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Vincenzo Fogliano » 9.Carbohydrate and polysaccharides


Carbohydrates

Required background knowledge

  • Monosaccharides (pentoses and exoses)
  • Disaccharides, Olisaccharides → Reducing power and anomeric carbon
  • Polysaccharides
  • Aldoses and chetoses
  • Phosphoric esthers
  • Linear and Cyclic structure (furanose and pyrane rings)

Reduction of an aldose produces a polyole


Reduction of chetose produces polyalcol epimers


Mild oxydation produces the corresponding acids


Monosaccharides are linked by C-O-C bonds named glycosidic bonds

Glycosidic bond can be alpha or beta and hydrolysed by specific enzymes.

Glycosidic bond can be alpha or beta and hydrolysed by specific enzymes.


α-D-glucopiranosil-4-D-glucopiranose

α-D-glucopiranosil-4-D-glucopiranose

α-D-glucopiranosil-4-D-glucopiranose


Structure of sucrose and lactose


Polysaccharides

Functions in plants and animals.

Are usually made up of the same monosaccharides (homopolysaccharides).

Spatial configuration in fibres or sheets (not the same as in protein!).

Main polysaccharides are:

  • starch;
  • glycogen;
  • cellulose;
  • chitine.

Starch – amylose

Starch is formed by amylose and amylopectin.

Amylose is a linear polymer of glucose linked with α 1→4 glicosydic bonds.

In the form of granules which are often very hard.

Different starches in different vegetables.


Starch – amylopectin

Amylopectin is in a branched polymer with a ramification every 20 residues (average).

It has α 1→4 and α1→6 glycosidic bonds.

Technology: amylose is compact and water penetrates slowly into amylose-rich starch granules (such as beans).

Amylopectin is more sticky than amylose.

Nutrition: degradation of amylopectins is faster than that of amylose. Why?

Maltodextrins.

Glycogen

Glycogen is the polymer for the storage of glucose in animals.

It is found in the muscles and in the liver.

It has different physiological functions in different tissues.

It is highly branched (a ramification every 8 residues) Branched polymers are quickly degraded.

Glycogen structure: more than 10.000 residues per molecule


Cellulose

Has a structural function in vegetables.

Beta glycosidic bonds favour the formation of an extended rigid structures.

Cellulose fibres can be hydrolysed only by organisms with beta hydrolases: the cellulase.

Beta glycosidic bonds

Beta glycosidic bonds


Chitin

Chitin is a homopolymer formed by units of N-acetyl glucosamine linked by β 1→ 4 glycosidic bonds.

It constitutes the external backbone of insects and molluscs.

Chitin

Chitin


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