Download Part II Biochemistry

Part II
Biochemistry
The branch of chemistry that deals with the different molecules,
their structure, composition, and chemical processes in the living cell
IUG, Fall 2013
Dr. Tarek Zaida
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Carbohydrates
 Carbohydrates occur in all plants and animals and are essential
to life.
 Through photosynthesis, plants convert atmospheric carbon dioxide to
carbohydrates, mainly cellulose, starch, and sugars.
 Cellulose is the building block of rigid cell walls and woody tissues in
plants, whereas starch is the chief storage form of carbohydrates for
later use as a food or energy source.
 Some plants (cane and sugar beets) produce sucrose, ordinary table
sugar.
 Another sugar, glucose, is an essential component of blood.
 Two other sugars, ribose and 2-deoxyribose, are components of the
genetic materials RNA and DNA.
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• The word carbohydrate arose because molecular formulas of
these compounds can be expressed as hydrates of carbon.
• Glucose, for example, has the molecular formula C6H12O6,
which might be written as C6(H2O)6.
• Carbohydrates are now defined as:
1. polyhydroxyaldehydes,
2. polyhydroxyketones, or
3. substances that give such compounds on hydrolysis.
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• Carbohydrates have two functional groups: the
hydroxyl group and the carbonyl group.
• Carbohydrates are usually classified according to
their structure as:
1. monosaccharides,
2. Oligosaccharides,
3. polysaccharides.
• The term
comes from Latin
• (saccharum, sugar) and refers to the sweet taste
of some simple carbohydrates.
• The three classes of carbohydrates are related to
each other through hydrolysis
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For example, hydrolysis of starch, a polysaccharide, gives first maltose and
then glucose.
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• Monosaccharides (or simple sugars, as they
are sometimes called) are carbohydrates that
cannot be hydrolyzed to simpler compounds.
• Polysaccharides contain many
monosaccharide units—sometimes hundreds
or even thousands.
• Usually, but not always, the units are identical.
• Two of the most important polysaccharides,
and
, contain linked units of
the same monosaccharide, glucose.
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• Oligosaccharides (from the Greek oligos, few)
contain at least
and generally no more than a
few linked monosaccharide units.
• They may be called disaccharides, trisaccharides,
and so on, depending on the number of units,
which may be the same or different.
, for example, is a disaccharide made of
two glucose units.
, another disaccharide, is made of two
different monosaccharide units:
and
.
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• Monosaccharides are classified according to the
number of carbon atoms present
• (
,
,
,
, and so on) and
according to whether the carbonyl group is present as
an aldehyde (
) or as a ketone (
).
• There are only two trioses:
and
• Each has two hydroxyl groups, attached to different
carbon atoms, and one carbonyl group.
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• Glyceraldehyde is the simplest aldose
Dihydroxyacetone is the simplest ketose.
• Each is related to glycerol in that each has a
carbonyl group in place of one of the hydroxyl
groups.
• Other aldoses or ketoses can be derived from
glyceraldehyde or dihydroxyacetone by adding
carbon atoms, each with a hydroxyl group.
• In aldoses, the chain is numbered from the
aldehyde carbon. In most ketoses, the carbonyl
group is located
at C-2.
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Naming Monosaccharides
• Carbohydrate nomenclature is unique
we do not name monosaccharides using the
IUPAC rules.
• Monosaccharide names end in ―ose
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Naming Monosaccharides
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Naming Monosaccharides
Examples:
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Name each of the following monosaccharides as an
aldose or ketose & according to its number of C atoms.
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• 1. Plants catch up CO2 from air & H2O
from soil
• 6CO2 + 6H2O
enzymes,
chlorophyll
sun light
C6H12O6 + 6O2
glucose
• The above reaction is called
photosynthesis (formation of
carbohydrates from CO2, H2O and sun
light)
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2. The carbohydrates produced during
photosynthesis are monosaccharides.
• Plants build disaccharides from
monosaccharides:
C6H12O6 + C6H12O6
C12H22O11 + H2O
glucose
glucose
Disaccharide
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• 3. Plants as well as animals can combine many
molecules of monosaccharides into large
polysaccharide molecules:
nC6 H12O6
polymerization
(C6H10O5)n + nH2O
Polysaccharides in plants are:
- Cellulose in stalk and stem
- Starch in roots and seeds
- Mono & disaccharides are found in fruits
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Oxygen – Carbon dioxide cycle
• 1. Plants pick up CO2 from air, water from soil
to build carbohydrates (monosaccharides).
• 2. Animals can not do that, so they have to
rely on plants to get their carbohydrates
Krebs cycle
• CO6H12O6 + 6O2
6CO2 + 6H2O + energy
• The above reaction is the reverse of photosynthesis.
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• During Photosynthesis:
 Energy from the sun needed for aerobic
reactions.
During catabolism of carbohydrates in animals
same amounts of energy is liberated from
oxidation of food.
So energy from metabolism in animals comes
from the sun through plants that store solar
energy in carbohydrates.
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Stereoisomerism
Stereoisomers:
• Are compounds having the same molecular
formula but a different structural formula.
• Stereoisomers have structures that are mirror
image of one another
• If you put your left hand on top of your right
hand, you will see that they are not
superimposable
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• Some molecules are superimposable with
their mirror images and others are not:
H
H-C-H
H
methane
H
H-C-H
H
Mirror image
Both methane and mirror image are said to be
superimposable (
)
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• In general any object with a plane of
symmetry is achiral
• Any object or molecule does not have a plane
of symmetry it is chiral
• If there are 4 different atoms attached to a
central carbon atom, it is said to be chiral
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Stereoisomers in Carbohydrates
• Carbohydrates are chiral molecules since they
have carbon atoms carrying four different
groups.
• The simplest three carbon sugar is
glyceraldehyde.
• This sugar exists as a pair of enantiomers.
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• Two forms of glyceraldehyde (D and L) have
the same physical properties except they
behave differently in the way they rotate
polarized light and the way they are affected
by catalysts.
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• Remember: Compounds with n chiral carbon atoms has
a maximum of 2n possible stereoisomers and half that
many pairs of enantiomers (mirror images).
•This aldotetrosose, has 2 chiral carbon atoms and a total
of 22 = 4 possible stereoisomers (2 pairs of
enantiomers).
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The D and L Forms of Sugars:
Drawing Sugar Molecules
• Fischer Projections (or open chain) represent
three-dimensional structures of stereoisomers
on a flat page.
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• A chiral carbon atom is represented in the
Fisher projection as the intersection of two
crossed lines
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• Bonds that point above the page are shown as
• horizontal lines.
• Bonds that curve behind and below the page are
shown as vertical lines.
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Fischer Projections of Sugar Molecules
In a Fischer projection, the aldehyde or
ketone carbonyl group of a monosaccharide
is always placed toward the top of the page.
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Fischer Projections of Sugar Molecules
• Example
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• Fischer projection of glucose:
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Monosaccharides are divided into two families:
D form and L form sugars.
• D: the –OH group on the chiral C furthest from
the C=O comes out of the plane of paper and
points to the right.
• L: the –OH group on the chiral C furthest from
the C=O comes out of the plane of paper and
points to the left.
• The D and L relate directly only to the position
of –OH group on the bottom carbon in a
Fischer projection.
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D: the –OH group on the chiral C furthest from the C=O
comes out of the plane of paper and points to the right.
L: the –OH group on the chiral C furthest from the C=O
comes out of the plane of paper and points to the left.
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Chiral Drugs
• Many enzymes will interact with only one
particular enantiomer
• In human body, enzymes will react with
carbohydrates of D- form, but will react with
proteins of L-form.
• Some drugs are enantiomers:
1. Ibuprofen used in pain relief
2. Indinavir used in Aids
3. Levofloxacin used as antibiotic
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some important pentoses and hexoses, and
their derivatives
1. D-glucose, also called dextrose
or blood sugar , is the most
important monosaccharide in
human metabolism.
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2. D-fructose, or fruit sugar, is
most common natural ketose.
Honey contains about is 38%
fructose
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• In deoxy sugars a hydrogen atom replaces one or
more of the -OH groups in a monosaccharide.
• D-ribose and its derivative D-2-deoxyribose
(deoxy = minus one oxygen atom) are found in
various coenzymes and in DNA.
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The Cyclic Form of Monosaccharides
• – A monosaccharide contains both an alcohol
and an aldehyde group.
• – It can react with itself to form a cyclic
hemiacetal.
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