Download Introductory Chemistry: Concepts & Connections 4th Edition

Introductory Chemistry:
Concepts & Connections
4th Edition by Charles H. Corwin
Chapter 20
Biochemistry
Christopher G. Hamaker, Illinois State University, Normal IL
© 2005, Prentice Hall
Introduction to Biochemistry
• There are 30 elements which are essential for life.
• The study of the chemistry of living things is
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
Chapter 20
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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.
Chapter 20
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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. Chapter 20
6
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 carboxy
group (-COOH) attached to a carbon atom with
side chain (R) attached to the a-carbon.
• There are twenty naturally occurring amino acids
in human proteins. Each one has a different side
chain.
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The 20 Natural Amino Acids
Neutral Amino Acids
Acidic Amino Acids
Basic Amino Acids
9
Polypeptides
• In proteins, amino acids are linked together with
peptide linkages or a peptide bond.
• In a dipeptide, two amino
acid units are joined
together by a peptide bond.
• In a polypeptide, up to 50
amino acids are linked
together.
• Human insulin is composed
of 48 amino acids.
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Primary Structure of Proteins
• Proteins are polypeptides composed of hundreds
or thousands of amino acid units.
• The primary structure of a protein is the sequence
of amino acids. Individual amino acids are
represented by the colored 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
structures, 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 structure in proteins is a result of
hydrogen bonds.
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13
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 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 it the substrate.
Chapter 20
16
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:
ES → E + P1 + P2
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Enzyme Reactions
• The active site of an enzyme has a shape designed
to specifically bind its substrate.
• Once the reaction has occurred, the active site is
free to bind 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 a 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, and aldehyde or ketone group, and a few
hydroxyl groups.
• A simple sugar is referred to as a monosaccharide.
• 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 5 carbon atoms
and 1 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.
Chapter 20
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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 3-dimentional structures.
Starch
Cellulose
Chapter 20
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Lipids
• Unlike most other biological compounds, lipids
are not water soluble.
• Lipids include
– Fats and oils
– Waxes
– Steroids
– The water insoluble vitamins A, D, E, and K
Chapter 20
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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.
Chapter 20
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Triglycerides
• 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.
Chapter 20
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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 and 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 and example of an
unsaturated triglyceride from a vegetable oil.
Chapter 20
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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:
deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA).
• A nucleic acid is a polymer composed of many
repeating units, each of which is called a
nucleotide.
Chapter 20
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Nucleotides
• Each nucleotide consists of three similar
component molecules: a five-carbon sugar, a
nitrogen-containing base, and a molecule of
phosphoric acid.
• The five-carbon sugars are similar in RNA and
DNA. The two sugars are identical except for a
hydroxyl group. The structures are shown here:
Chapter 20
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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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Nucleotide Structures
• Nucleotides for DNA and RNA are the sugar with
a phosphoric acid and the nitrogen base attached.
• Shown here are structures of a DNA and RNA
nucleotide.
Chapter 20
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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.
Chapter 20
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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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Conclusions
• There are 4 major types of biological molecules:
– Proteins are composed of amino acids.
– Carbohydrates are composed of simple sugar
molecules.
– A lipid is a water insoluble biological molecule.
– A nucleic acid is a polymer composed of a
sugar molecule, a nitrogen base, and phosphoric
acid.
Chapter 20
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Conclusions 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 where the reaction would
normally take many years.
Chapter 20
37
Conclusions 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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