Karbohidrat 2

Pengantar Biokimia Gizi

Polysaccharides(also called Glycans)

Most carbohydrates

found in nature occur

as polysaccharides.

Formation of di- and polysaccharide bonds

Dehydration

synthesis of a

sucrose molecule

formed from

condensation of a

glucose with a

fructose

Lactose:

Maltose:

Disaccharides

• Composed of two monosaccharide units by glycosidic link from C-1 of one unit and -OH of second unit

• 13, 14, 1 6 links most common but 1 1 and 1 2 are possible

• Links may be a or b

• Link around glycosidic bond is fixed but anomeric forms on the other C-1 are still in equilibrium

Polysaccharides

Primary Structure:

Sequence of residues

N.B.

Many are homopolymers. Those that

are heteropolymers rarely have >3,4

different residues

Secondary & Tertiary Structure

• Rotational freedom

• hydrogen bonding

• oscillations

• local (secondary) and overall (tertiary) random coil, helical conformations

Movement around bonds:

Tertiary structure - sterical/geometrical conformations

• Rule-of-thumb: Overall shape of the chain is

determined by geometrical relationship within each

monosaccharide unit

b(14) - zig-zag - ribbon like

b(1 3) & a(14) - U-turn - hollow helix

b(1 2) - twisted - crumpled

(16) - no ordered conformation

Ribbon type structures

(a) Flat ribbon type conformation: Cellulose

(b) Buckled ribbon type conformation: Alginate

Hollow helix type structures

• Tight helix - void can be filled by including

molecules of appropriate size and shape

• More extended helix - two or three chains may

twist around each other to form double or triple

helix

• Very extended helix - chains can nest, i.e., close

pack without twisting around each other

Amylose forms inclusion complexes with iodine, phenol,

n-butanol, etc.

The liganded amylose-iodine complex:

rows of iodine atoms (shown in black)

neatly fit into the core of the amylose

helix.

N.B. Unliganded amylose normally

exists as a coil rather than a helix in

solution

Tertiary Structure: Conformation Zones

Zone A: Extra-rigid rod: schizophyllan

Zone B: Rigid Rod: xanthan

Zone C: Semi-flexible coil: pectin

Zone D: Random coil: dextran, pullulan

Zone E: Highly branched: amylopectin, glycogen

Quarternary structure -aggregation of ordered structures

Aggregate and gel formation:

• May involve

• other molecules such as Ca2+ or sucrose

• Other polysaccharides (mixed gels)

Polysaccharides – 6 case studies

1. Alginates

2. Pectin

3. Xanthan

4. Galactomannans

5. Cellulose

6. Starch

1. Alginate

Source: Brown seaweeds (Phaeophyceae,

mainly Laminaria)

Linear unbranched polymers containing b-

(14)-linked D-mannuronic acid (M) and a-

(14)-linked L-guluronic acid (G) residues

Calcium poly-a-L-guluronate left-handed helix view down axis

view along axis, showing the hydrogen bonding and

calcium binding sites

Different types of alginates -different properties e.g. gel strength

Polyguluronate: - gelation through addition of

Ca2+ ions – egg-box

Polymannuronate – less strong gels,

interactions with Ca2+ weaker, ribbon-type

conformation

Alternating sequences – disordered structure,

no gelation

Properties and Applications

• High water absorption

• Low viscosity emulsifiers and shear-thinning thickeners

• Used in pet food chunks, onion rings, stuffed olives and pie fillings, wound healing agents, printing industry (largest use)

2. Pectin

• Cell wall

polysaccharide in

fruit and

vegetables

• Main source -

citrus peel

Partial methylated poly-a-(14)-D-galacturonic acid residues

(‘smooth’ regions), ‘hairy’ regions due to presence of

alternating a -(12)-L-rhamnosyl-a -(14)-D-galacturonosyl

sections containing branch-points with side chains (1 - 20

residues) of mainly L-arabinose and D-galactose

Properties and applications

• Main use as gelling agent (jams, jellies)

– dependent on degree of methylation

– high methoxyl pectins gel through H-bonding

and in presence of sugar and acid

– low methoxyl pectins gel in the presence of

Ca2+ (‘egg-box’ model)

• Thickeners

• Water binders

• Stabilizers

3. Xanthan

Extracellular polysaccharide from Xanthomonas campestrisb-(14)-D-glucopyranose backbone with side chains of -(31)-a-linked D-mannopyranose-(21)-b-

D-glucuronic acid-(41)-b-D-mannopyranose on alternating residues

Properties and applications

• double helical conformation

• pseudoplastic

• shear-thinning

• thickener

• stabilizer

• emulsifier

• foaming agent

• forms synergistic gels with galactomannans

4. Galactomannans

b-(14) mannose (M) backbone with a-(16) galactose (G) side chains

• Ratio of M to G depends on source

– M:G=1:1 - fenugreek gum

– M:G=2:1 - guar gum

– M:G=3:1 - tara gum

– M:G=4:1 - locust bean gum

Guar gum - obtained from endosperm of Cyamopsis

tetragonolobus

Locust bean gum - obtained from seeds of carob tree (Ceratonia

siliqua)

Properties and applications

• non-ionic

• solubility decreases with decreasing galactose

content

• thickeners and viscosifiers

• used in sauces, ice creams

• LBG can form very weak gels

5. Cellulose

b-(14) glucopyranose

• found in plants as microfibrils

• very large molecule, insoluble in aqueous and most

other solvents

• flat ribbon type structure allows for very close

packing and formation of intermolecular H-bonds

• two crystalline forms (Cellulose I and II)

• derivatisation increases solubility (hydroxy-propyl

methyl cellulose, carboxymethyl cellulose, etc.)

Properties and

applications

6. Starch Some homopolysaccharides are stored forms of fuel

Electron micrographs of starch and glycogen granules

Other homopolysaccharides (e.g. cellulose and chitin) serve as structural

elements in plant cell walls and animal exoskeletons.

Amylose and amylopectin, the polysaccharides of starch

amylopectin

occurs every

24 to 30 residues

Strands of amylopectin form double helical

structures with each other or with amylose strands

Bacterial and algal cell walls contain structural heteropolysaccharides

Peptidoglycan

(alternating b1→4-linked GlcNac-Mur2Ac crosslinked by short peptides)

The structure of agarose

exact structure depends

on the bacterial speciesLysozyme kills bacteria by hydrolyzing the b1→4 bond.

Repeating units of some common glycosaminoglycans of extracellular matrix

linear polymers composed of

repeating disaccharide units

Glucoronic acidN-Acetylglucosamine

Proteoglycans

glycosaminoglycan-containing macromolecules

of the cell surface and extracellular matrix

Proteoglycan structure

Two families of membrane proteoglycans

Mammalian cells have

at least 30 types

of proteoglycans.

Basic unit: a core protein

with covalently attached

glycosaminoglycan(s).

Heparan sulfate bind a variety of extracellular

ligands and thereby modulate the ligands interaction with

specific receptors of the cell surface

Interactions between cells

and the extracellular matrix

Cross-linked meshwork

that gives the whole

extracellular matrix strength

and resilience

Roles of

oligosaccharides

in recognition

and adhesion

at the cell surface