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Chapter 17
Amino Acids and Proteins
17.1 Functions of Proteins
17.2 Amino Acids
17.3 Amino Acids as Acids and Bases
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Functions of Proteins
Proteins perform many different functions.
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17.3 Amino Acids
Amino acids:
 Are the building blocks of proteins.
 Contain a carboxylic acid group and an amino
group on the alpha () carbon.
 Have different side groups R that give each
amino acid unique characteristics.
R side chain
|
H2N—C —COOH
General structure of an
|
-amino acid
H
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Nonpolar Amino Acids

Amino acids are classified as nonpolar when the
R groups are H, alkyl, or aromatic.
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Polar Amino Acids

Amino acids are classified as polar when the R
groups are alcohols, thiols, or amides.
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Acidic and Basic Amino Acids


Amino acids are classified as acidic when the R
group is a carboxylic acid.
Amino acids are classified as basic when the R
group is an amine.
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17.2 Chiral Objects



Chiral compounds have the
same number of atoms
arranged differently in space.
A chiral carbon atom is
bonded to four different
groups.
Your hands are chiral. Try to
superimpose your thumbs,
palms, back of hands, and
little fingers.
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Mirror Images


The mirror images of chiral compounds cannot
be superimposed.
When the H and I atoms are aligned, the Cl and
Br atoms are on opposite sides.
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Achiral Structures are
Superimposable

When the mirror image of an achiral structure
is rotated, the structure can be aligned with the
initial structure. Thus this mirror image is
superimposable.
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Some Everyday Chiral and
Achiral Objects
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Learning Check
Identify each as a chiral or achiral compound.
Cl
H C CH3
CH2CH3
A
Cl
H C CH3
Cl
H C CH3
H
Br
B
C
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Solution
Identify each as a chiral or achiral compound.
Cl
H C CH3
CH2CH3
Cl
H C CH3
H C CH3
H
Br
B
C
A
Chiral
Cl
Achiral
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Chiral
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Zwitterions



Both the –NH2 and the –COOH groups in an
amino acid undergo ionization in water.
A zwitterion forms that has + and – charge.
At the isoelectric point (pI), the + and – charges in
the zwitterion are equal.
NH2—CH2—COOH
Glycine
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+
H3N—CH2—COO–
Zwitterion of glycine
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Amino Acids as Acids

In solutions more basic than the pI, the —NH3+
in the amino acid donates a proton.
+
OH–
H3N—CH2—COO–
Zwitterion
at pI
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H2N—CH2—COO–
Negative ion
Higher pH
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Amino Acids as Bases

In solution more acidic than the pI, the COO- in
the amino acid accepts a proton.
+
H+
H3N—CH2—COO–
Zwitterion
at pI
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+
H3N—CH2—COOH
Positive ion
Low pH
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pH and Ionization


Acidic amino acids such as aspartic acid have a
second carboxyl group that can donate and accept
protons.
The pI for aspartic acid occurs at a pH of 2.8.
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Electrophoresis




Electrophoresis separates amino acids
according to their isoelectric points.
The positively charged amino acids move
towards the negative electrode.
The negatively charged amino acids move
toward the positive electrode.
An amino acid at its pI will not migrate in
either direction.
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Separation of Amino Acids

When electrophoresis is completed, the amino
acids are identified as separate bands on the
filter paper or thin layer plate.
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Learning Check
CH3
|
+
H3N—CH—COOH
CH3
|
H2N—CH2—COO–
(1)
(2)
Which structure represents:
A. Alanine at a pH above its pI?
B. Alanine at a pH below its pI?
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Solution
CH3
|
+
H3N—CH—COOH
CH3
|
H2N—CH2—COO–
(1)
(2)
Which structure represents:
A. Alanine at a pH above its pI? (2)
B. Alanine at a pH below its pI?
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(1)
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The Peptide Bond

A peptide bond is an amide bond between the
carboxyl group of one amino acid and the amino
group of the next amino acid.
O
CH3 O
||
|
||
+
+
H3N—CH2—C—OH + H3N—CH—C—O–
O H CH3 O
||
|
|
||
+
H3N—CH2—C—N—CH—C—O–
peptide bond
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A Dipeptide


A peptide is named from the free amine (NH3+)
using -yl endings for the names of the amino acids.
The last amino acid with the free carboxyl group
(COO-) uses its amino acid name.
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Learning Check
Write the names and three-letter abbreviations of
the amino acids in the tripeptides that could form
from two glycine and one alanine.
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Solution
Write the names and three-letter abbreviations of
the amino acids in the tripeptides that could form
from two glycine and one alanine.
Gly-Gly-Ala Glycylglycylalanine
Gly-Ala-Gly Glycylalanylglycine
Ala-Gly-Gly Alanylglycylglycine
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Learning Check
Write the name of the following tetrapeptide using
amino acid names and three-letter abbreviations.
CH3
CH3
H3N
S
CH CH3
SH
CH2
CH3 O
CH O
CH2 O
CH2 O
CH C N
CH C N
CH C N CH C O
H
H
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H
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Solution
CH3
CH3
H3N
S
CH CH3
SH
CH2
CH3 O
CH O
CH2 O
CH2 O
CH C N
CH C N
CH C N CH C O
H
H
Ala
Leu
Cys
H
Met
Ala-Leu-Cys-Met
Alanylleucylcysteinylmethionine
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17.7 Primary Structure


A polypeptide containing 50 or more amino acids
is called a protein.
The primary structure of a protein is the sequence
of amino acids in the peptide chain.
CH3
CH3
H3N
S
CH CH3
SH
CH2
CH3 O
CH O
CH2 O
CH2 O
CH C N
CH C N
CH C N CH C O
H
H
Ala-Leu-Cys-Met
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H
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Primary Structures


The nonapeptides oxytocin and vasopressin have
similar primary structures.
Only the amino acids at positions 3 and 8 differ.
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Insulin
Insulin:
 Was the first
protein to have its
primary structure
determined.
 Of humans has a
primary structure
that is similar to
the insulin of pigs
and cows.
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17.9 Secondary Structure: Alpha
Helix

The secondary structures of proteins
indicate the arrangement of the polypeptide
chains in space.
The alpha helix is a three-dimensional
arrangement of the polypeptide chain that
gives a corkscrew shape like a coiled
telephone cord.
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Alpha Helix

The coiled shape
of the alpha helix
is held in place by
hydrogen bonds
between the amide
groups and the
carbonyl groups of
the amino acids
along the chain.
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Secondary Structure: Pleated
Sheet
The pleated sheet:
 Holds proteins in a
parallel arrangement
with hydrogen bonds.
 Has R groups that
extend above and
below the sheet.
 Is typical of fibrous
proteins such as silk.
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Secondary Structure: Triple Helix



A triple helix:
Consists of three alpha helix chains.
Contains large amounts glycine, proline,
hydroxy proline and hydroxylysine that contain
–OH groups for hydrogen bonding.
Is found in collagen, connective tissue, skin,
tendons, and cartilage.
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Essential Amino Acids
Essential amino acids:
 Are the ten amino
acids that are not
synthesized by the
body.
 Must be obtained
from the diet.
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Essential Amino Acids
Essential amino acids are:
 Found in milk and eggs
(complete proteins).
 Not all found in grains
and vegetables
(incomplete proteins).
 Obtained by combining
two or more vegetables
that provide
complementary proteins.
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Learning Check
Indicate the type of structure as:
1) primary
2) alpha helix
3) beta pleated sheet
4) triple helix
A. Polypeptide chains held side by side by H bonds.
B. Sequence of amino acids in a polypeptide chain.
C. Corkscrew shape with H bonds between amino
acids.
D. Three peptide chains woven like a rope.
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Solution
Indicate the type of structure as:
1) primary
2) alpha helix
3) beta pleated sheet
4) triple helix
A. 3 Polypeptide chains held side by side by H bonds.
B. 1 Sequence of amino acids in a polypeptide chain.
C. 2 Corkscrew shape with H bonds between amino
acids.
D. 4 Three peptide chains woven like a rope.
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17.10 Tertiary Structure
The tertiary structure:
 Gives a specific overall shape to a protein.
 Involves interactions and cross links between
different parts of the peptide chain.
 Is stabilized by
Hydrophobic and hydrophilic interactions
Salt bridges
Hydrogen bonds
Disulfide bonds
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Tertiary Structure
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Tertiary Structure

The interactions
of the R groups
give a protein its
specific threedimensional
tertiary
structure.
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Globular Proteins
Globular proteins:
 Have compact,
spherical shapes.
 Carry out synthesis,
transport, and
metabolism in the
cells.
 Such as myoglobin
store and transport
oxygen in muscle.
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Myoglobin
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Fibrous Proteins
Fibrous proteins:
 Consist of long, fiber-like shapes.
 Such as alpha keratins make up hair, wool,
skin, and nails.
 Such as feathers contain beta keratins with
large amounts of beta-pleated sheet structures.
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Learning check
Select the type of tertiary interaction as:
1) disulfide
2) ionic
3) H bonds
4) hydrophobic
A. Leucine and valine
B. Two cysteines
C. Aspartic acid and lysine
D. Serine and threonine
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Solution
Select the type of tertiary interaction as:
1) disulfide
2) ionic
3) H bonds
4) hydrophobic
A. 4 Leucine and valine
B. 1 Two cysteines
C. 2 Aspartic acid and lysine
D. 3 Serine and threonine
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17.11 Quaternary Structure



The quaternary structure
contains two or more
tertiary subunits.
Hemoglobin contains two
alpha chains and two
beta chains.
The heme group in each
subunit picks up oxygen
for transport in the blood
to the tissues.
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Summary of Structural Levels
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Learning Check
Identify the level of protein structure as:
1) Primary
2) Secondary
3) Tertiary
4) Quaternary
A.
B.
C.
D.
E.
Beta pleated sheet
Order of amino acids in a protein
A protein with two or more peptide chains
The shape of a globular protein
Disulfide bonds between R groups
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Solution
Identify the level of protein structure
1. Primary
2. Secondary
3. Tertiary
4. Quaternary
A. 2 Beta pleated sheet
B. 1 Order of amino acids in a protein
C. 4 A protein with two or more peptide chains
D. 3 The shape of a globular protein
E. 3 Disulfide bonds between R groups
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17.12 Chemical Properties of
Proteins
Protein hydrolysis:
 Splits the peptide bonds to give smaller
peptides and amino acids.
 Occurs in the digestion of proteins.
 Occurs in cells when amino acids are needed
to synthesize new proteins and repair tissues.
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Hydrolysis of a Dipeptide


In the lab, the hydrolysis of a peptide requires
acid or base, water and heat.
In the body, enzymes catalyze the hydrolysis of
proteins.
OH
CH3 O
+
H3N CH C N
heat,
+
H2O, H
CH2 O
CH C OH
OH
H
+
H3N
CH3 O
CH COH
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CH2 O
+
+ H3N CH C OH
51
Denaturation
Denaturation involves the disruption of
bonds in the secondary, tertiary and
quaternary protein structures.
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Denaturation




Heat and organic compounds break apart H
bonds and disrupt hydrophobic interactions.
Acids and bases break H bonds between polar
R groups and disrupt ionic bonds.
Heavy metal ions react with S-S bonds to form
solids.
Agitation such as whipping stretches chains
until bonds break.
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Applications of Denaturation




Denaturation of protein
occurs when:
An egg is cooked.
The skin is wiped with
alcohol.
Heat is used to cauterize
blood vessels.
Instruments are sterilized in
autoclaves.
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54
Learning Check
What are the products of the complete
hydrolysis of the peptide Ala-Ser-Val?
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Solution
The products of the complete hydrolysis of the
peptide Ala-Ser-Val are:
alanine
serine
valine
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56
Learning check
Tannic acid is used to form a scab on a burn.
An egg becomes hard boiled when placed in
hot water. What is similar about these two
events?
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57
Solution
Acid and heat cause the denaturation of
protein. They both break bonds in the
secondary and tertiary structures of protein.
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58
Chapter Summary




20 different amino acids occur in proteins.
Amino acid side chains have acidic or basic
functional groups, or neutral groups that are
either polar or nonpolar.
Each amino acid has a isoelectric point – the
pH at which the numbers of positive and
negative charges in a solution are equal.
A molecule can be identified as a chiral
molecule if it contains a carbon atom that has
four different groups.
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59
Chapter Summary Contd.




Chiral molecules do not have a mirror plane.
Mirror images of chiral molecules are nonsuperimposable.
All amino acids except glycine have four different
groups bonded to the -carbon, as a result they are
chiral.
Proteins are polymer of amino acids
(polypeptides).
Primary structure of a protein is the sequence in
which the amino acids are connected by peptide
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60
Chapter Summary Contd.


Primary structures are written with the amino
terminal amino acid on the left and carboxyl
terminal amino acid on the right. Noncovalent
interactions between side chain groups determine
the overall three dimensional shape of a protein
molecule.
Secondary structure of a protein is the regular
repeating three dimensional structures held
together by hydrogen bonding between the
backbone atoms within a chain or in adjacent
61
chain.
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Chapter Summary Contd.




Tertiary structure of a protein is the overall three
dimensional shape of a folded protein.
Quaternary structure of a protein is the structure
that incorporates more than one peptide chain.
The peptide bonds are broken by hydrolysis.
Denaturation is the loss of overall structure while
retaining its primary structure.
Agents that can cause denaturation of protein
includes heat, mechanical agitation, polar organic
solvents, detergents, salts, etc.
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62

End of Chapter 17
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