Central molecules of biochemistry include: Nucleic Acid (DNA/RNA)
What is a carbohydrate?
The term Carbohydrate was given to a family of related compounds found in all living
organisms based on early experimental observations.
All members of this family produced water and a carbon residue when heated
Carbohydrates have four primary functions in the support of life
To provide chemical energy for warmth, movement, sight, thought,
To supply cells with stored chemical energy
To provide cells with a carbon backbone for use in the biosynthesis of other
cellular components such as amino acids
To serve as structural components required in DNA, RNA and other
We still have not answered the initial question, what is a carbohydrate?
Misinterpretation of these results lead to the belief that these compounds
where hydrated forms of carbon and thus “carbo - hydrate”
Carbohydrates are polyhydroxylated hydrocarbons. These are hydrocarbons
with many OH functional groups.
The most common carbohydrates found in biological systems belong to one of two
The names of these groups reflect the second functional group found in most
The simplest members of these two families are glyceraldehyde and
Glyceraldehyde, like many other organic molecules exists in two different 3dimensional forms known as stereoisomers. Stereoisomers, unlike structural isomers,
are identical in both chemical formula and structure. Stereoisomers differ only in the
spatial 3-D arrangement of atoms around a central atom and in most cases the central
atom is carbon
When one chiral carbon is present in a molecule, two different spatial arrangements
of atoms are possible . We call each of these arrangements a configuration.
There are two possible configurations of glyceraldehyde. — R and S
When two chiral carbons are found in the same compound, there exists the possibility
of four different stereoisomers; two sets of enantiomers
The maximum number of possible stereoisomer for a compound can be
Stereoisomers = 2n
Where n is the number of chiral carbons
In 1891 A German chemist named Emil Fischer established the absolute
configurations of 12 out of 16 different stereoisomers of C6 aldoses. He presented 2D structures of his work which represented 3-D configurations. These structures are
now called Fischer Projections.
The two different enantiomers of glyceraldehyde can be drawn as shown above
using Fischer projections
When the OH group on the last chiral carbon falls on the right the
enantiomer is called the D isomer after the Greek word for right “dexios”
When the OH group on the last chiral carbon falls on the left the
enantiomer is called the L isomer after the Latin word for left “laevus”
Optical rotation: All chiral compounds interact with light and are said to be Optically
Interestingly, when a sample of pure D-glyceraldehyde is placed into a
polarimeter, the polarized light beam is rotated to the right. When the L-isomer
is used, the light is rotated equally but in the opposite direction
Right rotation --- dextrorotatory (d) or (+)
left rotation --- levorotatory (l) or (-)
The optical rotation d or (+) does not always correlate with the D configuration
nor does the l or (-) rotation correlate with L configuration.
Do not confuse light rotation (d,l) with Fischer notation (D,L)!
It just so happens that D-glyceraldehyde does rotate light to the right (d).
Therefore, the entire notation for D-glyceraldehyde is given as:
The L-isomer is written:
Finally, all living organisms have the unique ability to distinguish left and right
molecules. In some instances they accept only the left and exclude the right or visa
versa. All living organisms use only D-sugars and exclude all L-sugars. Plants make
only D-sugars. L-sugars are tasteless and in some cases may be toxic to animals.
Life is Chiral
Properties of Monosaccharide
The word monosaccharide is derived from the Greek word “sacharon” for sugar and
mono for one. The word sugar is synonymous with sweet tasting, however different
monosaccharides have varying degrees of sweetness.
All carbohydrates are solids at room temperature and they are very soluble in
water due to the many OH groups which can form H-bonds with water
The most common biologically significant monosaccharides are usually aldoses but
ketoses are also important.
The most important aldoses include:
The most important ketoses include:
Diastereomers exist for carbohydrates with two or more chiral carbons
A requirement for diastereomerism is the monosaccharides must be of the same
family with the same number of carbons
Glucose and Galactose are both 6 - Carbon aldoses
Fructose is also a 6 - Carbon sugar, but it’s a ketose not an aldose
Epimers: are a special set of diastereomers which differ at only one chiral
carbon out of several in the entire structure
Glucose and Galactose are diastereomers but they are also epimers
Monosaccharides greater than 4 carbons in length spontaneously close to form cyclic
rings. In fact the open chain form is consider a very minor component of any given
The cyclic structure is formed by an internal chemical reaction that results in
what is called a hemiacetal for aldoses and a hemiketal for ketoses
The hemiacetal or hemiketal structure predominates in an aqueous
solution and is therefore considered the most biologically significant
The cyclic structure for all 6-C aldoses is named after the organic
structure of a pyran ring. The added suffix “ose” places the structure in
the carbohydrate family ---- pyranose
Fructose is a 6-C monosaccharide which forms a five member ring,
closed by the formation of a hemiketal. The five member ring is named
after an organic cyclic ether called “furan”
Formation of a hemiacetal or hemiketal involves two functional groups — an
alcohol and a carbonyl (see diagram above)
The second to last alcohol in the Fischer diagram is the nucleophile and
the carbonyl carbon is the target.
Since a carbonyl group is trigonal planar in its geometry, the OH group
can attack it on either side.
Attack at the carbonyl of glucose will push the newly formed OH
at position C1of glucose or C2 of fructose up or down
C1 of glucose and C2 of fructose are called the anomeric carbons because
two different configurations (anomers) can be formed at these positions
during the formation of the cyclic ring.
If the newly formed OH group at C1 of glucose or C2 of fructose is
on the same side of the ring as C6, then the anomer is called $
If the newly formed OH group at C1 of glucose or C2 of fructose is
on the opposite side of the ring as C6, then the anomer is called "
Biologically important monosaccharides include:
Ribose & deoxyribose
most important nutritionally and the most abundant in nature.
Glucose is referred to as “blood sugar” because it is the
monosaccharide which is transported by the blood to peripheral
tissues. All other consumed sugars are converted into glucose by
the liver before they are released into the blood stream
fructose is commonly found abundantly in fruits, honey and corn
syrup. Fructose is also a constituent of sucrose (table sugar).
Fructose is converted mainly into glucose by the liver before it is
used as a source of energy by other cells.
ribose and deoxyribose are C5 aldoses which are used
mainly as structural units for RNA, DNA and other
Galactose: galactose is a C6 sugar which is related to glucose. The only
difference between glucose and galactose is the absolute
configuration around chiral carbon C4. However, galactose is
toxic! Galactose must be converted into glucose before it can be
used by animals.
Both of these are important metabolic intermediates
in the generation of cellular energy. The carbon
structures are also used to build fat and proteins
Reactions of Monosaccharides
All sugars that can be oxidized by a weak oxidizing agent are called reducing sugars.
All monosaccharide members of both the ketose and aldose families are reducing
sugars and will give a positive Benedict’s test.
Benedict’s reagent is a deep blue solution containing Cu+2 ions. When
Benedict’s solution is heated with a ketose or aldose, the Cu+2 ions are reduced
to Cu+1 and precipitate as Cu2O. The copper I oxide is a brick red solid that can
easily be identified.
Phosphate esters are formed by a catalyzed chemical reaction that links an inorganic
phosphate to one of the OH groups of the monosaccharide
For glucose and fructose phosphorylation usually occurs at either or both the C1
or C6 position
Phosphorylation has two important biological functions
Confers a net negative charge on the molecule. The ionic sugar is not
allowed to escape from the cell where it is needed
The energy of the phosphoester bond contributes to the reactivity of
several intermediates in utilization of sugar for energy
Glycoside formation is a general term used for the reaction of a monosaccharide with
another alcohol. The other alcohol may be a simple alcohol like methanol or ethanol
or a complex alcohol like another monosaccharide.
The reaction of a hemiacetal (C1 glucopyranose) or hemiketal (C2
fructofuranose) with an alcohol under slightly acidic conditions leads to the
formation of the full acetal or ketal
The newly formed (C-O-C) bond is called the glycosidic linkage
Glycosidic linkages are also important in linking one of the OH groups from
one monosaccharide with the hemiacetal or hemiketal portion of another
monosaccharide thus forming a disaccharide or polysaccharide structure
Disaccharides are sugars which are made of two monosaccharides linked together
through a glycosidic bond
Disaccharides are nutritionally important sugars.
Maltose is comprised of two glucose monomers linked together by what
is called an " (1,4) glycosidic bond
is a hydrolyzed form of starch which is isolated from germinating
grain such as wheat or barley
commonly known as milk sugar because it is found lactating
Lactose is composed of galactose and glucose monomers linked
together by what is called a $ (1,4) glycosidic bond
is common table sugar isolated from mainly sugar cane and sugar
Sucrose is composed of a glucose and fructose monomer linked
together by what is called an "1, $2 glycosidic bond
Neither the glucose nor the fructose monomer in sucrose contains a
hemiacetal or hemiketal due to the glycosidic bond. Sucrose will not
react with Benedict’s solution and thus is not a reducing sugar
A polysaccharide is a substance containing many monomeric subunits of glucose
linked together in a long polymer. Polysaccharides are not very soluble in water but
they will form a colloidal dispersion when heated
is the major stored chemical energy source in plants.
Starch is a polymer of glucopyranose subunits linked together via " (1,4)
glycosidic bonds; essentially an extended form of maltose
Amylose is composed of long unbranched chains of glucose from 1000
to 2000 monomers in length
Amylopectin is composed of long chains of glucose with random
branching side chains of glucose. Amylopectins are huge molecules
containing up to 105 glucose monomers
All starches react with I2 to form a deep blue to blackish color
Glycogen: is used as a stored energy source in animals and is comparable to
starch; especially amylopectin
Glycogen is is a polymer composed of glucopyranose subunits linked
together via " (1,4) glycosidic bonds with branching side chains of
glucose. Glycogen is more highly branched than is amylopectin
Glycogen is stored mainly in the liver and muscle cells where it can be
used as a quick source of energy when needed.
is a structural polysaccharide found mainly in the cell wall of
plants. Cellulose is the single most abundant organic compound
on this earth.
Cellulose is is a linear polymer composed of glucopyranose subunits
linked together via $ (1,4) glycosidic bonds. The main structural
difference between amylose and cellulose is the linkage between the
monomers of glucose
Animals possess an enzyme, called amylase, which catalyzes the
hydrolysis of starch. However animals do not contain a catalyst which
promotes the breakdown of cellulose
Cellulose in carnivores passes through the digestive tract essentially
intact. Cellulose provides the roughage which aids in the passage of
Herbivores possess bacteria which provide the animal with digestive
enzymes that hydrolyze cellulose. In this way the released sugars can be
used by the animal as a source of energy
Inherited Diseases of Sugar Metabolism
Galactosemia is a rare hereditary disease which is marked by the inability to
efficiently convert galactose into glucose. If galactose is not remove immediately
from an galactosemic infant’s diet, significant brain damage or death may result.
Interestingly, galactose is required as a structural unit for different tissues.
Lactose Intolerance is commonly observed in many humans, especially adults. An
important enzyme need to split the disaccharide, lactose, into glucose and galactose is
missing. Lactose cannot be absorbed through the lumen of the small intestine. The
excess sugar is used by bacteria. The result is gas, bloating and diarrhea due to
Glycoproteins are found throughout the cell and in different organelles but are mainly
located at the outer surface of the plasma membrane in animal cells.
Attachment of the sugar to the protein is either through an Asn, Thr or Ser
O-linked attached to the OH group of Thr or Ser
N-linked attached to the NH2 group of Asn
Glycoproteins can have a single monosaccharide attached or a more complex
structure containing several common and modified sugar groups
Modified Monosaccharides found in glycoprotein structures include:
Glycotransferase enzymes facilitate the synthesis of the carbohydrate structure on the
Most of the construction of the sugar structure takes place in the lumen of the
ER and in the golgi apparatus. Many glycoproteins are targeted to cellular
organelles, the plasma membrane or for export.
Missing or faulty transferases have been linked to several disease states. In
some cases the missing transferases cause different benign phenotypes such as
Functions of glycosylation have been discussed in the section of amino acid
Glycosylation and Blood typing
All of the common blood types differ mainly in the carbohydrate structure
attached to the blood group antigen. The following structures have been
determined for the A, B and O blood types
Note that all blood types have the same core carbohydrate structure with only
one modification. O-blood types do not express the gene for the transferase
which places the final sugar residue. Like wise B and A type express different
transferases which place slightly different sugar residues at the end of the
A-type individuals make antibodies against B-type structures and visa versa.
O-type individuals make antibodies against both A and B. Interestingly, no
antibodies are found against O structures in all “normal” individuals. Thus Otype blood can be given to nearly all people.
Core structures are common among many different glycoproteins, not just blood-type
Lectins: Are proteins found in plants and animals which bind (specifically) with
carbohydrate structures and promote cell-cell adhesion. A lectin usually has two or
more carbohydrate binding sites. Lectins in bacteria facilitate cell adhesion to the
tissues that they infect such as the ureter. Cranberries possess anti-adhesion
compounds which prevent urinary track infections!