Cyclic Structure of Fructose

1
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

?-D-Fructose
?-D-Fructose
2
Dehydration of Carbohydrates The Molisch Test

• Carbohydrates, like most alcohols, undergo
  dehydration reactions in the presence of
  concentrated sulfuric acid. Pentoses (five
  carbon sugars) give furfural, and ketohexoses and
  aldohexoses give substituted furfurals.

3
Oxidation of Monosaccharides

• Recall that Benedicts reagent (CuSO4) can
  oxidize aldehydes with adjacent hydroxyl groups
• The blue Cu2 ions in the Benedicts 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
  Benedicts reagent
• So, both aldoses and ketoses, in open chain form,
  can be oxidized by Benedicts reagent to form
  carboxylic acids
• Sugars that can be thus oxidized are called
  reducing sugars

4
Oxidation to Aldonic Acids

• the aldehyde group of an aldose is oxidized under
  basic conditions to a carboxylate anion
• the oxidation product is called an aldonic acid
• any carbohydrate that reacts with an oxidizing
  agent to form an aldonic acid is classified as a
  reducing sugar (it reduces the oxidizing agent)

5
Oxidation to Uronic Acids

• Enzyme-catalyzed oxidation of the primary alcohol
  at C-6 of a hexose yields a uronic acid
• enzyme-catalyzed oxidation of D-glucose, for
  example, yields D-glucuronic acid

6
D-Glucuronic Acid

• D-glucuronic acid is widely distributed in the
  plant and animal world
• in humans, it is an important component of the
  acidic polysaccharides of connective tissues
• it is used by the body to detoxify foreign
  phenols and alcohols in the liver, these
  compounds are converted to glycosides of
  glucuronic acid and excreted in the urine

7
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)

8
Reduction to Alditols

• sorbitol is found in the plant world in many
  berries and in cherries, plums, pears, apples,
  seaweed, and algae
• it is about 60 percent as sweet as sucrose (table
  sugar) and is used in the manufacture of candies
  and as a sugar substitute for diabetics
• these three alditols are also common in the
  biological world

9
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

?-D-Glucose Methanol
Methyl-?-D-glucoside
10
Formation of Glycosides

• a cyclic acetal derived from a monosaccharide is
  called a glycoside
• the bond from the anomeric carbon to the -OR
  group is called a glycosidic bond
• mutarotation is not possible in a glycoside
  because an acetal, unlike a hemiacetal, is not in
  equilibrium with the open-chain
  carbonyl-containing compound
• glycosides are stable in water and aqueous base,
  but like other acetals, are hydrolyzed in aqueous
  acid to an alcohol and a monosaccharide
• glycosides are named by listing the alkyl or aryl
  group bonded to oxygen followed by the name of
  the carbohydrate in which the ending -e is
  replaced by -ide

11
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

12
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

13
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 cows
  milk)

• Because the glucose is a hemiacetal, it can
  undergo mutorotation, and so lactose is a
  reducing sugar

14
Hydrolysis of Lactose

• Some people dont produce enough lactase, the
  enzyme that hydrolyzes lactose, and so cant
  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

15
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

16
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)

17
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
• The net reaction is
• C6H12O6 ? 2C2H5OH 2CO2

18
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

19
Plant Starch (Amylose and Amylopectin)

• Starch contains a mixture of amylose and
  amylopectin
• Amylose is an unbranched polymer (forms ?-helix)
  of D-glucose molecules linked by ?-1,4-glycosidic
  bonds
• Amylopectin is like amylose, but has extensive
  branching, with the branches using
  ?-1,6-glycosidic bonds

20
Glycogen and Cellulose

• Glycogen (animal starch) is like amylopectin,
  except its 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 dont have the enzyme to break down
  cellulose
• - some animals have microorganisms that do have
  the enzyme

21
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