Cyclic Structure of Fructose
• As a ketohexose, fructose forms a 5-membered ring when
the hydroxyl on C-5 reacts with the carbonly on C-2
CH2OH
C O
HO C H
CH2OH
O
H C OH
H C OH
CH2OH
CH2OH
O
OH
OH
CH2OH
OH
OH
OH
OH
CH2OH
D-Fructose
-D-Fructose
-D-Fructose
Oxidation of Monosaccharides
• Recall from Ch.15 that Benedict’s reagent (CuSO4) can oxidize
aldehydes with adjacent hydroxyl groups
• The blue Cu2+ ions in the Benedict’s reagent are reduced to form a
brick-red precipitate, Cu2O
• Normally, ketones are not oxidized, however ketones with an
adjacent hydroxyl group can rearrange to the aldehyde during
reaction with Benedict’s reagent
• So, both aldoses and ketoses, in open chain form, can be
oxidized by Benedict’s reagent to form carboxylic acids
• Sugars that can be thus oxidized are called reducing sugars
O
O
H
C
C
H C OH
HO C H
H C OH
OH
H C OH
+ Cu2+
HO C H
H C OH
H C OH
H C OH
CH2OH
CH2OH
D-Glucose
D-Gluconic acid
+ Cu2O(s)
Reduction of Monosaccharides
• Reduction of the carbonyl group of a monosaccharide (in
open-chain form) produces a sugar alcohol, or alditol
• D-Glucose is reduced to D-glucitol (also called D-sorbitol)
using hydrogenation (H2 and a metal catalyst)
Formation of glycosides
• Recall that an alcohol can react with a hemiacetal to form
an acetal (a di-ether)
• When an alcohol reacts with a cyclic hemiacetal of a
monosaccharide the cyclic acetal product is called a
glycoside
• The new ether bond is called a glycosidic bond
• Monosaccharides are linked together by glycosidic bonds
to form disaccharides and polysaccharides
• Alkyl glycosides can not undergo mutarotation, and so are
not reducing sugars
CH2OH
CH2OH
O
O
OH
+ HOCH3
OH
glycosidic bond
O CH3
OH
OH
OH
OH
OH
-D-Glucose
Methanol
Methyl--D-glucoside
Disaccharides
• A disaccharide is formed when a hydroxyl group on one
monosaccharide reacts with the anomeric carbon of
another monosaccharide to form a glycosidic bond
• Each disaccharide has a specific glycosidic linkage
(depending on which hydroxyl reacts with which anomer)
• The three most common disaccharides are maltose, lactose
and sucrose
• When hydrolyzed using acid or an enzyme, the following
monosaccharides are produced:
Acid
Maltose + H 2O
or
enzyme
D-Glucose + D-Glucose
Acid
Lactose + H 2O
or
enzyme
D-Glucose + D-Galactose
Acid
Sucrose + H 2O
or
enzyme
D-Glucose + D-Fructose
Maltose
• Maltose (malt sugar or corn sugar) consists of two glucose
molecules linked by an -1,4-glycosidic bond
• It comes from partial hydrolysis of starch by the enzyme
amylase, which is in saliva and also in grains (like barley)
• Maltose can be
fermented by yeast to
produce ethanol
• Maltose is also used
in cereals, candies and
malted milk
• Because one of the
glucose molecules is a
hemiacetal, it can
undergo mutorotation,
and so maltose is a
reducing sugar
Lactose
• Lactose (milk sugar) consists of one glucose molecule and
one galactose molecule linked by a -1,4 glycosidic bond
• It comes from milk products (about 4-5% of cow’s milk)
• Because the
glucose is a
hemiacetal, it can
undergo
mutorotation, and
so lactose is a
reducing sugar
Hydrolysis of Lactose
• Some people don’t produce enough lactase, the enzyme
that hydrolyzes lactose, and so can’t digest lactose
• Many adults become lactose intolerant, and develop
abdominal cramps, nausea and diarrhea
• Lactase can be added to milk products (or taken as a
supplement) to combat this problem
Sucrose
• Sucrose (table sugar) consists of one glucose molecule and
one fructose molecule linked by an ,-1,2-glycosidic bond
• Sucrose is the most abundant disaccharide and is
commercially produced from sugar cane and sugar beets
• Because the glycosidic bond in sucrose involves both
anomeric carbons, neither monosaccharide can undergo
mutorotation, and so sucrose is not a reducing sugar
CH2 OH
O
CH2 OH
O
1
OH
OH
OH
1
OH
OH
OH
OH
alpha-D-Glucose
O
+
CH2 OHO
OH
OH
OH 2
CH2 OH O
OH 2
CH2 OH
beta-D-Fructose
OH
Sucrose
CH2 OH
alpha,beta-1,2-glycosidic bond
Hydrolysis of Sucrose
• Sucrose is hydrolyzed by the enzyme sucrase, which is
secreted in the small intestine
• The glucose and fructose can then be absorbed into the
bloodstream (disaccharides are too large to be absorbed)
Fermentation
• A fermentation is defined as an energy-yielding metabolic
pathway with no net change in the oxidation state of
products as compared to substrates
• Yeast can ferment glucose, fructose, maltose and sucrose
• Ultimately, glucose is converted to pyruvate through
glycolosis, and the pyruvate is then converted to CO2 and
ethanol by a two-step enzymatic process
H+
O
NADH + H+
CO2
NAD+
O
O
OH
O
Pyruvate
pyruvate
decarboxylase
H
Acetaldehyde
• The net reaction is:
C6H12O6  2C2H5OH + 2CO2
alcohol
dehydrogenase
Ethanol
Polysaccharides
• A polysaccharide is a polymer consisting of hundreds to
thousands of monosaccharides joined together by
glycosidic linkages
• Three biologically important polysaccharides are starch,
glycogen and cellulose
- all three are polymers of D-glucose, but they differ in the
type of glycosidic bond and/or the amount of branching
• Starch and glycogen are used for storage of carbohydrates
- starch is found in plants and glycogen in animals
- the polymers take up less room than would the individual
glucose molecules, so are more efficient for storage
• Cellulose is a structural material used in formation of cell
walls in plants
Plant Starch (Amylose and Amylopectin)
• Starch contains a mixture of amylose and amylopectin
• Amylose is an unbranched polymer (forms -helix) of Dglucose molecules linked by -1,4-glycosidic bonds
• Amylopectin is like amylose, but has extensive branching,
with the branches using -1,6-glycosidic bonds
Glycogen and Cellulose
• Glycogen (animal starch) is like amylopectin, except it’s even
more highly branched
- animals store glycogen in the liver (about a one-day supply in
humans) and use it to maintain fairly constant blood sugar
levels between meals
• Cellulose is an unbranched polymer of D-glucose molecules
linked by -1,4-glycosidic bonds
- cellulose forms -sheets of parallel strands held together by
hydrogen bonding
- we don’t have the enzyme to break down cellulose
- some animals have microorganisms that do have the enzyme
Iodine Test for Starch
• The presence of starch can easily be identified using iodine (I2)
• Rows of iodine atoms form in the core of the -helix of
amylose, forming a dark blue complex
• Because amylopectin, glycogen and cellulose do not form helices, they do not complex well with iodine, so do not show
the blue color (they show a purple or brown color)
• Monosaccharides do not interact with the iodine, so no color is
produced