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Carbohydrates chemistry
Dr : Abdel
naser
Badawy
‐
Carbohydrates are organic molecules found in nature,
constituting one of the four major classes of biomolecules.
‐‐
The other three are proteins, nucleic acids and lipids.
-- Saccharides
(saccharo
is Greek for ―sugar)
•
Carbohydrates are aldehyde
or ketone
compounds with multiple
hydroxyl groups.•
The basic molecular formula
(C.H2
O)n
where n = 3 or more.•
The term ―carbohydrate comes from the observation that when you heat sugars, you get carbon and water (hence, hydrate of carbon).
Classification
They are classified according to the number of structural units into:
1‐Monosaccharides=
They are the simplest carbohydrates that can not
be hydrolysed
into simpler units.
2‐
Disaccharides=
produce 2 molecules of monosaccharide on
hydrolysis.
3‐
Oligosaccharides=
produce three to ten monosaccharide units on
hydrolysis
4‐
Polysaccharides=
: produce more than 10 monosaccharide units on
hydrolysis
Monosaccarides-1
•
* Def :
They are the simplest carbohydrate unites which can not be hydrolyzed to a simpler form
•
* General formula:
(CH2
O)n
n≥3
:* Nomenclature
•
1-
According to active group in the sugar:–
If monosaccharide contains aldehyde
group (CHO) → it's
called aldose.–
And if contain ketone
group
(c=o) → it's called ketose.
According to the number of carbon atoms (n):‐2
If sugar contains
3 carbons → it's called triose, 4c→ tetrose 5c→ pentose
6c→ hexose
7c→ heptose
Three Carbon Four Carbon
By combining the two methods , we find that:-33c-Aldotriose -ketotriose4c-Aldotetrose -ketotetrose5c-Aldopentise -ketopentose6c-Aldohexose -ketohexose
Five Carbon Six Carbon
The structure of glucose can be represented in one of the following ways:
1.The straight – chain (open ‐
chain) structural formula :
Aldohexose
can account for some of the properties of glucose, but
can not explain some reaction
D-glucose
2. The cyclic structure
accounts for the remainder of the chemical
properties of glucose. This cyclic structure can be represented in two
forms:
a. Fischer projection formula :where the aldehyde
group reacts with
an alcohol group on the same sugar to form a hemiacetal
ring.
C
H – C – OH
HO – C – H
H – C – OH
H – C
CH2OH
α‐D‐Glucopyranose
H OH
O
C
H – C – OH
HO –C – H
H – C – OH
H – C
CH2OH
β ‐D‐Glucopyranose
oH H
O
b. Haworth formula:
Where the cyclic structure is represented in pyranose
(six –
membered) and
furanose
(five –
membered) rings resembling pyran
and furan rings.
Here oH
and H written above and below instead of Right and left
so what is on Right → below left → above
except last carbon (which close the ring) in which
left →below Right
→above
α‐D‐Glucopyranose
An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon
Hemiacetals or Hemiketals
:c) Boat and chair forms
represents the three dimensional configuration of sugar in nature.
formshaworthFructose in open chain &
carbon, Asymmetric Anomeric carbon):chiralcarbon atom (
•
It is that carbon atom attached to four different groups or atoms.
•
Formation of a ring results in the creation of an anomeric
carbon
at carbon 1 of an aldose
or at carbon 2 of a ketose.
Reducing sugars•
If the oxygen on the anomeric
carbon (the carbonyl group) of a sugar is not attached to any other structure, that sugar is a reducing sugar.
•
Only the state of the oxygen on the anomeric
carbon determines
if the sugar is reducing or nonreducing—the other hydroxyl groups on the molecule are not involved
Reducing sugars
•
A reducing sugar can react with chemical reagents (for example, Benedict's solution) and reduce the reactive component, with the anomeric
carbon
becoming oxidized.
Any compound having one or more asymmetric carbon atom
shows two properties:
1.
Optical activity.
2.
Stereoisomerism
Optical activity-1
•
Def.
It is the ability of the compound to rotate plane polarized light to the right or to the left.
•
If the compound rotates plane polarized light to the right, it is called dextrorotatory, d or (+).
•
If it rotates plane polarized light to the left, it is called levorotatory, l or (-).
•
The direction of rotation is independent of the stereochemistry
•
of the sugar, so it may be designated D(−), D(+), L(−), or L(+).
•
For example, the naturally occurring form of fructose is the D(−) isomer.
••
dextrorotatorydextrorotatory sugar (d or +): sugar (d or +): glucose, glucose, galactosegalactose
and starchand starch
••
levorotatorylevorotatory sugar (l or sugar (l or --): ): fructose fructose and invert sugarand invert sugar..
2‐
Stereoisomerism
Compound
that
have
the
same
structural
formula
but
differ
in
spatial
configuration
are
known
as
stereoisomers
and
the
phenomenon is called stereoisomerism.
The
number
of
possible
isomers
of
a
compound
depends
on
the
number of asymmetric carbon atoms (n) and is equal to 2n.
Glucose, with four asymmetric carbon atoms, has 16 isomers.
•
The more important types of isomers include –
D and L isomers,
–
pyranose
and furanose
ring structures, –
alpha and beta anomers,
–
epimers
and –
aldose-ketose
isomers.
IsomersIsomersAre compounds which have the same molecular Are compounds which have the same molecular
weight, same percentage composition, and differ weight, same percentage composition, and differ in their physical and chemical properties.in their physical and chemical properties.
--
StereoisomersStereoisomers: are compounds that have the : are compounds that have the same structural formula but differ in spatial same structural formula but differ in spatial configuration (arrangement of atoms and groups configuration (arrangement of atoms and groups of atoms in space around the asymmetric of atoms in space around the asymmetric carbon(scarbon(s) i.e. different configuration).) i.e. different configuration).
1. D and L isomers ):enantiomers(
•
A special type of isomerism is found in the pairs of structures that are mirror image of each other.
•
These mirror images are called enantiomers
and the two members of the pair are designated as a D and an L-sugar.
•
A monosaccharide is designated D if the hydroxyl group on the highest numbered asymmetric carbon= prelast
carbon
is drawn to
the right as in D- glyceraldehyde
and L if the
hydroxyl group on the highest numbered asymmetric carbon is drawn to the left as in L-
glyceraldehyde.
‐
The majority of the sugars in humans are D‐sugars.
‐
Two exceptions are L‐fucose
(in glycoproleins) and L‐iduronic
acid
(in glycosaminoglycans).
ring structures: furanoseand Pyranose2.
The monosaccharides
are either pyran
(a six‐membered
ring) or furan (a five‐membered
ring).
For glucose in solution, more than 99% is in the pyranose
form.
:anomers3. Alpha and beta
The ring structure of an aldose
is a hemiacetal, since it is
formed by combination of an aldehyde
and an alcohol group.
Similarly, the ring structure of a ketose
is a hemiketal. The cyclic
structure is retained in solution, but isomerism (C6
H12
O6
) occurs
about position 1, the carbonyl or anomeric
carbon atom, to give
a mixture of α‐glucopyranose
(38%) and β‐glucopyranose
(62%).
•
The cyclic α
and β
anomers
of a sugar in solution are in equilibrium with each other, and can be spontaneously interconverted
(a process called mutarotation)
: Epimers4.
Isomers differing as a result of variations in configuration of
the OH and H on carbon atoms 2, 3, and 4 of glucose are known
as epimers. Biologically, the most important epimers
of
(C6
H12
O6
) glucose are mannose and galactose, formed by
epimerization at carbons 2 and 4, respectively.
Epimers of glucose
D-
glucose D-
galactose Epimer at C4
D-
mannoseEpimer at C2
isomerism: ketose‐Aldose5.
Fructose has the same molecular formula as glucose
(C6
H12
O6
) but differs in its structural formula, since there is keto
group in position 2, the anomeric
carbon of fructose, whereas
there is aldehyde
group in position 1, the anomeric
carbon of
glucose
Are Physiologically Important:MonosaccharidesMany
xylose, ribuloseribose, e.g:Pentoses1
, fructose, mannosegalactoseglucose, : Hexoses‐2
) are formed as metabolic sedoheptulose(carbon sugar ‐seven‐3
intermediates in the pentose phosphate pathway.
g carboxylic acid derivatives of glucose are important, includin‐4
formation and in glucuronide(for glucuronate‐Da.
glycosaminoglycans)
)glycosaminoglycans(in iduronate‐Lb.
acid pathway)theuronic(an intermediate in gulonate‐Lc.
Derived Sugars
Sugars:Deoxy1.
Lack an Oxygen Atom =Deoxy
sugars are those in which a hydroxyl
group has been replaced by hydrogen. Examples:
a. deoxyribose
in DNA.
b. L‐fucose
occurs in glycoproteins.
c. 2‐deoxyglucose is used experimentally as an inhibitor of glucose
metabolism..
):Hexosamines2. Amino Sugars (
Are compounds in which OH at C2 is replaced by NH2
1. D‐glucosamine, a constituent of hyaluronic
acid ,
2. D‐galactosamine(chondrosamine), a constituent of chondroitin;
3. Several antibiotics (eg, erythromycin) contain amino sugars
believed to be important for their antibiotic activity
Galactosamine
3.Sugar acids
1.1.
Produced by oxidation of Produced by oxidation of carbonyl carbon to carboxylic carbonyl carbon to carboxylic group.group.
2.2.
Or by oxidation of last hydroxyl Or by oxidation of last hydroxyl carbon to carboxylic group.carbon to carboxylic group.
3.3.
Or by oxidation of both.Or by oxidation of both.
1.Aldonic1.Aldonic
CHO
C OHH
C HHO
C OHH
C OHH
CH2OH
COOH
C OHH
C HHO
C OHH
C OHH
CH2OH
bromine water, O2
D-GluconicacidD-Glucose
UronicUronic--22
CHO
C OHH
C HHO
C OHH
C OHH
CH2OH
CHO
C OHH
C HHO
C OHH
C OHH
COOH
Dil. Nitric acid
D-GlucuronicacidD-Glucose
H2O2
AldaricAldaric--33
CHO
C OHH
C HHO
C OHH
C OHH
CH2OH
COOH
C OHH
C HHO
C OHH
C OHH
COOH
Conc. Nitric acid
D-Glucaric acidD-Glucose
O2
Glycoside Formation
•
The hemiacetal
and hemiketal
forms of monosaccharides
can react with alcohols
to form acetal
and ketal
structures called glycosides. The new carbon-oxygen bond is called the glycosidic
linkage.
AcetalGlycosides=
The OH of anomeric
carbon of monosaccharides
can react with either:
1‐
Nitrogen of amines
(The anomeric
carbon atom of sugar can be
linked to the nitrogen atom of an amine by N‐
glycosidic
bond) e.g
OH
of ribose linked to nitrogen of nitrogenous base to form nucleotides.
OH of other -2 compound :
•
A-
May be carbohydrate
( Monosaccharides
can be linked to
each other by O-
glycosidic
bonds to form disaccharides, oligosaccharides and polysaccharides).
•
(monosaccharide+ monosaccharide)=hemiacetal+hemia
cetal=acetal.•
b-
May be non carbohydrate
e.g
glycolipid, glycoprotein. •
The non carbohydrate component of a glycoside is called aglycone.
O- and N-glycosides
•
If the group on the non- carbohydrate molecule
to which the sugar is attached is an -OH group, the structure is an O-glycoside
•
All sugar-sugar glycosidic
bonds are O-
type linkages
O- and N-glycosides
•
If the group is an -NH2 , the structure is an N-
glycoside
–
purines
and pyrimidines (found in nucleic acids),
–
aromatic rings (such as those found in steroids and bilirubin),
–
proteins (found in glycoproteins
and
glycosaminoglycans),
Naming glycosidic bonds
•
Glycosidic
bonds between sugars are named according to –
numbers of the connected carbons (1-4, 1-6),
and
–
position of the anomeric
hydroxyl
group of the sugar involved in the bond.
Naming glycosidic bonds
•
this anomeric
hydroxyl group is in the α
configuration, the linkage is an α-bond.
•
If it is in the β configuration, the
linkage is a β-bond.
Naming glycosidic bonds
•
Lactose, for example, is synthesized by forming a glycosidic
bond between carbon
1 of β-galactose
and carbon 4 of glucose. –
The linkage is, therefore:
β(1 →4)
glycosidic
bond. •
Because the anomeric
end of the
glucose residue is not involved in the glycosidic
linkage it (and,
therefore, lactose) remains a reducing sugar.
Naming of O‐
glycosidic
bond() carbohydrate and carbohydrate
‐
Glycosidic
bonds between sugars are named according to:
1‐
The numbers of the connected carbons
2‐
The position of the anomeric
hydroxyl group of the sugar.
If the anomeric
hydroxyl group is in α
configuration the link is α‐
bond and if it’s in β, the link is β‐
bond. Examples
In lactose β
1 of galactose
bind to C4 of glucose by β
1
→ 4
galactosidic
bond.
Examples of glycosides:
1‐
Disaccharides as maltose, lactose and sucrose
2‐
Polysaccharides.
3‐
Glycolipids.
4‐Glycoproteins.(may be O‐
or N‐
glycosidic
link)
5‐Nucleotides as ATP, GTP, UTP where aglycon
is purine
or
pyrimidine
bases (N‐
glycosides)
The glycosides that are important in medicine
1. cardiac glycosides all contain steroids as the aglycone.
2. Other glycosides include antibiotics such as streptomycin.
Disaccharides
‐
Disaccharide consists of two sugars joined by an O‐
glycosidic
bond.
‐
The most abundant disaccharides are sucrose, lactose and maltose.
‐
Other disaccharides include isomaltose, cellobiose
and trehalose.
‐The disaccharides can be classified into homo disaccharides and
hetero disaccharides.
‐A) Homo disaccharides: are formed of the same monosaccharide units
and include maltose, isomaltose, cellobiose
and trehalose.
‐(B) Hetero disaccharides: are formed of different monosaccharide
units and include: sucrose, lactose.
Lactose:Lactose:It is formed of -galactose
and -glucose linked by
-1,4-glucosidic linkageContain free anomeric
carbon so reducing sugar
It may appear in urine in late pregnancy and during lactation.
OOH
H HH
OHH
OH
CH2OH
H
OH OH ..... H
H ..... OHH
OHH
OH
CH2OH
H
and Lactose-Galactose Glucose
1 4O
SucroseSucrose•
α
D-glucopyranose
and
β
D fructofuranose
by α 1-
2 glycosidic
bond
•
No free aldehyde
or keton
gp
so non
reducing sugar•
hydrolysed
to glucose
and fructose by sucrase (invertase) enzyme.
•
* Sucrose is dextrorotatory +66.5.
OH
OH
H
H
OHH
OH
CH2OH
H 1
Sucrose
-Glucose
-FructoseCH2OH
O
H
CH2OH
OH H
H OH
O
2
Maltose (malt sugar):Maltose (malt sugar):It consists of 2 It consists of 2 --glucose units linked by glucose units linked by
--1,41,4--glucosidic linkage,glucosidic linkage,Contain free anomeric
carbon so reducing sugar.
OH
OH
H
H
OHH
OH
CH 2OH
H
OH OH ..... H
H ..... OHH
OHH
OH
CH 2OH
H
O
and Maltose -Glucose Glucose
1 4
B. B. TrehaloseTrehalose::It is formed of 2 -glucose units linked by -1,1-
glucosidic linkage. Not Contain free anomeric carbon so non reducing sugar
Present in a highly toxic lipid extracted from Mycobacterium tuberculosis.
OH
OH
H
H
OHH
OH
CH 2OH
H
O
H H
OHHO H2C
H
OHH
OH H
O11
Trehalose -Glucose -Glucose
Disaccharides Components Reduction 1-
sucrose =
cane sugar = beet sugar = table sugar
α
D- glucopyranose
and β
Dfructofuranose. by α
1-
2
glycosidic
bond
No free aldehyde
or
keton
gp
so non reducing sugar
hydrolysed
to glucose and fructose by sucrase
(invertase) enzyme.* Sucrose is dextrorotatory +66.5.
2-
Lactose (milk sugar)
β
D galactose and α
D glucose.
by β
1-4 glycosidic
bond
Contain free anomeric
carbon so reducing sugar
* It may appear in urine in late pregnancy and during lactation.
Disaccharides *Components Bond Reduction
1-
Maltose 2αD-glucose α
1-4 glycosidic Contain free anomeric
carbon so reducing sugar.
4-
Trehalose 2αD-glucose α
1-1 glycosidic Not Contain free anomeric
carbon so non reducing sugar
Oligosaccharides
•
Oligosaccharides contain from 3 to 10 monosaccharide units.
•
Raffinose
An oligosaccharide found in peas and beans
Polysaccharides (glycans)
‐
Polysaccharides consist of more than 10 monosaccharide units and /
or their derivatives
Classification According to structure:
1‐
Homo polysaccharides (Homo glycans): contain only one type of
monosaccharide molecule.
E.g. starch, glycogen, dextrin, cellulose, inulin
and chitin
2‐
Hetero polysaccharides: contain more than one type of
monosaccharides.
E.g. glycosaminoglycan, glycoprotein.
1. Starch
‐
is a homopolymer
of glucose forming an α‐
glucosidic
chain, called a
glucosan
or glucan.
‐It is the most abundant dietary carbohydrate in cereals, potatoes,
legumes, and other vegetables.
‐The two main constituents are amylose
(15–20%), which has a
nonbranching
helical structure) and amylopectin
(80–85%), which consists of branched chains composed of 24–30
glucose residues united by 1 → 4linkages in the chains and by 1 → 6
linkages at the branch points.
:Dextrins2. Are intermediates in the hydrolysis of starch.
••
AmyloseAmylose::. . Straight chain compound Straight chain compound present in the form glucose units present in the form glucose units linked by linked by --1,41,4--glucosidic bond of a glucosidic bond of a helix formed of a large number of helix formed of a large number of --
glucose.glucose.
OH H
H
OHH
OH
CH 2OH
H
OH H
OHH
OH
CH 2OH
H
O
Am ylose
14
nO O
1 4
It forms the inner part of starch granules
:Glycogen3.
‐
is the storage polysaccharide in animals.
‐
It is a more highly branched structure than amylopectin, with chains
of 12–14 α‐D‐glucopyranose
residues in α[1 → 4]‐glucosidic linkage),
with branching by means of α(1 → 6)‐glucosidic bonds
:Inulin4.
is a polysaccharide of FRUCTOSE
(and hence a fructosan)
found in plants.
It is readily soluble in water .
is used to determine the glomerular
filtration rate.
Cellulose 5.
‐Is the chief constituent of the framework of plants.
‐It is insoluble
‐consists of β‐D‐glucopyranose
units linked by β(1 → 4) bonds to form
long, straight chains strengthened by cross‐linked hydrogen bonds.
Cellulose cannot be digested by mammals because of the absence
of an enzyme that hydrolyzes the β
linkage. It is an important
source of “bulk”
in the diet. So prevent constipation.
‐
Starch
Cellulose
6. Chitin
is a structural polysaccharide in the exoskeleton of crustaceans
and insects and also in mushrooms.
It consists of N‐acetyl‐D‐glucosamine
units joined by
β
(1 →4)‐glycosidic
linkages
Glycosaminoglycans
(GAGs)
(Mucopolysaccharides)
‐
Glycosaminoglycans
are long linear (unbranched)
heteropolysaccharide
chains generally composed of a repeating
disaccharide unit (acidic sugar‐amino sugar)n.
‐The
amino
sugar
is
either
D‐glucosamine
or
D‐galactosamine
in
which
the
amino
group
is
usually
acetylated,
and
sometimes
sulphated.
There are 6 types:
1. heparin. 2. heparan
sulphate. 3. dermatan
sulphate
4. keratan
s 5. Chondroitin
s 6. hyaluronic
acid
)mucopolysaccharides(Glycosaminoglycans
All of the glycosaminoglycans
except hyaluronic
acid and heparin are
found covalently attached to protein, forming proteoglycan
monomers.
Their property of holding large quantities of water and occupying
space lubricating other structures, is due to the large number of OH
groups and negative charges on the molecules, which, by repulsion,
keep the carbohydrate chains apart.
Glycoproteins
‐Are proteins to which oligosaccharides are covalently attached.
‐
Oligosaccharide chains formed mainly of sialic
acids and L‐fucose.
‐
Sialic
acids are N‐
or O‐derivatives of neuraminic
acid .
‐
Neuraminic
acid is a nine‐carbon sugar derived from mannosamine
and pyruvate.
•
Glycoproteins
have many functions: •
1-
Soluble as enzymes, hormones and
antibodies. •
2-
In lysosomes
•
3-
attached to the cell membrane (The membrane bound glycoproteins) participate in:–
a-
cell surface recognition (by other cells,
hormones, viruses)–
b-
Cell surface antigenicity
(as blood gp
antigens)
Proteoglycans Glycoprotein Carbohydrate components
Glycosaminoglycans-
Repeating disaccharide unit
-
Linear (unbranched) -
long
-
Contain uronic
acids (glucuronic and iduronic
-
N-
acetyl hexosamine-
Contain hexoses
as galactose
(in
keratin sulphate)-
contain sulphate
-
No pentoses-
No deoxy
sugar
Oligosaccharides-
No repeating units
-
Branched- short-
contain sialic
acid derivatives
(NANA)-
N-
acetyl hexasamine
-
Contain hexoses
as galactose and mannose
- No sulphate-
Contain pentose
-Contain deoxy
sugar as L-fucose
2-
Tissue distribution and functionsStructural-
Cartilage
-
Bone-
Tendons
-
Cell membrane-
Cornea
Functional and structural - mucines-
Blood groups antigens
-
some hormones- enzymes-
Immunoglobulins
and
receptors
AcidNeuraminic-Acetyl–NANA=N
-
Neuraminic
Acid=sialic
acid = mannosamine + pyruvic
acid
= amino sugar acid-
NANA found in glycoproteins.
Fucose-L
galactose-L–deoxy -= 6
=Methyl pentose
Found in glycoprotein
THE END!
Thank You