Carbohydrates Structure and Biological Function Entire chapter 8

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CarbohydratesStructure and Biological
Function(Entire chapter 8)

• Monosaccharides and their strucutres
• Reactions of Glucose and Other Sugars
• Disaccharides and Polysaccharides
• Glycoproteins

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 Biological functions of carbohydrates 1)   
Energy source Glucose is the major biological
fuel.
2)    Ribose and deoxyribose are the building
blocks of RNA and DNA.   3)    Some large
sugars (sellulose) are important structure
materials for plant and bacteria cell walls.
  4)    Can form glycoproteins or glycolipids,
which act as cell surface markers.
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• Carbohydrates all have CO and -OH functional
  groups.
• Classified based on
• Size of carbon chain
• Number of sugar units
• Location of CO
• Stereochemistry

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• Types of carbohydrates based on number
• of sugar units
• Monosaccharides a single sugar unit
• (simple sugar 3- 7 carbons)
• Disaccharides - two simple sugar units
• Polysaccharides - more than 10 units

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Monsaccharides General formula (CH2O)n. n
number of carbon.
Most monosaccharides in nature contain 3 - 7
carbons. They are triose, tetrose, pentose,
hexose, and heptose).
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According to where the carbonyl group is located,
monosaccharides can be divided into

• Aldose - polyhydroxyl aldehyde (aldehyde sugar)
• with the carbonyl group (CO) at the
• first carbon position, which forms an
• aldehyde group (CHO).
• Ketose - polyhydroxyl ketone (ketone sugar).
• Carbonyl group is at C2 position

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Most monosaccharides have OH group on each C
except for the carbonyl C
H CO H-
C-OH H- C-OH H- C-OH
CH2OH
?
Aldose and pentose
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Common expression of carbohydrate structure -
Fischer projection. In this system, the center
of the cross is a tetrahedral carbon.
H CO
H-C-OH CH2OH
Glyceraldehyde simplest monosaccharide with 3
carbons (triose). Its also an aldose, or
aldotriose which has an aldehyde group at C1
position.
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H CO
H-C-OH H-C-OH H-C-OH
CH2OH
CH2OH CO
HO-C-H H-C-OH
H-C-OH CH2OH
Aldose Ketose (- CHO) (- CO)
(Based on the location of CO)
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• Based on the number of carbon atoms in the chain

H CO
H-C-OH H-C-OH H-C-OH
H-C-OH CH2OH
H CO
H-C-OH H-C-OH
H-C-OH CH2OH
H CO
H-C-OH H-C-OH
CH2OH
H CO
H-C-OH CH2OH
triose tetrose pentose
hexose All can be aldose or ketose.
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In glyceraldehyde molecule, the asymmetric
carbon, C2 is called a chiral center. There
can be two stereoisomers, the D and L forms.
Naturally existing glyceraldehyde is in the D
configuration.
H CO
H-C-OH CH2OH
Chiral center
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Any other sugars with same configuration as
D-glyceraldehyde is classified as D sugar,
otherwise L sugar. (Fig. 8.4).
Most naturally occurred sugars are in their
D-forms. D-glucose, D- ribose (Amino acids ?)

Conventionally, the chiral center farthest from
the carbonyl carbon determines D or L forms of
a sugar.
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CH2OH CO
HO-C-H H-C-OH
H-C-OH CH2OH
H CO
H-C-OH CH2OH
D-glyceraldehyde D-fructose aldotriose
sugar ketohexose sugar
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H CO
H-C-OH H-C-OH
HO-C-H HO-C-H
CH2OH
H CO
H-C-OH H-C-OH H-C-OH
CH2OH
D-ribose L-mannose aldopentose sugar
aldohexose sugar
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• Stereoisomers
• Have different spatial arrangements.
• D- or L- forms are mirror images that cant be
  overlapped.

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L- and D- glyceraldehyde
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Some important monosaccharides

• D-glyceraldehyde Simplest sugar
• D-glucose Most important in diet
• D-fructose Fruit sugar
• D-galactose Milk sugar
• D-ribose Used in RNA

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D-glyceraldehyde

• Three carbon
• Aldotriose

H CO
H-C-OH CH2OH
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D-glucose

• An aldohexose sugar.
• Common names include dextrose, grape sugar, blood
  sugar.
• Most abundant organic compound found in nature.

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D-fructose

• Also called fruit sugar.
• A ketohexose.
• Sweetest of all simple sugars.

CH2OH CO
HO-C-H H-C-OH
H-C-OH CH2OH
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Carbohydrates in cyclic structures

• Monohydrates with five C or more spend most of
  their time in cyclic structure.

• When the aldehyde or ketone groups interact
• with the -OH in the other end of the molecule,
• a ring is formed.
• aldehyde ? hemiacetal
• ketone ? hemiketal

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Intramolecular cyclization
?
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Fischer vs. Haworth projections
? -D-glucose
C
H
OH
CH
OH
2
H
C
OH
H
O
H
C1
H
O
C
H
HO
H
OH
C
OH
H
OH
OH
C
HO-CH2
H
OH
H
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•  
• Based on the position of OH on C-1, each
• cyclic sugar can have two stereoisomers,
• ? and ?.
• ? - OH group is under the plane (or it is down
• compared to CH2OH) (trans).
• ? - OH group is above the plane (or it is up
• compared to CH2OH (cis).

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? -D - glucose
CH
OH
2
O
OH
H
? - D - glucose
H
H
OH
OH
H
H
OH
In solution, D-glucose exists in a mixture of
three forms, 33 of ? rings, 66 of ? ring and
1 of open chain.
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• This can also happen
• to ketose (fructose).

CH2OH
CH
OH
2
O
H
OH
OH
CH
2
??(trans)
H
OH
C
O
H
OH
C
H
HO
C
OH
H
C
OH
H
OH
CH
OH
2
O
CH
OH
H
OH
2
??(cis)
H
CH2OH
H
OH
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D-galactose

• (Common part of lactose, a disaccharide)

O
H
OH
H
H
OH
O
H
C
OH
H
C
OH
H
OH
H
C
H
HO
C
H
HO
C
OH
H
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D-glucose vs. D-galactose
D-glucose D-galactose
O
Can you find a difference? Your body can! You
cant digest galactose - it must be converted to
glucose first.
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D-ribose

• A part of the backbone of RNA.
• When the C-2 OH is removed,
• the sugar becomes
• deoxyribose.

H CO
H-C-OH H-C-OH
H-C-OH CH2OH
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Reactions of glucose and other monosaccharides

• Oxidation-Reduction. Required for their complete
  metabolic breakdown.
• Esterification. React with acid to form esters.
• Amino derivatives.
• Glycoside formation - Linkage of monosaccharides
  to form polysaccharides.

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1). Oxidation-reduction (redox) Reaction Redox
reactions are the most important part of energy
production.  
For an open chain form of glucose, oxidation
changes the aldehyde (COH) group into a
carboxylic acid group (COOH).
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(Oxidized form)
(Reduceded form)
O
HO
C
C
OH
H
C
H
HO
C
OH
H
C
OH
H
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In a redox reaction, if one reactant (glucose)
becomes oxidized (gaining an O atom or losing a
H), the other has to be reduced (loosing an O,
or gaining a H).
The molecule that gains O or loses H (glucose) is
called a reducing agent. The molecule that
gains H or loses O is called an oxidizing agent.
Fir example, glucose and NAD. By losing H to
NAD, glucose is oxidized, NAD is reduced to
NADH. Glucose- reducing agent.
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Reducing sugars are those that contain free
aldehyde group (aldose) and are capable of
reducing metal ions (Cu ? Cu). In a redox
reaction, an aldehyde sugar will gain an O and
becomes oxidized (-COH ? COOH). In the mean
time, it acts as a reducing agent to make the
other reactant reduced.
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CH2OH CO
HO-C-H H-C-OH
H-C-OH CH2OH
D-glucose D-fructose Reducing
sugar nonreducing sugar
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The oxidizing agents, Benedicts reagent (CuSO4),
Tollen's reagent (Ag(NH3)2) and Cu are used
to identify the presence of reducing sugars
(urine glucose).
Glucosered Cu glucoseox
Cu 2Cu O2 ? Cu2O
(red precipitate)
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Benedicts Reaction
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An open chained aldose can be oxidized into an
acid (COOH). Oxidation of a cyclic aldose will
produce a a cyclic ester (lactone). (Fig. 8.10)
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In aerobic respiration pathways, the enzyme that
catalyzes redox reactions is dehydrogenase, which
uses NAD and NADH as coenzymes. NAD - an
oxidizing agent that can receive
electrons. NADH - a reducing agent that can
donate electrons. NAD 2e NADH H
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2). Esterification (phosphorylation) When -OH
group in an alcohol (ROH) or carbohydrate react
with phosphoric acid, phosphate esters can be
produced.  
The most important phosphate esters are
D-glucose 6-phosphate, D-ribose 5-phosphate and
D-deoxyribose 5 phosphate. Phosphorylated
carbohydrates are important derivatives of sugar.
 
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Esterification

• Esters are formed by reaction of hydroxyl groups
  (alcohols) with acids.

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• The most important biological esters of
  carbohydrates are phosphate esters.

O
O
P
OH
HO
R
OH

P
OH
HO
O - R
OH
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In a living cell, phosphate esters are not
produced by acid, instead, by transferring
phosphoryl (P) group form ATP to a carbohydrate
molecule. and the reaction is catalyzed by
kinases.  

• Example Phosphorylation of glucose
• D-glucose ATP glucose-6-phosphate
  ADP
• (P- group is transferred to C6 on glucose)

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3). Amino Derivatives of Sugars When -OH group
of a carbohydrates is replaced by an amino group
? amino sugar.   Two naturally occurred amino
sugars are D-2-aminoglucose (glucosamine) and
D-2-aminogalactose (galactosamine). (which
OH?)
Derivatives of glucosamine, N-acetylglusamine
and N-acetylmuramic acid, are two components of
bacteria cell wall.  
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The replacement of a hydroxyl group on a
carbohydrate results in an amino sugar.
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• Functions of amino sugars.
• Structural components of bacterial cell walls.
• As a component of exoskeleton of some organisms.
  (Chitin polymer of N-acetyl-glusamine).

• A major structural unit of chondroitin sulfate
• (derivative of glucosamine), a component of
• cartilage
• Component of glycoprotein and glycolipids.

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4). Glycoside Formation An O-glycosidic bond
can be formed between two monosaccharides, when
two -OH groups react with each other. The
product is called a glycoside.  
A glycosidic bond can link C1 of one carbohydrate
and C4 of the second carbohydrate.
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Glycoside formation

• or ? -OH group on one cyclic monosaccharide
• can form O-linkage with the one on another sugar.
• glycosidic bond
• (oxygen bridge)
• sugar -O- sugar

O
O
OH
H
H
H
H
H
H
H
OH
OH
H
OH
OH
OH
H
OH
H
OH
O
H
H
H
O
H
H
OH
o
H
H
OH
OH
H
OH
OH
H
H
OH
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Glycosidic bonds
Two types of glycosidic bonds can be formed,
depending on the position of the C-1 OH (? or ?
configuration) (Fig. 8.16) Glycosidic bond
- linkage between a ? C-1 OH and a ? C-4 OH
is called ? (1 ? 4) linkage.
? bonds ? bonds
C1
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Disaccharides (glycosides) sucrose, lactose,
maltose
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Lactose

• Milk sugar - dimer of ?-D-galactose and
  either ? or ? - D-glucose.

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For disaccharides, the sugar is either ? or ?
based on the form of the remaining C-1 OH.
C1
?-lactose
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• Lactase
• Enzyme required to hydrolyze lactose.
• Lactose intolerance
• Lack or insufficient amount of the enzyme.
• If lactase enters lower
• GI, it can cause gas
• and cramps.

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?-Maltose

• Malt sugar. Not common in nature except in
  germinating grains.

?-D-glucose and ?-D-glucose, ? (1 4)
linkage.
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• It is called ?-maltose because C-1 on ?-D-glucose
  is in the ? position.

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• Uses for ?-maltose
• Ingredient in infant formulas.
• Production of beer.
• Flavoring - fresh baked aroma.
• It can be hydrolyzed by
• maltose H2O 2 glucose

maltase
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Sucrose

• Disaccharide of ?-glucose and
  ?-fructose.

An unique linkage
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This is a special case The linkage between the
glucose and fructose is between two carbonyl
carbons.
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How sweet it is!

• Sweetness relative to sucrose
• lactose 0.16
• galactose 0.32
• maltose 0.33
• sucrose 1.00
• fructose 1.73
• aspartame 180

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• Polysaccharides
• Carbohydrate polymers
• - Chains of monosaccharides linked up by
• O-glycosidic bonds.
• Different polysaccharides starch, glycogen,
• cellulose, depending on the length degree of
• branching sequence of the monosaccharides
• and types of glycosidic bonds.
•  

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Functions of Polysaccharides

• Storage Polysaccharides
• Energy storage - starch and glycogen
• Structural Polysaccharides
• Provide protective cell walls cellulose
• Structural Peptidoglycans
• Bacterial cell walls

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1. Starch

• Energy storage in plants
• Long repeating chain of ?-D-glucose
• (up to 4000 units)

• Mixture of amylose and Amylopectin
• Amylose - major form of starch, straight chain of
  
• glucose linked up by ? (1? 4) glycosidic bonds.
• Amylopectin - branched structure with side chains
  
• linked by ?(1? 6) glycosidic bonds.

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Amylose

• Straight chain that forms coils ? (1 4)
  linkage.

Amylase is the enzyme that digests amylose.
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Amylose starch

• Straight chain that forms coils ? (1 4)
  linkage. Most common type of starch.

Coiled structure
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Amylopectin

• Branched structure due to crosslinks.

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2. Glycogen

• Energy storage of animals (animal starch).
• Stored in liver and muscles as granules.
• Highly branched polymer, similar to amylopectin.
• (but bigger)

? (1 6) linkage at crosslink
c
c
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The enzyme glycogen phosphorylase is the enzyme
that cleaves glucose from glycogen for energy
supply.
(Is phosphorylated glycogen phosphorylase the
more active or less active form?)
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 3. Cellulose Structural component of wood and
plant fibers. Cellulose is an unbranced polymer
of glucose linked up by ? (1?4) glycosidic bond.
 
Many long cellulose chains lined up in a parallel
arrangement and associate with each other
through H-bonds. The bundles of cellulose form
strong and rigid frameworks that provides
physical support for plant cells.
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Cellulose

• Most abundant polysaccharide.
• ? (1 4) glycosidic linkages.
• Result in long fibers - for plant structure.

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Celullase, the enzyme that digest ? (1-4)
glycosidic bonds between glucoses, is produced
by some fungi, bacteria and termites. These
organisms can extract glucose for nutrition
supply.
Cattle, sheep, goats can also use cellulose by
producing sellulose. Humans ?
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4. Mucopolysaccharides

• These materials provide a thin, viscous,
  jelly-like coating to cells extracellular
  matrix.
• The most abundant form is hyaluronic acid.
• Alternating units of
• N-acetylglucosamine and
• D-glucuronic acid.

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5. Structural peptidoglycans

• Bacterial cell walls are composed of unbranched
  polymer of alternating units of
  N-acetylglucosamine and N-acetylmuramic acid.
• Peptide crosslinks between the polymer

R
O
O
H
H
O
H
H
H
H
OR
O
OH
O
H
H
NH
H
NH
H
C
O
C
O
Peptide crosslink
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• Polymer of N-acetylgluco-
• samine and N-acetylmuramic
• Acid crosslinked with
• peptide bridges

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6. Glycoproteins

• Proteins that carry covalently bound carbohydrate
  units.
• Biological functions serve as cell surface
  marker to be involved in
• immunological protection
• cell-cell recognition
• blood clotting
• host-pathogen interaction

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• Carbohydrates only account for 1-30 of the total
  weight of a glycoprotein.
• The most common monosaccharides are
• glucose mannose
• galactose fucose
• sialic acid
• N-acetylgalactosamine
• N-acetylglucosamine

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Glycoprotein structure

• Linking sugars to proteins.
• O-glycosidic bonds using hydroxyl groups of
• serine and threonine
• N-glycosidic bonds
• using side chain amide
• nitrogen of asparagine residue

threonine
polypeptide chain
asparagine
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