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INTRODUCTORY CHEMISTRY
Concepts and Critical Thinking
Sixth Edition by Charles H. Corwin
Chapter 20
Biochemistry
by Christopher Hamaker
© 2011 Pearson Education, Inc.
Chapter 20
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Introduction to Biochemistry
• There are 30 elements that are essential for life.
• The study of the chemistry of living things is
called biochemistry.
• Biological compounds are often large and complex
with molar masses greater than 1,000,000 g/mol.
• These large molecules are polymers of smaller
molecules.
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Biological Compounds
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Proteins
• Proteins are naturally occurring polymers
composed of many amino acids.
• An amino acid has both an amine and a
carboxylic acid functional group.
• Amino acids are linked together by amide bonds,
which are referred to as peptide linkages.
© 2011 Pearson Education, Inc.
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Carbohydrates
• A carbohydrate is either a simple sugar or a
polymer composed of many simple sugars.
• A carbohydrate usually contains either an
aldehyde or ketone functional group and several
alcohol groups.
• Starch is a carbohydrate polymer composed of
glucose units linked together by glycoside
linkages.
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Lipids
• A lipid is a water-insoluble compound usually
composed of an alcohol and one or more
carboxylic acid molecules.
• Fats and oils are esters of glycerol, an alcohol that
has three –OH groups.
• As a result, each molecule
of a fat or an oil contains
three ester groups from
three carboxylic acid
molecules joined to one
glycerol molecule.
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Nucleic Acids
• A nucleic acid is a biochemical polymer composed
of a very large number of individual units.
• Each unit in the nucleic acid
contains a sugar molecule
attached to an organic
nitrogen-containing molecule
and an attached phosphate
group.
• The units are attached together
by phosphate linkages.
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Amino Acids
• Proteins are composed of amino acids.
• An amino acid has an amine (–NH2) and a
carboxyl group (–COOH) attached to a carbon
atom with a side chain (R–) attached to the acarbon.
• There are 20 naturally
occurring amino acids in
human proteins. Each
one has a different side
chain (R–).
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The 20 Natural Amino Acids
Neutral Amino Acids
Acidic Amino Acids
Basic Amino Acids
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Primary Structure of Proteins
• Proteins are polypeptides composed of hundreds
or thousands of amino acid units (amide bonds).
• The primary structure of a protein is the sequence
of amino acids. Individual amino acids are
represented by the ovals in the figure below.
• The replacement of one amino acid by another in
the primary structure of a protein can completely
alter its biological activity.
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Secondary Structure of Proteins
• When the peptide chain twists and bends, proteins
acquire a secondary structure.
• There are two primary types of secondary
structure: a-helix and pleated sheet.
• An a-helix is analogous to that in a coiled
telephone cord.
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Pleated Sheet
• In a pleated sheet, the chains run antiparallel to
each other. It looks like a sheet of paper folded in
an accordion shape.
• Secondary structures in proteins are a result of
hydrogen bonds.
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Tertiary Structure of Proteins
• The overall three-dimensional structure of a
protein is referred to as its tertiary structure.
• The tertiary structure of a protein may be long and
extended, or compact and folded.
• The tertiary structure
of a protein is held
together by
intermolecular
forces.
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Enzymes
• An enzyme is a protein that acts as a biological
catalyst.
• Enzymes are incredibly selective for specific
molecules.
• An enzyme can speed up a biochemical reaction so
that the rate is a million times faster than it would
be in the absence of the enzyme.
• Many reactions catalyzed by enzymes would be
too slow without the enzyme to sustain life.
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Enzyme Mechanisms
• The location where the reaction occurs on the
enzyme is called the active site.
• The molecule that reacts is the substrate.
• We can use the lock-and-key model to describe
enzyme mechanisms.
• In the model, the key
is the enzyme and the
lock is the substrate.
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Enzyme Reactions
• An enzyme reaction takes place in two steps.
• First, the substrate (S) binds to the active site on
the enzyme (E).
Step 1:
E + S → ES
• Second, the enzyme releases two or more products
(P1 and P2).
Step 2:
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ES → E + P1 + P2
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Enzyme Reactions, Continued
• The active site of an enzyme has a shape
specifically designed to bind its substrate.
• Once the reaction has occurred, the active site is
free to bind to another substrate molecule and
repeat the reaction.
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The Enzyme Ptyalin
• The enzyme ptyalin is in human saliva and
catalyzes the breaking down of starch molecules
into sugar units.
• The reaction would take about two years to occur
without the enzyme.
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Carbohydrates
• The word carbohydrate means “hydrates of
carbon,” and many have the empirical
formula CH2O.
• Carbohydrate names
usually end in the suffix
-ose.
• Carbohydrates have an
aldehyde or ketone
functional group and
several hydroxyl groups.
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Simple Sugars
• A simple sugar molecule typically has three to six
carbons, an aldehyde or ketone group, and a few
hydroxyl groups.
• A monosaccharide
with an aldehyde
group is an
aldose, and one
with a ketone
group is a
ketose.
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Aqueous Structures of Sugars
• In aqueous solution, sugar molecules usually exist
in ring structures.
• Glucose forms a cyclic structure by joining a
hydroxyl group to an aldehyde group.
• The resulting ring structure has five carbon atoms
and one oxygen atom.
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Disaccharides
• A disaccharide is two
monosaccharide
molecules joined
together.
• In the formation of a
disaccharide, two simple
sugars split out water and
are joined together by a
special –O– bond called
a glycoside linkage.
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Polysaccharides
• A polysaccharide is a polymer of many
monosaccharide linkages joined by glycoside
linkages.
• Starch and cellulose are both polysaccharides
composed only of glucose units, but with slightly
different three-dimentional structures.
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Lipids
• Unlike most other biological compounds, lipids
are not water soluble.
• Lipids include:
– Fats and oils
– Waxes
– Steroids
– Water-insoluble
vitamins A, D, E, and K
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Fatty Acids
• A fatty acid is a carboxylic acid with a long
hydrocarbon chain.
• Fatty acids can be saturated or unsaturated.
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Triglycerides (Triglycerols)
• A triglyceride is a lipid formed from glycerol, a
trihydroxy alcohol, and three fatty acids.
• When three fatty acids react with glycerol, a
triglyceride is formed with three ether linkages.
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Fats
• In triglycerides obtained from animals, the fatty
acids are mostly saturated and have few double
bonds.
• A semisolid lipid obtained from an animal source
is a fat.
• The following structure is an example of a
saturated triglyceride from animal fat.
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Oils
• A liquid lipid obtained from a plant source is an
oil.
• Triglycerides obtained from plants have mostly
unsaturated fatty acid chains.
• The following structure is an example of an
unsaturated triglyceride from a vegetable oil.
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Nucleic Acids
•
•
•
Nucleic acids are biochemical compounds found
in every living cell.
Nucleic acids contain the genetic information
responsible for the reproduction of a species.
There are two basic types of nucleic acids:
1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)
•
A nucleic acid is a polymer composed of many
repeating units, each of which is called a
nucleotide.
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Nitrogen Bases
• There are five nitrogen bases.
• Three are found in both DNA and RNA, one is
found only in DNA, and one is found only in
RNA.
• Their structures are shown below:
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DNA and RNA
• DNA and RNA are polymers
of individual nucleotides. A
segment is shown here.
• A DNA molecule is actually
two strands of DNA wound
together in a spiral structure
called a double helix.
• The double strand is held
together by hydrogen bonds
between nitrogen pairs.
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DNA Double Helix
• An adenine (A) on one strand always hydrogen
bonds to a thymine (T) on the other strand.
• Also, a cytosine
(C) on one strand
always hydrogen
bonds to a
guanine (G) on
the other strand.
• These interactions
give DNA its
characteristic
structure.
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Chapter Summary
•
There are four major types of biological
molecules:
1. A protein is composed of amino acids.
2. A carbohydrate is composed of simple sugar
molecules.
3. A lipid is a water-insoluble biological molecule.
4. A nucleic acid is a polymer composed of a sugar
molecule, a nitrogen base, and a phosphoric acid.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
• An enzyme is a catalyst for biological reactions.
• Enzymes work by a lock-and-key mechanism,
where only a specific substrate fits into the
enzyme to react.
• Reactions catalyzed by enzymes can be completed
in a matter of seconds, whereas the reaction would
normally take many years.
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Chapter Summary, Continued
• A triglyceride is an ester of glycerol and three
different fatty acids.
• A fat is a triglyceride from an animal source. An
oil is a triglyceride from a plant source.
• A nucleic acid is a molecule that carries genetic
information.
• DNA and RNA are the two basic types of nucleic
acids.
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