharide

The peptidoglycan monomer of a gram-(+) bacterium: e.g. Staphylococcus aureus

The peptidoglycan monomer of a gram-(-) bacterium: e.g. Escherichia

coli

Comparison between gram-(-) and gram-(+) bacteria peptidoglycan framework

a. The peptidoglycan monomer

b. The peptidoglycan structure

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