lecture 14 carbohydrates complete to be taught
DESCRIPTION
TRANSCRIPT
Carbohydrates or Saccharides
• most abundant biological molecules• contain primary three atoms - C, H, O
• general formula, (CH2O)n where n > 3
• monosaccharide = basic carbohydrate unit• polysaccharide = polymeric form• structural variation of carbohydrates is
fundamental to their biological activity
1. Monosaccharides
2. Polysaccharides
3. Glycoproteins
Outline:
Monosaccharides - definitions
• Monosaccharides are aldehyde or ketone derivatives of straight-chain polyhydroxy alcohols containing at least 3 carbon atoms.
• Aldose = carbohydrate containing an aldehyde
• Ketose = carbohydrate containing a ketone
Carbon atomsin alcohol Name
3 triose 4 tetrose 5 pentose 6 hexose 7 heptose . .
Monosaccharides - 3, 4, 5 carbon aldoses
H
CHO
C OH
C OHH
C OHH
CH2OH
H
CHO
C OH
C HHO
C OHH
CH2OH
CHO
C HHO
C OHH
C OHH
CH2OH
D-Ribose(Rib)
D-Arabinose(Ara)
D-Xylose(Xyl)
H
CHO
C OH
C OHH
CH2OH
D-Erythrose
HO
CHO
C H
C OHH
CH2OH
D-Threose
H
CHO
C OH
CH2OH
D-Glyceraldehyde
Aldotriose Aldotetroses
Aldopentoses
Fischerprojections
Monosaccharides - 6 carbon aldoses
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
CHO
C HHO
C HHO
C OHH
C OHH
CH2OH
CHO
C OHH
C HHO
C HHO
C OHH
CH2OH
CHO
C OHH
C OHH
C OHH
C OHH
CH2OH
D-Allose D-Glucose(Glc)
D-Mannose(Man)
D-Galactose(Gal)
Aldohexoses
Monosaccharides - ketoses
CH2OH
C O
C OHH
CH2OH
D-Erythrulose
CH2OH
C O
CH2OH
Dihydroxyacetone
Ketotriose Ketotetrose
CH2OH
C O
C OHH
C OHH
CH2OH
CH2OH
C O
C HHO
C OHH
CH2OH
D-Ribulose D-Xylulose
Ketopentoses
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
C OHH
C HHO
C OHH
CH2OH
D-Fructose D-Sorbose
Ketohexoses
Configuration and Conformation
Alcohols can react
with aldehydes or
ketones to form
hemiacetals or
hemiketals.
R OH R' C
O
H
+ C
OH
HO
R'
R
R OH R' C
O
R''
+ C
OH
R''O
R'
R
hemiacetal
hemiketal
The hydroxy group and the aldehyde (or ketone) of
monosaccharides can react intramolecularly to form
cyclic hemiacetals (or hemiketals).
Haworth Projections
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
D-Glucose
H O
OH
H
OH
H
OHH
OH
CH2OH
HH
OH
H
H
OHH
OH
CH2OH
HOH
O=
OH
CH2OH
H
HOH2C
OH H
H HOO
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
D-Fructose
O
CH2OH
H
HOH2C
OH H
H HOOH
=
-D-Glucopyranose
-D-Fructofuranose
Cyclic Sugars have two Anomeric Forms
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
D-Glucose
H O
OH
OH
H
H
OHH
OH
CH2OH
H
-D-Glucopyranose
H O
OH
H
OH
H
OHH
OH
CH2OH
H
-D-Glucopyranose
Monosaccharides are conformationally variable
HOO
H
H
HO
H
H
OHHOH
OH
HO
OH
OH
H
OH
OH
HH
OH
H
HOO
H
H
HO
H
HO
OHHH
OH
OHO
H
OH
H
OH
OH
HH
OH
H
Two chair conformations of -D-glucopyranose:
Two chair conformations of -D-glucopyranose:
Monosaccharide Derivatives
H
CHO
C OH
C HHO
C OHH
C OHH
COOH
D-Glucuronic acid
H
COOH
C OH
C HHO
C OHH
C OHH
CH2OH
D-Gluconic acid
Oxidations:
Reductions:
H
CH2OH
C OH
C OHH
C OHH
CH2OH
H
CH2OH
C OH
C HHO
C OHH
CH2OH
D-Ribitol D-Xylitol
H
CH2OH
C OH
CH2OH
D-Glycerol
H
HO
OH
HH
OHH
OH
OHHO
H H
myo-Inositol
H O
HO
H
OHH
NH2H
OH
CH2OH
H
-D-Glucosamine
Amino Sugars:
HO O
H
H
OHH
NH2H
OH
CH2OH
H
-D-Galactosamine
Glycosides
H O
HO
H
OHH
OHH
OH
CH2OH
H
-D-Glucose
+ CH3OH
( R-OH )
H O
HO
H
OCH3
H
OHH
OH
CH2OH
H
Methyl--D-Glucoside
H O
HO
OCH3
HH
OHH
OH
CH2OH
H+ + H2O
Methyl--D-Glucoside
( O-R )
( O-R )
The anomeric carbon can condense with alcohols
to form -glycosides and -glycosides.
Polysaccharides (or Glycans)
• Homopolysaccharides - consist of one type of monosaccharide
• Heteropolysaccharides - consist of more than one type of monosaccharide
• Polysaccharides form branched as well as linear polymers.
• Complete description of an oligosaccharide includes:
- identity of the monosaccharides- anomeric forms- linkages of all its component monosaccharides
Disaccharides - simplest polysaccharides
Lactose
occurs naturally in milk
disaccharide of galactose and glucose
Systematic Name: O--D-galactopyranosyl-(14)-D-glucopyranose
(14) designates that the glycosidic bond links C1 of galactose with C4 of glucose
Lactose has a free anomeric carbon on the glucose end. This is an example of a reducing sugar, and it can readily reduce mild oxidizing agents.
1 1
6 6
H O OH
HH
OHH
OH
CH2OH
H
-D-Glucose
HO O
H HH
OHH
OH
CH2OH
H
-D-Galactose
O
Sucrose
Sucrose
most abundant dissacharide
common table sugar
major form of carbohydratetransported in plants
Systematic Name: O--D-glucopyranosyl-(12)--D-fructofuranoside
H O H
H
OHH
OH
CH2OH
H
-D-Glucose
HO CH2OH
HHOH2C
OH H
H HOO
-D-Fructose
O
The anomeric carbon of glucose and the anomeric carbon of fructose participates in the glycosidic bond.
1 2
6
6
5
Structural Polysaccharide - Cellulose
• Plants have rigid cell walls to withstand osmotic pressure differences and for load-bearing functions.
• Cellulose is the primary structural component of plant cell walls.
• Cellulose is a linear polymer of up to 15,000 D-glucose residues linked by (14) glycosidic bonds.
H O
HH
OHH
OH
CH2OH
H
-D-Glucose
O
H O
HH
OHH
OH
CH2OH
HO[ ]
-D-Glucose
n
Storage Polysaccharide - Starch
• Starch is a mixture of glycans that plants synthesize as their principal food reserve.
• Composed primarily of -amylose and -amylopectin.• The -amylose is a linear polymer of glucose residues
linked by (14) bonds.
n
H O
H
OHH
OH
CH2OH
H
-D-Glucose
H O
H
OHH
OH
CH2OH
H
H
[ ]-D-Glucose
O
H
O
-amylose
Fig. 8-10Saliva contains amylase which hydrolyzes the (14) bonds of starch.
Amylopectin and Glycogen- branched polysaccharide
• Amylopectin in plants and Glycogen in animals consist mainly of (14)-linked glucose residues but is a branched molecule with (16) branch points.
n
O
H
OHH
OH
CH2OH
H
H
]
O
H
O
H
OHH
OH
CH2
H
H
[ O
H
O
O
. . . .
. . . .
. . . .
[
(16) branch points every24 to 30 glucose residues
Amylopectin in plants
n
O
H
OHH
OH
CH2OH
H
H
]
O
H
O
H
OHH
OH
CH2
H
H
[ O
H
O
O
. . . .
. . . .
. . . .
[
(16) branch points every8 to 12 glucose residues
Glycogen in animals
(16) branch point (16) branch point
Glycoproteins
• Many proteins are actually glycoproteins with carbohydrates (varing from <1% to >90%).
• Polypeptide chain is synthesized under genetic control.• The carbohydrate is added by posttranslational modification.
Proteoglycans = complex mixture consisting of proteins and glycosaminoglycans.
a proteoglycan
Fig. 8-13