Carbohydrates saccharides are the single most abundant class of biomolecules in nature'

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• Carbohydrates (saccharides) are the single most
  abundant class of biomolecules in nature.

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Sugars and Sweetness
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Aspartame (Equal) sweetness 180
Sucralose (Splenda) sweetness 600
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Sweet Biotechnology
New
Old
corn (60 starch)
extract starch, incubate with a-amylase
glucoamylase
sugarcane, beets (12-20 of dry wt. is sucrose)
corn syrup (mostly glucose)
extract, evaporate H2O, refine
incubate with glucose isomerase, purify
high fructose corn syrup (55 fructose, 40
glucose)
table sugar (sucrose)
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• Carbohydrates can be classified as
• Monosaccharides
• (glucose, fructose, ribose) sugars, sweet
  tasting
• have important biological roles themselves and
  serve as building blocks for oligosaccharides and
  polysaccharides
• Oligosaccharides smaller than a polysaccharide
  but larger than a monosaccharide (2-20
  monosaccharide residues)
• Disaccharide (lactose, sucrose) sugars, sweet
  tasting
• Polysaccharides hundreds to thousands of
  monosaccharide units bonded together.
• (starch, cellulose, glycogen, chitin).

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• IUPAC names are not commonly used for
  carbohydrates.
• Most common names of carbohydrates end in ose.
• Glucose, fructose, lactose, sucrose, cellulose
• Most polysaccharides do not end in ose
• Starch, chitin, glycogen
• Dietary carbohydrates are classified as
• Simple monosaccharides and disaccharides
• Complex polysaccharides

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• Carbohydrates are either polyhydroxyaldehydes or
  polyhydroxyketones, containing two or more
  hydroxyl groups and an aldehyde or ketone
  functional group.
• Monosaccharides have one of the following
  structures

• In the aldoses, the carbonyl is C1 in the
  ketoses, the carbonyl is C2.

• Monosaccharide names combine two kinds of
  prefixes with ose.

• Example ribose, 5-carbon sugar contains a
  aldehyde group is also called a aldopentose.

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Stereoisomerism of Aldoses

• All D-aldoses have the hydroxyl farthest from the
  carbonyl group pointing to the right (blue
  hydroxyl).

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Stereoisomerism of Ketoses

• All D-ketoses have the hydroxyl group farthest
  from the carbonyl carbon pointed to the right
  (blue hydroxyl)

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Facts About Some Important Monosaccharides

• D-Glucose is the most abundant monosaccharide in
  nature.
• Also called blood sugar or dextrose.
• Found in combined forms starch, cellulose,
  glycogen, chitin, lactose, and sucrose.
• D-Fructose (levulose) is bonded to glucose to
  form sucrose, the sugar in fruits and table
  sugar.
• D-Galactose is bonded to glucose to form lactose,
  the sugar in milk.

• Each D-aldose and D-ketose theoretically has an
  L-enantiomer, however, these are almost never
  found in nature.
• Degradative and synthetic enzymes almost always
  recognize only the D-enantiomers of sugar
  molecules (principle of chiral recognition).

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Facts About Some Important Monosaccharides

• D-ribose, D-xylose, and 2-Deoxyribose are
  important pentoses.
• D-ribose Component of RNA
• 2-Deoxyribose Component of DNA
• In 2-deoxyribose, the OH at C2 is replaced by
  H. It is an example of a deoxysaccharide or
  deoxysugar.

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• The OH group of an alcohol and the carbonyl
  group of an aldehyde or ketone can undergo an
  acid catalyzed reaction to form a hemiacetal

• Sugar molecules contain both an alcoholic
  functional group and an aldehyde or ketone
  functional group and form cyclic hemiacetals.
• In the case of D-glucose, the ring formed is
  called a pyranose ring.

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• The formation of the hemiacetal structure
  converts the carbonyl carbon into a new
  tetrahedral stereocenter.

• This carbon is called the hemiacetal carbon. It
  can be recognized because it has two different
  oxygen groups attached
• -OR an ether linkage
• -OH an alcohol
• Two stereoisomers are possible at the hemiacetal
  carbon called ? and ? configurations. D-glucose
  consists of an equilibrium mixture of these two
  hemiacetals 36 ? and 64 ?.
• These stereoisomers are diastereomers of each
  other and are called anomers.

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• The ? anomer of D-glucose has a cis relationship
  between the OH at C1 and the CH2OH at C5.
• The ? anomer of D-glucose has a trans
  relationship between the OH at C1 and the CH2OH
  at C5.
• The acyclic form of glucose (open-chain
  structure) contributes less than 0.2 of the
  equilibrium mixture and is not stable compared to
  the hemiacetal forms of glucose (? 36 ? 64).

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• D-Fructose and other ketohexoses form 5-membered
  cyclic hemiacetals called furanoses.

• Note the trans relationship in the ? anomer and
  the cis relationship in the ? anomer between the
  OH and CH2OH groups.

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• These 5-membered and 6-membered cyclic hemiacetal
  structures are called Haworth structures or
  Haworth projections.
• The bold bonds at the bottom of each structure
  are meant to appear closer to the viewer (above
  the plane of the screen) than the C-O or O at the
  top of the ring which is meant to appear farther
  from the viewer (below the plane of the screen).
• Haworth projections are not always drawn with
  bold bonds.

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• Monosaccharides are crystalline solids at room
  temperature and are very soluble in water where
  they can form highly viscous solutions.
• Monosaccharides are slightly soluble in alcohols
  (methanol, ethanol) and are insoluble in less
  polar solvents (ethers, hydrocarbons).
• Almost all monosaccharides taste sweet.

• A solution of a monosaccharide consists of the ?
  anomer, ? anomer, and acyclic structure. These
  structures interconvert rapidly and form an
  equilibrium mixture.
• Usually, only a single anomer is drawn which
  represents both, when drawing the structure of a
  mono- or disaccharide in a chemical equation.

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The Oxidation of the Aldehyde Group

• The aldehyde group of an aldose can be oxidized
  by Cu2 complexed with citrate ion in alkaline
  solution (Benedicts solution) to a carboxylic
  acid.
• This oxidation occurs with the acyclic (open
  chain) form of the aldose and not with the
  hemiacetal form.

• The COOH will actually be ionized, -COO-, in
  alkaline solution.

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• Aldoses and ketoses are called reducing sugars
  because they are able to reduce Cu2 in
  Benedicts reagent.
• Benedicts reagent is used to test for the
  presence of glucose in urine. If glucose is
  present, the blue Cu2 color of the reagent is
  replaced by a red Cu2O precipitate.
• The small amount (0.2) of acyclic form removed
  by oxidation is quickly replaced via the
  ?-acyclic-? equilibrium according to LeChatliers
  principle.
• The glucose oxidase test is specific for
  D-glucose, not all aldoses and ketoses.
• D-glucose is oxidized by O2 to D-gluconic acid
  and hydrogen peroxide by the enzyme glucose
  oxidase.
• The H2O2 produced, oxidizes added o-toluidine to
  give colored products.

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Acetal Formation The Production of Glycosides

• Monosaccharides can be converted into glycosides
  by the acid-catalyzed reaction of the hemiacetal
  OH group with an alcohol.
• Reaction occurs preferentially at the hemiacetal
  OH because it is the most reactive hydroxyl
  present.

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• Acetals of carbohydrates are called glycosides,
  and the C-O-C bond at the anomeric carbon is
  called a glycosidic linkage.
• In the acid-catalyzed laboratory reaction, a
  mixture of both ? and ? glycosidic linkages is
  formed. In living systems one of the other
  glycosidic linkage is enzymatically formed, never
  both.
• Glycosides are not reducing sugars because they
  are not in equilibrium with an open chain form.
  For the same reason, they do not undergo
  mutarotation.
• In the presence of water and an acid catalyst or
  the appropriate enzyme, glycosides can be
  hydrolyzed to the saccharide and alcohol.
• Polysaccharides consist of monosaccharides linked
  together by glycosidic bonds. The presence of ?
  versus ? glycosidic linkages in polysaccharides
  is important to their structure and function.

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Other Derivatives of Monosaccharides

• Phosphate Esters
• ATP is used to store and release energy

• 2-Deoxyribose-5-phosphate is a building block in
  DNA.

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• Disaccharides are two monosaccharides joined by a
  glycosidic linkage.
• The glycosidic linkage in a disaccharide results
  from a dehydration reaction between the anomeric
  hydroxyl group of one monosaccharide and one of
  the alcoholic groups of the other monosaccharide.

• To characterize a disaccharide

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Maltose

• Also known as corn sugar, or malt sugar.
• Produced by the partial hydrolysis of starch by
  the enzyme amylase.
• In maltose, two D-glucose molecules are linked by
  an ?(1?4) linkage.

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• Because maltose has a free hemiacetal OH, it is
  a reducing sugar.
• The ? in ?-D-maltose refers to the ?-anomer of
  maltose, not the configuration of the anomeric
  hydroxyl in the glycosidic bond.

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Cellobiose

• Cellobiose is a product of the partial hydrolysis
  of cellulose.
• Cellobiose is identical to maltose except that
  the two glucose molecules are connected by a
  ?-anomeric linkage rather than an ?-anomeric
  linkage.
• Cellobiose is a reducing sugar since one of the
  sugars has a free hemiacetal.

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Lactose

• Lactose, or milk sugar, constitutes about 4-8 of
  mammalian milk.
• Lactose contains galactose and glucose joined by
  a ?(1?4) glycosidic linkage.

• Lactose is a reducing sugar since the glucose has
  a free hemiacetal.
• Lactose intolerance and galactosemia are two
  significant hereditary diseases involving lactose
  consumption (see Box 18.2).

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What about trans fats?
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Sucrose

• The most abundant disaccharide in nature.
• Sucrose is formed by acetal formation between the
  hemiacetal OH groups of both ?-D-glucose and
  ?-D-fructose. The linkage is ?,?(1?2)

• Because both hemiacetal OH groups are involved
  in the glycosidic linkage, sucrose is not a
  reducing sugar.

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• Polysaccharides contain large numbers (hundreds
  or thousands) of monosaccharides residues (repeat
  units) bonded together.
• Starch and glycogen serve as storage forms for
  D-glucose.
• Cellulose and chitin serve as biological
  structural materials.
• Polysaccharides differ in the following ways

Example Amylose
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Starch and Glycogen

• Starch and glycogen are storage polysaccharides
  for D-glucose.
• Starch Plants
• Amylose 10-30 of starch, unbranched, ?(1?4)
  linkages

• Amylopectin 70-90 of starch, branched, ?(1?4)
  linkages and ?(1?6) branches, branches have
  branches, MW 1,000,000 or more.

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• Glycogen Animals
• Glycogen is similar to amylopectin but more
  highly branched.
• Amylose, amylopectin, and glycogen all contain a
  single free hemiacetal hydroxyl and hundreds or
  thousands of hemiacetal glycosidic linkages.
• The percentage of free hemiacetal OH groups is
  so small that none of these molecules give a
  positive test with Benedicts solution. They are
  all non-reducing sugars.

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Starch and Glycogen in Digestion and Metabolism

• Starch is the principle carbohydrate in our diet
  and is digested to D-glucose
• Amylase hydrolyzes amylose and parts of
  amylopectin to maltose in the digestive tract.
• Maltase cleaves maltose to D-glucose.
• The result of amylase digestion of amylopectin is
  dextrin which contains the remaining ?(1?6)
  linkages. Dextrin is hydrolyzed by dextrinase to
  D-glucose.
• Some of the D-glucose from starch is used
  immediately for energy (glycolysis).
• The excess D-glucose is stored in the liver and
  skeletal muscles as glycogen (glycogenesis).
• Any D-glucose still in excess is converted to fat
  and deposited in the fat tissues.

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• When required for energy or biosynthesis,
  D-glucose is released from a glycogen molecule
  (glycogenolysis).
• Removal of D-glucose from glycogen can be very
  rapid because it can be removed from all of the
  tips of the glycogen branches simultaneously

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Cellulose

• Cellulose is a structural polysaccharide, and is
  the most abundant organic compound in the
  biosphere (50 of all organic carbon).
• Cellulose is a linear polymer of D-glucose
  monomers connected with ?(1?4) linkages.

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• The properties of cellulose are much different
  from those of ?-amylose or starch.
• Starch swells and forms a colloidal suspension
  when placed in water.
• Cellulose (wood) is insoluble and retains its
  shape and most of its physical strength when
  placed in water.
• Cellulose molecules exist in an extended chain
  conformation and pack side to side to form
  ribbons. The ribbons pack side to side and on top
  of each other to form fibers.
• All of the cellulose molecules are held together
  in a fiber by intermolecular hydrogen bonding.

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• Humans cannot digest cellulose because we lack
  the enzyme cellulase which cleaves ?(1?4)
  linkages. Even so, cellulose in the diet has some
  beneficial effects.
• The main nutritional carbohydrate for grazing
  animals is cellulose (grass and other plants).
  These animals cannot digest cellulose directly,
  but have symbiotic microorganisms in their
  digestive tracts that secrete cellulase into the
  animals digestive tracts.
• Starch can be differentiated from cellulose by
  adding a few drops of I2 solution. I2 forms a
  dark blue solution in the presence of starch
  which does not form with cellulose.

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Cell Recognition Glycolipids and Glycoproteins

• Cells interact with and recognize other cells
  through a process called cell recognition. Cell
  recognition is accomplished through saccharides
  attached to the cell surfaces.
• These saccharides, usually oligosaccharides, are
  present as glycolipids and glycoproteins. The
  lipid or protein part of the molecule is
  integrated into the cell-membrane structure with
  the saccharide part located on the external
  membrane surface.

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• The ABO blood group types are A, B, AB, and O.
  These result from three types of antigens
  (containing saccharide molecules) A, B, and O.
• There are only two types of antibodies anti-A
  and anti-B. There is no anti-O.

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Plant and animal life is possible only because of
photosynthesis. Photosynthesis traps the energy
of sunlight and uses it to transform carbon
dioxide and water into organic compounds and
molecular oxygen.
About 20 of photosynthesis on Earth takes place
in land plants and 80 takes place in the oceans.

• The energy stored in the carbohydrates created
  through photosynthesis ultimately supplies the
  energy for all animal life on earth.
• While utilizing carbohydrates for energy, animals
  recombine the carbohydrates with oxygen and
  return carbon dioxide and water to the
  environment, thus completing a cycle.
• Plants and animals are thus interdependent
  through a carbon cycle and an oxygen cycle.