Structure, properties and function of saccharidesVladimíra Kvasnicová

SACCHARIDES(carbohydrates, glycides)

= polyhydroxyaldehydesor

polyhydroxyketonesD-glucose D-fructose

•monosaccharides

•oligosaccharides

•polysaccharides

17 kJ/g

Saccharidesmonosaccharides:

C3 - glyceraldehyde (aldotriose), dihydroxyacetone (ketotriose); smallest saccharides, both are important intermediates of a human metabolism

C5 - ribose, 2-deoxyribose (aldopentoses); components of nucleotides: found in nucleic acids (ribose in RNA, deoxyribose in DNA)

C6 - glucose, galactose, mannose (aldohexoses), fructose (ketohexose); components of oligo- and-polysaccharides, metabolic intermediates

disaccharides (= oligosaccharides composed of 2 monosaccharides):

• sucrose (= saccharose) consists of glucose and fructose

• lactose (= milk sugar) consists of galactose and glucose

• maltose (= malt sugar) consists of two glucoses

polysaccharides (= glycans):

• starch = storage polysacharide of plants, consists of linear amylose and branched amylopectin; it is a glucan = polymer of glucose

• glycogen = storage polysaccharide of animal cells („animal starch“); it is a branched polymer of glucose (resembels amylopectin)

• cellulose = structural polysaccharide of plants, linear, water insoluble glucan

heteroglycosides = complex saccharides

• proteoglycans

• glycoproteins

• glycolipids

cellextracellular fluid

The figure has been adopted from: J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme 2005

bacteria

These molecules contain also

non-sugar part (protein or lipid)

MONOSACCHARIDES

GLUCOSE – central role

The figures have been adopted from Harper´s Biochemistry a z http://www.vuw.ac.nz/staff/paul_teesdale-spittle/organic/chiral_web/images/fig1_5d.gif (April 2007)

chiral carbon

Saccharidesstructure of monosaccharides

• they contain one aldehyde group (aldoses) or one keto group - always at the 2nd carbon (ketoses) and two or more alcohol (hydroxy) groups

• the terminal hydroxy group (the last one in a molecule) is a „primary alcohol group“, it can be oxidized in two steps to carboxylic acid group

• all other hydroxy groups in a molecule (with exception of the first one in a molecule of ketose, which is also primary) are „secondary alcohol groups“

• aldehyde group can be either oxidized to a carboxylic acid group or reduced to a primary alcohol group; keto group can be only reduced, producing a secondary alcohol group (sugar-derived acids or sugar alcohols are formed)

• all carbons of a sugar molecule containing secondary alcohol groups are „chiral carbons“ (C*) - they form two possible spatial arrangements: with the alcohol group oriented either right or left

• a number of possible isomers of a monosaccharide can be calculated as 2n, where n = number of C* in the molecule; e.g. aldohexose contains 4 C*, so 24 = 16; it means that there are 16 different aldohexoses from which 8 are D-saccharides and 8 are L-saccharides (D- and L- saccharides are mirror images of each other); only one of these 16 isomers is called D-glucose

• ketoses contain by one C* less than aldoses (they have 2 primary alcohol gr.)

The figure is found at http://astrobiology.berkeley.edu/Mars101/pix/image003.jpg (April 2007)

The figure is found at http://webphysics.davidson.edu/Alumni/bedenius/liqcry/chiral.gif (April 2007)

number of isomers for n(C*) = 2n

n = 1, 2, 3, ...

Classification of monosaccharides

fructoseglucosehexoses (C6)

ribuloseribosepentoses (C5)

erythruloseerythrosetetroses (C4)

dihydroxyacetoneglyceraldehydetrioses (C3)

ketoses (>C=O)aldoses (-CHO)

Isomers of monosaccharides

The figures have been adopted from Harper´s Biochemistry

1) D- and L- izomers

= mirror images(enantiomers)

nature important:D-monosaccharides

2) pyranoses and furanoses

• pyranoses: aldohexoses

• furanoses: fructoseribose

glucopyranosepredominates

The figures have been adopted from Harper´s Biochemistry

3) αααα and ββββ anomers

only cyclicmolecules

• during dissolving of a saccharide in water the equilibrium between anomers is established = mutarotation(optical rotation of anomers is not the same)

ββββ-D-Glc

αααα-D-Glc

The figure is found at http://www.cbs.umn.edu/bmbb/brl/carb/CarbTJSA/Haworth/TwoAnomers.jpg (October 2007)

These isomers differ in an arrangement of the -OH group

formed by a cyclization of a monosaccharide.

The anomeric -OH group is the most reactive one in a molecule.

1) αααα-D-glucopyranose 2) ββββ-D-glucopyranose5) D-glucose

3) αααα-D-glucofuranose 4) ββββ-D-glucofuranose

All possible structures of glucose,

(2) predominates in aqueous solutions

4) aldo-/keto- isomers

aldose / ketose

glyceraldehyde / dihydroxyacetone

ribose / ribulose

glucose /fructose

The figures have been adopted from Harper´s Biochemistry

5) epimers

= isomers of saccharides differing in orientation of only one –OHgroup in space

Man = 2-epimer of Glc

Gal = 4-epimer of Glc

The figures have been adopted from Harper´s Biochemistry

The figures have been adopted from Harper´s Biochemistry

Gal is found in lactose (milk sugar)

Gal and Man and their derivatives

are found inheteroglycosides

Physiologically important monosaccharides

ALDOSES

The figures have been adopted from Harper´s Biochemistry

KETOSES

The figures have been adopted from Harper´s Biochemistry

Physiologically important monosaccharides

Derivates of monosaccharides

1) sugar alcohols are formed by reduction of the carbonyl group

glucose → glucitol (= sorbitol)

fructose

mannose → mannitol

galactose → galactitol

D-Glucitol

2) oxidation of saccharides produces acids:

• aldaric acids (glucaric)both C1 and C6 is oxidized = dicarboxylic a.

• aldonic acids (gluconic)oxidation of C1

• alduronic (glucuronic)oxidation of C6

3) deoxysaccharides are formed by reduction of secondary –OH group

2-deoxy-D-ribose

4) amino saccharides contain one amino group instead of one –OH group

D-glucose amine

The figures have been adopted from Harper´s Biochemistry

The figure has been adopted from J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme 2005

amino saccharides are often acetylated

(found in heteroglycosides)

5) esters are formed by esterification

• with H3PO4

(intermediatesof metabolism)

• with H2SO4 (foundin proteoglycans)

The figure has been adopted from J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme 2005

(ester is formed by a reaction of an alcohol group with an acid)

6) glycosides are formedby reaction with alcoholsor amines

• O-glycosidic bond(oligo and polysaccharides,

connection to proteins)

• N-glycosidic bond(in nucleic acids,connection to proteins)

The figure has been adopted from J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme 2005

The figure is found at http://www.nmc.edu/~koverbaugh/bio115/Image14.gif (October 2007)

O-glycosidic bond

The figure is found at http://www.thebestlinks.com/images/f/f5/ATP.png (October 2007)

N-glycosidic bond

Adenosine triphosphate (ATP)

ribose

DISACCHARIDES

SALAME

• SAcharose (= sucrose)

• LActose

• MaltosE

saccharose has not reducing properties

The figure has been adopted from J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme 2005

α-Glc(1→4)Glc β-Gal(1→4)Glc α-Glc(1→2)β- Fru

free anomeric (= hemiacetal) hydroxyl ⇒ reducing properties

Numbers in parenthesis denote which carbon of each sugar is involved in the bond

POLYSACCHARIDES

• homopolysaccharidesstarch, glycogen, cellulose, inuline

• heteropolysaccharidesglycoproteins, proteoglycans

• branched

• unbranched

• storagestarch, glycogen,inuline

• structuralcellulose, proteoglycans

amylose (maltose)n amylopectine

STARCH (Glc)n

The figures have been adopted from Harper´s Biochemistry

α(1→4) glycosidic bonds α(1→4) glycosidic bonds

α(1→6) glycosidic bonds

GLYCOGEN (Glc)n

The figure is found at http://students.ou.edu/R/Ben.A.Rodriguez-1/glycogen.gif (October 2007)

nonreducing end reducing end

OH

(anomeric -OH group is blocked by the bond) (anomeric -OH group is free: the aldehyde group can reconstitute after opening the cycle of the sugar)

CELLULOSE

ββββ-Glc(1→4)Glc

The figures are found at http://web.chemistry.gatech.edu/~williams/bCourse_Information/6521/carbo/glu/cellulose_int_2.jpghttp://www.kjemi.uio.no/14_skole/modul/Evina_organisk/Org_K3fig14_cellulose.JPG (October 2007)

The figure is found at http://www.grandmeadows.com/archives/truth1.gif (October 2007)

PROTEOGLYCANS

core protein

+glycosaminoglycans

(GAG)

(aminosugar-uronic acid)nGAG is a polymer of these sugar derivatives

http://departments.oxy.edu/biology/Stillman/bi322/030101/F9_23.gif (October 2008)

The figure has been adopted from J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme 2005

GLYCOPROTEINS