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CARBOHYDRATES
STRUCTURES AND FUNCTIONS
Erwin D. Abueva, M.D.
August 2009
INTRODUCTION
Carbohydrates are aldehyde or ketone derivatives
of the polyhydric alcohols and their derivatives
 In animal cells, this biomolecule serves as an
important source of energy for vital activities in the
form of glucose and glycogen
 Some carbohydrates have specific functions

Ribose in the nucleoprotein
 Galactose in certain lipids
 Lactose in milk
 Cellulose in fibers

CLASSIFICATION OF CARBOHYDRATES
 Monosaccharides
 Disaccharides
 Oligosaccharides
 Polysaccharides
MONOSACCHARIDES
the simplest unit of carbohydrate molecule
which cannot be further hydrolyzed
 consist of only one sugar and are usually
colorless, water-soluble, crystalline solids
 are the building blocks of disaccharides like
sucrose (table sugar) and polysaccharides
(cellulose and starch)
 further classified as either aldoses or ketoses
 may be subdivided according to the number of
carbon atoms they possess

PROPERTIES OF MONOSACCHARIDES
 Asymmetry
of Carbohydrates
 When a compound has more than
one asymmetric center, each center
is viewed in turn, and a
representation is constructed
showing the relative positions of
the substituents of the various
asymmetric centers.
DEFINITION OF TERMS
Stereoisomers: compounds that have the same
structural formula but differ in spatial configuration
(glucose, galactose, and fructose are isomers)
 Enantiomers: isomers that are
nonsuperimposable mirror images of each other
(D-sugar and L-sugar)
 Epimers: two sugars that differ in configuration
around 1 specific carbon (galactose and glucose
are epimers on carbon 4)
 Anomers: isomeric forms of sugar that differ in
configuration around carbonyl carbon (C1 in
aldose and C2 in ketose)

ISOMERS OR ENANTIOMERS?
GLUCOSE
MANNOSE
ISOMERS OR ANOMERS?
GLUCOSE
FRUCTOSE
ENANTIOMERS OR EPIMERS?
GLUCOSE
GALACTOSE
MONOSACCHARIDES
Number of
Carbons
Generic Name
Examples
3
Triose
Glyceraldehyde,
Dihydroxyacetone
4
Tetrose
Erythrose
5
Pentose
Ribose, Ribulose,
Xylulose
6
Hexose
Glucose, Galactose,
Mannose, Fructose
7
Heptose
Sedoheptulose
9
Nonose
Neuraminic acid or Sialic
acid
MONOSACCHARIDES

HEXOSES






Monosaccharide with 6 carbon atoms
Classified by functional group, with aldohexoses
having an aldehyde at position 1, and ketohexoses
having a ketone at position 2
Glucose is found in fruit juices, in hydrolysis of starch,
cane sugar, maltose, and lactose
Mannose is found in plant mannans and gums
Fructose are found in fruit juices. It can be converted
to glucose in the liver and so used in the body
Galactose is used in the synthesis of lactose in the
mammary gland to make milk.
MONOSACCHARIDES
GLUCOSE
Principal sugar in
blood
 Major metabolic fuel
 Dextrose or grape
sugar
 Found in fruit juices
 White crystalline solid
 Soluble in water

MONOSACCHARIDES
GALACTOSE
Less soluble and
less sweet than
glucose
 In plants, constituent
of pectin

MONOSACCHARIDES
FRUCTOSE
Levulose or fruit
sugar
 Sweetest of all
sugars

MONOSACCHARIDES

Cyclization of monosaccharides
Monosaccharides exist in the open chain (acyclic) form
and in ring form
 Formation of a hemiacetal (or hemiketal) ring results in
the creation of anomeric carbon at C1 of an aldose and
C2 of a ketose
 the anomers are designated as alpha- and beta- Dglucose.
 These anomeric forms are important in enzymatic
reactions.

MONOSACCHARIDES
Alpha-D-glucose
Beta-D-glucose
MONOSACCHARIDES

The vast majority of the sugars in humans are Dsugars





D-glucose: dextrose or blood sugar; 6-member ring
(stable)
D-galactose: stereoisomer of glucose: converted to
glucose during metabolism
D-fructose: readily converted to glucose in metabolism
by isomerization; forms a 5-member ring rather than a
6-member ring
D-ribose: constituent of RNA, ATP, and coenzymes
2-deoxyribose-D-ribose: missing OH at #2 position of
ribose (substituted by H); found in DNA; forms a 5member ring
MONOSACCHARIDES
Representation of sugar conformation:
 Fischer projection: the carbon is written
vertically, with C1 at the top, and the hydroxyl
and hydrogen substituents written to the sides
 Haworth projection: Carbon is written farthest
to the right, the plane of the ring is flat on the
paper, and the hydroxyl and hydrogen groups
are project either “above” or “below” the plane
drawn
MONOSACCHARIDES
Fischer projection
Haworth projection
MONOSACCHARIDES

PENTOSES
 Has 5 carbon atoms
 Either has an aldehyde functional group in
position 1 (aldopentoses), or a ketone
functional group in position 2 (ketopentoses)
 Ribose is an important constituent of RNA,
deoxyribose in DNA, in enzymes and in high
energy molecules like ATP
 Arabinose and Xylose, which are found in
wood gums, are important constituents of
glycoproteins
MONOSACCHARIDES - PENTOSES
D-Ribose
D-Arabinose
D-Xylose
D-Lyxose
MONOSACCHARIDES
Oxidation-Reduction Reactions

Oxidation of the –CH2-OH group at carbon 6 produces a
–uronic acid
 Example: (1) glucose to glucoronic acid
(2) galactose to galacturonic acid

Reduction of the carbonyl carbon (aldehyde or keto
group) produces a new alcohol group. Such compounds
are called polyols.
 Example: (1) glucose is reduced to sorbitol
(2) fructose is reduced to mannitol
(3) ribose is reduce to deoxyribose
MONOSACCHARIDES
MONOSACCHARIDES

Formation of Derivatives

A. Glycoside Formation
Any monosaccharide unit can react with a
non-carbohydrate unit (aglycone) such as
alcohol, glycerol, a sterol or a phenol to give
a glycoside
 Glycosides are nonreducing sugars and will
not react with Tollen’s, Benedict’s nor
Fehling’s reagents because the linkage with
the noncarbohydrate group is at C1

MONOSACCHARIDES

Formation of Derivatives

B. Exhaustive Methylation

Reaction with alcohols affects only the
anomeric carbon. The other hydroxyl groups
can also be methylated using dimethyl
sulfate
MONOSACCHARIDES

Formation of Derivatives

C. Formation of Amino Sugars
Most monosaccharides can acquire an amino
group at C-2. In addition, these amino
groups can be acetylated to form N-acetyl
derivatives.
 Amino sugars are frequently found in
glycoproteins (in cell membranes) and
proteoglycans

MONOSACCHARIDES

Formation of Derivatives

D. Formation of Sugar Sulfates
Some polysaccharides contain sulfates
esterified at C-2, C-4, and for C-6. These
polysaccharides are found mostly in
proteoglycans of the extracellular matrix.
 Ex. Chondroitin sulfates, keratan sulaftes,
dermatan sulfates
