Carbohydrates
o The name "carbohydrate" means a "hydrate of carbon.�
o The name is derived from the general formula of carbohydrate is Cx(H2O)y-x
and y may or may not be equal and range in value from 3 to 12 or more
o For example glucose is: C6(H2O)6 or is
more commonly written, C6H12O6.
o General names for carbohydrates include sugars, starches, saccharides,
and polysaccharides. The term saccharide is derived from the Latin word "
saccharum" from the sweet taste of sugars.
o The chemistry of carbohydrates most closely resembles that of alcohol,
aldehyde, and ketone functional groups. As a result, the modern definition of a
CARBOHYDRATE is that the compounds are polyhydroxy aldehydes or ketones.
o The chemistry of carbohydrates is complicated by the fact that there is a
functional group (alcohol) on almost every carbon. In addition, the
carbohydrate may exist in either a straight chain or a ring structure. Ring
structures incorporate two additional functional groups: the hemiacetal and
acetal.
o A major part of the carbon cycle occurs as carbon dioxide is converted to
carbohydrates through photosynthesis. Carbohydrates are utilized by animals and
humans in metabolism to produce energy and other compounds.
Synthesis
o Carbohydrates are initially synthesized in plants from a complex series
of reactions involving photosynthesis.
o
o Photosynthesis is a complex series of reactions carried out by algae,
phytoplankton, and the leaves in plants, which utilize the energy from the sun
o The simplified version of this chemical reaction is to utilize carbon
dioxide molecules from the air and water molecules and the energy from the sun
to produce a simple sugar such as glucose and oxygen molecules as a by product.
Biological
functions
o Store energy in the form of starch (photosynthesis in plants) or glycogen
(in animals and humans).
o Provide energy through metabolism pathways and cycles.
o Supply carbon for synthesis of other compounds.
o Form structural components in cells and tissues.
o intermediates in the
biosynthesis of other basic biochemical entities (fats and proteins)
o associated with other
entities such as glycosides, vitamins and antibiotics)
o form structural
tissues in plants and in microorganisms (cellulose, lignin, murein)
o participate in
biological transport, cell-cell recognition, activation of growth factors,
modulation of the immune system
Classification
of Carbohydrates
o Based on the location
of C=O functional group (aldehyde C=O or ketone C=O)
o number of carbon
atoms
o Number of sugar units
o Stereochemistry
Classification
based on the number of sugar units
w Monosaccharides -
simple sugars with multiple OH groups ;simplest carbohydrate that cannot be
broken down into smaller carbohydrate molecules
w Disaccharides - 2
monosaccharides covalently linked.
w Oligosaccharides - a
few monosaccharides covalently linked, more specifically 10 monomers
w Polysaccharides -
polymers consisting of chains of monosaccharide or disaccharide units
Monosaccharides
o Classification based
on C=O functionality
o Aldoses (e.g.,
glucose) have an aldehyde group at one end.
Ketoses
(e.g., fructose) have a keto group, usually at C2.
Monosaccharide
o Classification based
on number of Carbon atom
Classification
based on C=O functionality
Aldoses
o D-glyceraldehyde-product of breakdown of larger
monosaccharides
o D-erythrose and threose are not abundant in
nature,
o D-erythrose can be found in muscles of animals in
minimum amount in form of its derivative erythrose-4-phosphate
o D ribose-component of RNA
o D xylose and arabinose can be found in nuts.
o D xylose is sugar that is normally not metabolized
is used to detect carbohydrate malabsorption in the xylose excretion test
o D-2-deoxyribose is another aldopentose similar to that of
ribose
o Structure of D-ribose
and D-2-Deoxyribose are identical except that there is a hydrogen atom instead
of hydroxyl group in C-2 carbon atom of D-2-Deoxyribose
o Deoxyribose is a component of deoxyribosenucleic acid
(DNA), a compound which transmits genetic information from parent to offspring
by directing the synthesis of protein molecules
o D-glucose-a white crystalline solid found in the
fruit juice and honey and in blood of mammals
o D-glucose is also
known as grape sugar and blood sugar (dextrose)
o Deficiency in insulin
in the body results in increase in blood sugar level. Disorder related in the
insufficient production of insulin in the body is known as diabetes mellitus
o D-galactose-important monosaccharide that can� combined with glucose in the disaccharide
lactose, a component of milk.
o Galactosemia-congenital
disorder,a persons who are suffering on this disease are unable to metabolize
galactose because they lack enzymes that can degrade galactose.
Classification
based on C=O functionality
Ketoses
o D-fructose is the most important ketose in living organisms
o D-frucose-known as fruit sugar and considered as the sweetest tasting
sugar. Also known as the levulose because it rotates the plane polarized light
levorotatorily
Types of
stereoisomerisms exhibit by carbohydrate molecules
o Enantiomers- a special class of stereoisomers which are
not superimposable mirror image of each other. The two enantiomers of a
compound have the identical chemical properties but differ in characteristic
physical property, ability to rotate the plane of plane polarized light.
Rotation of plane of plane polarized light are clockwise (dextrorotatory,
positive rotation, +)� or
counterclockwise (levorotatory, negative, -) rotation
Compounds
posses enantiomeric property if it:
����������� -contain chiral or asymmetric carbon
center (4 different groups are attached to central carbon atom
����������� -mirror image of one another
����������� -not superimposable
Enantiomers
Consider
glyceraldehyde
o There is only
one� asymmetric C atom in glyceraldehyde
which can be found in C-2
o Using the formula 2n,the
number of possible stereoisomers can be calculated, where n is the number of
asymmetric C-atom
o Since there is only
one asymmetric C-atom, the possible number of stereoisomer is only 2, the
dextrorotatory and levorotatory stereoisomer
o For glyceraldehyde,
using the Fischer projections, the 2 stereoisomers are shown below
Epimer
o Epimer-molecules
differ only in position of one OH group
Structural
representation of sugars
o Fisher projection:
straight chain representation
o Haworth projection:
simple ring in perspective
o Conformational
representation: chair and boat configurations
Aldohexose: Pyranoses
o Open chain
monosaccharides form cyclic hemiacetals or hemiketals when an H atom from the
hydroxyl group adds to the carbonyl group. A new single bond forms between the
Hydroxyl O and the Carbonyl C
o Anomers-isomers of
sugar differ only in the anomeric position of OH
o Alpha and beta anomer
Cyclic Structure for
Glucose
Glucose
cyclic hemiacetal formed by reaction of -CHO with -OH on C5.
Glucose
forms an intra-molecular hemiacetal, as the C1 aldehyde� &��
C5 OH react, to�� form a 6-member
pyranose ring, named after pyran.
Cyclization
of glucose produces a new asymmetric center at C1. The 2 stereoisomers are
called anomers, a & b.
Haworth
projections represent the cyclic sugars as having essentially planar rings,
with the OH at the anomeric C1:
w �α (OH below the ring)
w �β (OH above the ring).
w
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.
Aldopentoses and Ketohexoses: Furanoses
o The cyclic forms of
aldopentoses and ketohexoses both contain five membered rings that include 4
carbon atoms and one O atom. To produce the hemiacetal form of an aldopentose,
the hydroxyl H atom on the C-4 atoms adds to the carbonyl.
o Pentoses and hexoses
can cyclize as the ketone or aldehyde reacts��
with a distal OH.
Cyclic Structure for
Fructose
Cyclic
hemiacetal formed by reaction of C=O at C2 with -OH at C5.
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 on glucose to form methyl glucoside
(methyl-glucopyranose).
Disaccharides
Maltose, a cleavage product of starch (e.g., amylose), is a
disaccharide with an a(1� 4) glycosidic link between C1 - C4 OH of 2 glucoses.
It
is the a anomer (C1 O points
down).
Sucrose (common name: table sugar, also called saccharose) is a disaccharide
(glucose + fructose) with the molecular formula C12H22O11.
Its systematic name is α-D-glucopyranosyl-(1→2)-β-D-fructofuranose. It is best known for its role in human
nutrition and is formed by plants but not by higher organisms
Lactose is a disaccharide
that consists of β-D-galactose and β-D-glucose molecules bonded
through a β1-4 glycosidic linkage. Lactose makes up around 2-8% of the
solids in milk. The name comes from the Latin word for milk, plus the -ose
ending used to name sugars.
Trehalose, also known as mycose,
is an alpha-linked (disaccharide) sugar found extensively but not abundantly in
nature. It can be synthesized by fungi, plants and invertebrate animals. It is
implicated in anhydrobiosis�the ability of plants and animals to withstand
prolonged periods of desiccation.
o The sugar is thought
to form a gel phase as cells dehydrate, which prevents disruption of internal
cell organelles by effectively splinting them in position. Rehydration then
allows normal cellular activity to be resumed without the major, generally
lethal damage, that would normally follow a dehydration/rehydration cycle.
Trehalose has the added advantage of being an antioxidant.
Cellobiose is a disaccharide
subunit of cellulose composed of two glucose molecules linked in a β(1→4) bond; which can be
hydrolyzed by bacteria or cationic exchangers. Cellobiose is an indispensable
part of fabric especially cotton, jute, and other cellulosic material.
o Cellobiose is also an
essential part of all plant structures. Its strong -OH bond interactions with
neighboring cellobiose molecules contributes greatly to the plant's structural
resilience. Most mammal do not possess the enzyme needed to break down
cellobiose into useable energy but some insects such as termites possess the
needed enzyme.
Lactulose
o galactose-b-(1,4)-fructose
o a semi-synthetic
disaccharide (not naturally occurring)
o not absorbed in the
GI tract
o used either as a
laxative (Chronulac) or in the management of portal systemic encephalopathy
(Cephulac)
o metabolized in distal
ileum and colon by bacteria to lactic acid, formic acid and acetic acid (remove
ammonia)
Oligosaccharides
o Trisaccharide:
raffinose (glucose, galactose and fructose)
o Tetrasaccharide:
stachyose (2 galactoses, glucose and fructose)
o Pentasaccharide:
verbascose (3 galactoses, glucose and fructose)
o Hexasaccharide:
ajugose (4 galactoses, glucose and fructose)
o
Polysaccharides
or glycans
o homoglycans (starch, cellulose, glycogen, inulin)
o heteroglycans (gums, mucopolysaccharides)
o characteristics:
o polymers (MW from
200,000)
o White and amorphous
products (glassy)
o not sweet
o not reducing; do not
give the typical aldose or ketose reactions)
o form colloidal solutions
or suspensions
Starch
o most common storage
polysaccharide in plants
o composed of 10 � 30%
a-amylose and 70-90% amylopectin depending on the source
o the chains are of
varying length, having molecular weights from several thousands to half a
million
Amylose
o Soluble starch,
polymer of D-glucose.
o Starch-iodide
complex, deep blue.
Amylopectin
Branched, insoluble fraction of starch.
Cellulose
o Polymer of β-D-glucose
attached by b(1,4) linkages
o Yields glucose upon
complete hydrolysis
o Partial hydrolysis
yields cellobiose
o Most abundant of all
carbohydrates
o Cotton flax: 97-99%
cellulose
o Wood: ~ 50% cellulose
o Gives no color with
iodine
o Held together with
lignin in woody plant tissues
Linear
structures of cellulose and chitin (2 most abundant polysaccharides)
Glycogen
o also known as animal
starch
o stored in muscle and
liver
o present in cells as
granules (high MW)
o contains both a(1,4)
links and a(1,6) branches at every 8 to 12 glucose unit
o complete hydrolysis
yields glucose
o glycogen and iodine
gives a red-violet color
o hydrolyzed by both a
and b-amylases and by glycogen phosphorylase
Inulin
o b-(1,2) linked
fructofuranoses
o linear only; no
branching
o lower molecular
weight than starch
o colors yellow with
iodine
o hydrolysis yields
fructose
o sources include
onions, garlic, dandelions and jerusalem artichokes
o used as diagnostic
agent for the evaluation of glomerular filtration rate (renal function test)
Chitin
o Chitin is the second
most abundant carbohydrate polymer
o Present in the cell
wall of fungi and in the exoskeletons of crustaceans, insects and spiders
o Chitin is used
commercially in coatings (extends the shelf life of fruits and meats)
Dextrins
o products of the
reaction of glucose and the enzyme transglucosidase from Leuconostoc
mesenteroides
o contains a (1,4), a
(1,6) and a (1,3) linkages
o MW: 40,000; 70,000;
75,000
o used as plasma
extenders (treatment of shock)
o also used as
molecular sieves to separate proteins and other large molecules (gel filtration
chromatography)
o components of dental plaques
o produced by the
partial hydrolysis of starch along with maltose and glucose
o dextrins are often
referred to as either amylodextrins, erythrodextrins or achrodextrins
o used as mucilages
(glues)
o also used in infant
formulas (prevent the curdling of milk in baby�s stomach)
Pectins
o pectins are
heteropolysaccharides found in the pulp of fruits (citrus, apples)
o on hydrolysis pectins
yield galacturonic acid, galactose, arabinose, methanol and acetic acid
o pectins are composed
of galactans and arabans
o used as gelling
agents (to make jellies)
Gums
o widely used in the
food and pharmaceutical industry
o used as: suspending
agents, gelling agents, thickening agents, emulsifiers, foam stabilizers,
crystallization inhibitors, adhesives, binding agents
o agar, tragacanth,
karaya, carrageenan, guar gum, gum arabic (acacia), furcellaran, sodium
alginate, locust bean gum