2.0 Kimia Makanan Carbohydrates
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Transcript of 2.0 Kimia Makanan Carbohydrates
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Carbohydrates
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Sources of Carbohydrates Animal products
Plants - major source
Photosynthesis
Minor except milk (lactose)
Starch: storage of energy
Cellulose: structural
6CO2 + 6H2O + 673 Kcal → C6H12O6 + 6O2
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Nutrient Composition of an
Adult Steer
Water 54%
Fat 26% Protein 15%
Ash 4.6%
Carbohydrate <1% little stored in the
animal body
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Carbohydrates (CHO) C:H:O (1:2:1)
The most abundant organic moleculesin nature
Sources
Major component of plant tissue
Comprise up to 70% or more of dry matter of forages
Make up less than 1% of the weight of animals
Sugars, starch, cellulose, gums
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Carbohydrates Carbohydrates are polyhydroxy
aldehydes or ketones
Aldehyde Ketone
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Classification Classified according to the number
of sugar molecules:
Most plants contain different typesof carbohydrates than animals
1.Monosaccharides - 1 unit2.Disaccharides - 2 units3.Oligosaccharides - 3 to 10 units
4.Polysaccharides - Greater than 10units
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Monosaccharides (CnH2nOn) Classified by the number of
carbon atoms:
3-C triose
4-C tetrose
5-C pentose 6-C hexose
nutritionally important
Sugars that contain four or more carbons exist primarily in cyclic form
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Monosaccharides Pentoses (5C)
Xylose and arabinose
Component in hemicellulose, glycoproteins
Ribose
Found in every living cell
Found in compounds involved in metabolism: ATP/ADP
Riboflavin
DNA/RNA
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Monosaccharides Hexoses (6c)
Glucose
Component of starch, cellulose, andglycogen
Major end-product of carbohydratedigestion in monogastrics
Primary form of sugar used for energy
Glucose, fructose, and galactose are among the most important monosaccharides in living organisms
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Monosaccharides Hexoses (6c)
Fructose
75% of sugars in honey Found in fruits and cane sugar
Galactose Component of milk sugar (lactose)
May be metabolized to glucose
Mannose Found after hydrolysis of plant mannosans and
gums; legumes
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Disaccharides Two monosaccharide molecules linked
by a glycosidic (or acetal) bond
Lactose (galactose + glucose)
Maltose (glucose + glucose)
Milk sugar Found only in milk
Intermediate product of starch hydrolysis Found in starch from melting of barley Alpha 1-4 linkage fundamental for starch
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Disaccharides Sucrose (glucose + fructose)
Common table sugar
Produced in leaves and stems of plants Found in sugar cane and sugar beets
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Disaccharides Cellulobiose (glucose + glucose)
Beta 1-4 linkage in cellulose
Does not exist freely in nature
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Oligo- and Poly-saccharides
Oligosaccharide
Chain of 3 –10 sugar molecules
Polysaccharide
Chain of 10+ sugar molecules
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Polysaccharides Heteropolysaccharide - composed of
two or more types of monosaccharides
Homopolysaccharides - composed of one type of monosaccharide
Starch
basic unit: alpha-D glucose principal sugar form in cereals
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Starch
Starch granules
Held together by H-bonds
Insoluble in water Raw starch is not well digested
Heat causes swelling of granules
‘Gelatinization’
Access for digestive enzymes
Retrograded starch -
indigestible crystalline formafter cooling
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Starch Three forms of starch:
1. Amylose Apha 1-4 linkages
Straight chain
14-30% of total plant starch
Water soluble
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Starch2. Amylopectin
Alpha 1-4 linkages
with alpha 1-6linkage at branchpoints
70-85% total plant
starch Not water soluble
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Starch3. Glycogen
Animal starch
Small amounts in liver andmuscle
Highly branched
Water soluble
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Carbohydrates
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General characteristics the term carbohydrate is derived from the
french: hydrate de carbone
compounds composed of C, H, and O (CH2O)n when n = 5 then C5H10O5
not all carbohydrates have this empiricalformula: deoxysugars, aminosugars
carbohydrates are the most abundantcompounds found in nature (cellulose: 100billion tons annually)
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General characteristics Most carbohydrates are found naturally in
bound form rather than as simple sugars
Polysaccharides (starch, cellulose, inulin, gums) Glycoproteins and proteoglycans (hormones, blood group
substances, antibodies)
Glycolipids (cerebrosides, gangliosides)
Glycosides
Mucopolysaccharides (hyaluronic acid)
Nucleic acids
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Functions sources of energy
intermediates in the biosynthesis of other
basic biochemical entities (fats and proteins) associated with other entities such as
glycosides, vitamins and antibiotics)
form structural tissues in plants and in
microorganisms (cellulose, lignin, murein) participate in biological transport, cell-cell
recognition, activation of growth factors,modulation of the immune system
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Classification of carbohydrates Monosaccharides (monoses or glycoses)
Trioses, tetroses, pentoses, hexoses
Oligosaccharides Di, tri, tetra, penta, up to 9 or 10
Most important are the disaccharides
Polysaccharides or glycans Homopolysaccharides
Heteropolysaccharides
Complex carbohydrates
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Monosaccharides - simple sugars with multiple
OH groups. Based on number of carbons (3, 4,5, 6), a monosaccharide is a triose, tetrose,pentose or hexose.
Disaccharides - 2 monosaccharides covalentlylinked.
Oligosaccharides - a few monosaccharidescovalently linked.
Polysaccharides - polymers consisting of chains
I
(CH2O)n or H - C - OHI
Carbohydrates (glycans) have the following
basic composition:
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Monosaccharides
Aldoses (e.g., glucose)have an aldehyde group atone end.
Ketoses (e.g., fructose) have
a keto group, usually at C2.
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose
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D vs L Designation
D & L designationsare based on the
configuration aboutthe singleasymmetric C inglyceraldehyde.
The lowerrepresentations areFischer Projections.
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehydeD-glyceraldehyde
L-glyceraldehydeD-glyceraldehyde
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Sugar Nomenclature
For sugars with morethan one chiral center,
D or L refers to theasymmetric C farthestfrom the aldehyde orketo group.
Most naturallyoccurring sugars are Disomers.
O H O H
C C
H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
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D & L sugars are mirrorimages of one another.
They have the samename, e.g., D-glucose& L-glucose.
Other stereoisomers have unique names,e.g., glucose, mannose,galactose, etc.
The number of stereoisomers is 2n
, where n isthe number of asymmetric centers.
The 6-C aldoses have 4 asymmetric centers. Thusthere are 16 stereoisomers (8 D-sugars and 8 L-
sugars).
O H O H
C C
H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
Hemiacetal & hemiketal
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Hemiacetal & hemiketalformation
An aldehydecan react withan alcohol toform ahemiacetal.
A ketone canreact with analcohol to form
a hemiketal.
O C
H
R
OH
O C
R
R'
OHC
R
R'
O
aldehyde alcohol hemiacetal
ketone alcohol hemiketal
C
H
R
O R'R' OH
"R OH "R
+
+
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Pentoses andhexoses cancyclize as the
ketone oraldehyde reactswith a distal OH.
Glucose forms anintra-molecularhemiacetal, asthe C1 aldehyde
& C5 OH react,to form a 6-member pyranosering, named after
pyran.
These representations of the cyclic sugars are called
Haworth projections.
H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
23
4
5
6
1 1
6
5
4
32
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose
(linear form)
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Fructose forms either
a 6-member pyranose ring, by reaction of the C2keto group with the OH on C6, or
a 5-member furanose ring, by reaction of the C2
keto group with the OH on C5.
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
H
OH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
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Cyclization of glucose produces a new asymmetriccenter at C1. The 2 stereoisomers are calledanomers, & .
Haworth projections represent the cyclic sugars ashaving essentially planar rings, with the OH at theanomeric C1:
(OH below the ring)
H O
OH
H
OHH
OH
CH2OH
H
-D-glucose
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose
23
4
5
6
1 1
6
5
4
3 2
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Because of the tetrahedral nature of carbon bonds,pyranose sugars actually assume a "chair" or
"boat" configuration, depending on the sugar.The representation above reflects the chair
configuration of the glucopyranose ring more
accurately than the Haworth projection.
O
H
HO
H
HO
H
OH
OHHH
OH
O
H
HO
H
HO
H
H
OHHOH
OH
-D-glucopyranose -D-glucopyranose
1
6
5
4
32
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Sugar derivatives
sugar alcohol - lacks an aldehyde or ketone; e.g.,ribitol.
sugar acid - the aldehyde at C1, or OH at C6, isoxidized to a carboxylic acid; e.g., gluconic acid,
CH2OH
C
C
C
CH2OH
H OH
H OH
H OH
D-ribitol
COOH
C
C
C
C
H OH
HO H
H OH
D-gluconic acid D-glucuronic acid
CH2OH
OHH
CHO
C
C
C
C
H OH
HO H
H OH
COOH
OHH
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Sugar derivatives
amino sugar - an amino group substitutes for ahydroxyl. An example is glucosamine.
The amino group may be acetylated, as inN -acetylglucosamine.
H O
OH
H
OH
H
NH2H
OH
CH2OH
H
-D-glucosamine
H O
OH
H
OH
H
NH
OH
CH2OH
H
-D- N -acetylglucosamine
C CH3
O
H
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N-acetylneuraminate (N-acetylneuraminic acid,also called sialic acid) is often found as a terminal
residue of oligosaccharide chains of glycoproteins.Sialic acid imparts negative charge toglycoproteins, because its carboxyl group tends todissociate a proton at physiological pH, as shown
here.
NH O
H
COO
OH
H
HOH
H
H
R
CH3C
O
HC
HC
CH2OH
OH
OH
N -acetylneuraminate (sialic acid)
R =
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Glycosidic Bonds
The anomeric hydroxyl and a hydroxyl of another
sugar or some other compound can join together,splitting out water to form a glycosidic bond:
R-OH + HO-R' R-O-R' + H2O
E.g., methanol reacts with the anomeric OH onglucose to form methyl glucoside (methyl-glucopyranose).
O
H
HO
H
HO
H
OH
OHHH
OH
-D-glucopyranose
O
H
HO
H
HO
H
OCH3
OHHH
OH
methyl--D-glucopyranose
CH3-OH+
methanol
H2
O
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Structural representation of
sugars Fisher projection: straight chain
representation
Haworth projection: simple ring inperspective
Conformational representation: chair
and boat configurations
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Rules for drawing Haworth
projections draw either a six or 5-membered ring
including oxygen as one atom
most aldohexoses are six-membered aldotetroses, aldopentoses,
ketohexoses are 5-membered
O O
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Rules for drawing Haworth
projections next number the ring clockwise starting next
to the oxygen
if the substituent is to the right in the Fisherprojection, it will be drawn down in theHaworth projection (Down-Right Rule)
OO
1
23
4
5
1
23
4
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Rules for drawing Haworth
projections for D-sugars the highest numbered
carbon (furthest from the carbonyl) is
drawn up. For L-sugars, it is drawndown
for D-sugars, the OH group at the
anomeric position is drawn down for and up for . For L-sugars is up and is down
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Optical isomerism A property exhibited by any compound
whose mirror images are non-
superimposable Asymmetric compounds rotate plane
polarized light
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POLARIMETRY Measurement of optical activity in chiral or
asymmetric molecules using plane polarized light
Molecules may be chiral because of certain atoms
or because of chiral axes or chiral planes
Measurement uses an instrument called a
polarimeter (Lippich type)
Rotation is either (+) dextrorotatory or (-)
levorotatory
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polarimetry
Magnitude of rotation depends upon:
1. the nature of the compound
2. the length of the tube (cell or sample container) usuallyexpressed in decimeters (dm)
3. the wavelength of the light source employed; usually
either sodium D line at 589.3 nm or mercury vapor lamp
at 546.1 nm
4. temperature of sample
5. concentration of analyte in grams per 100 ml
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[]DT
l x c
observed x 100=
D = Na D line
T = temperature oC
obs
: observed rotation in degree (specify solvent)
l = length of tube in decimeter
c = concentration in grams/100ml
[] = specific rotation
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CH2OH CH2OH6 6
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Cellobiose, a product of cellulose breakdown, is theotherwise equivalent anomer (O on C1 points up).
The (1 4) glycosidic linkage is represented as a zig-
zag, but one glucose is actually flipped over relative
H O
OH
H
OHH
OH
CH2OH
H
O H
OH
H
OHH
OH
CH2OH
H
O
HH
1
23
5
4
6
1
23
4
5
6
maltose
H O
OH
H
OHH
OH
CH2OH
H
O OH
H
H
OHH
OH
CH2OH
H
H
H
O1
23
4
5
6
1
23
4
5
6
cellobiose
Disaccharides:
Maltose, a cleavage
product of starch(e.g., amylose), is a
disaccharide with an
(1 4) glycosidic
link between C1 - C4OH of 2 glucoses.
It is the anomer
(C1 O points down).
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Other disaccharides include:
Sucrose, common table sugar, has a glycosidicbond linking the anomeric hydroxyls of glucose & fructose.
Because the configuration at the anomeric C of glucose is (O points down from ring), the linkageis (12).
The full name of sucrose is -D-glucopyranosyl-(12)--D-fructopyranose.)
Lactose, milk sugar, is composed of galactose & glucose, with (14) linkage from the anomeric
OH of galactose. Its full name is -D-
CH2OH CH2OH CH2OH CH2OHCH2OH6
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Polysaccharides:
Plants store glucose as amylose or amylopectin,
glucose polymers collectively called starch.
Glucose storage in polymeric form minimizesosmotic effects.
Amylose is a glucose polymer with (14) linkages.It adopts a helical conformation.
The end of the polysaccharide with an anomeric C1
not involved in a glycosidic bond is called the
H O
OH
H
OHH
OHH
O H
H
OHH
OHH
O
HH H O
O
H
OHH
OHH
H H O
H
OHH
OHH
OH
HH O
O
H
OHH
OHH
O
H
1
5
4
3
1
2
amylose
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Amylopectin is a glucose polymer with mainly(14) linkages, but it also has branches formed by
(16) linkages. Branches are generally longer thanshown above.
The branches produce a compact structure & provide
multiple chain ends at which enzymatic cleavage can
H O
OH
H
OHH
OH
CH2OH
H
O H
H
OHH
OH
CH2OH
H
O
HH H O
OH
OHH
OH
CH2
HH H O
H
OHH
OH
CH2OH
H
OH
HH O
OH
OHH
OH
CH2OH
H
O
H
O
1 4
6
H O
H
OHH
OH
CH2OH
HH H O
H
OHH
OH
CH2OH
H
H
O
1
OH
3
4
5
2
amylopectin
CH2OH CH2OHglycogen
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Glycogen, the glucose storage polymer in animals,is similar in structure to amylopectin.
But glycogen has more (16) branches.
The highly branched structure permits rapid glucoserelease from glycogen stores, e.g., in muscle duringexercise.
The ability to rapidly mobilize glucose is moreessential to animals than to lants.
H O
OH
H
OHH
OH
CH2OH
H
O H
H
OHH
OH
CH2OH
H
O
HH H O
OH
OHH
OH
CH2
HH H O
H
OHH
OH
CH2OH
H
OH
HH O
OH
OHH
OH
CH2OH
H
O
H
O
1 4
6
H O
H
OHH
OH
HH H O
H
OHH
OHH
H
O
1
OH
3
4
5
2
glycogen
CH2OH CH2OH CH2OH CH2OHCH2OH6
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Cellulose, a major constituent of plant cell walls,consists of long linear chains of glucose with
(14) linkages.
Every other glucose is flipped over, due to linkages.
This promotes intra-chain and inter-chain H-bonds
and
cellulose
H O
OH
H
OHH
OHH
O
H
OHH
OHH
O
H H O
O H
OHH
OHH
H O
H
OHH
OHH
H
OHH O
O H
OHH
OHH
O
H H H H
1
5
4
3
1
2
van der Waals interactions,that cause cellulose chains
to be straight & rigid, andpack with a crystallinearrangement in thick bundles - microfibrils.
See: Botany online
Schematic of arrangement of
cellulose chains in a microf ibril.
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Multisubunit Cellulose Synthase complexes in theplasma membrane spin out from the cell surfacemicrofibrils consisting of 36 parallel, interactingcellulose chains.
These microfibrils are very strong.
The role of cellulose is to impart strength andrigidity to plant cell walls, which can withstand highhydrostatic pressure gradients. Osmotic swelling isprevented.
cellulose
H O
OH
H
OHH
OH
CH2OH
H
O
H
OHH
OH
CH2OH
HO
H H O
O H
OHH
OH
CH2OH
H
H O
H
OHH
OH
CH2OH
H
H
OHH O
O H
OHH
OH
CH2OH
HO
H H H H
1
6
5
4
3
1
2