Chapter 19 Amino Acids and
Proteins
19.1 Proteins and Amino Acids
19.2 Amino Acids as Acids and Bases
1
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Functions of Proteins
� Proteins perform many different functions in the body.
TABLE 19.1
2
Amino Acids
Amino acids
� Are the building blocks of proteins.
� Contain a carboxylic acid group and an amino group
on the alpha (α) carbon.
� Are ionized in solution.
3
� Are ionized in solution.
� Each contain a different side group (R).
R R
│ + │H2N—C —COOH H3N—C —COO−
│ │H H
ionized form
Examples of Amino Acids
H
+ │H3N—C—COO−
│
H glycine
4
H glycine
CH3
+ │H3N—C—COO−
│H alanine
Types of Amino Acids
Amino acids are classified as
� Nonpolar (hydrophobic)
with hydrocarbon side
chains.
� Polar (hydrophilic) with
Nonpolar Polar
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� Polar (hydrophilic) with
polar or ionic side chains.
� Acidic (hydrophilic) with
acidic side chains.
� Basic (hydrophilic) with
–NH2 side chains.
Acidic Basic
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Nonpolar Amino Acids
A nonpolar amino acid has
� An R group that is H, an alkyl group, or aromatic.
6
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Polar Amino Acids
A polar amino acid has
� An R group that is an alcohol, thiol, or amide.
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Acidic and Basic Amino Acids
An amino acid is
� Acidic with a carboxyl R group (COO−).
� Basic with an amino R group (NH3+).
Basic Amino Acids
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Learning Check
Identify each as (P) polar or (NP) nonpolar.
+
A. H3N–CH2–COO− (Glycine)
CH
9
CH3
|
CH–OH
+ │B. H3N–CH–COO− (Threonine)
Solution
Identify each as (P) polar or (NP) nonpolar.
+
A. H3N–CH2–COO− (Glycine) (NP) nonpolar
CH
10
CH3
|
CH–OH
+ │B. H3N–CH–COO− (Threonine) (P) polar
Fischer Projections of Amino Acids
Amino acids
� Are chiral except for glycine.
� Have Fischer projections that are stereoisomers.
� That are L are used in proteins.
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L-alanine D-alanine L-cysteine D-cysteine
CH2SH
H2N H
COOH
CH2SH
H NH2
COOH
CH3
H NH2
COOH
CH3
H2N H
COOH
A zwitterion
• Has charged —NH3+ and COO- groups.
• Forms when both the —NH2 and the —COOH groups
in an amino acid ionize in water.
• Has equal + and − charges at the isoelectric point (pI).
Zwitterions and Isoelectric Points
12
• Has equal + and − charges at the isoelectric point (pI).
O O
║ + ║NH2—CH2—C—OH H3N—CH2—C—O–
Glycine Zwitterion of glycine
In solutions more basic than the pI,
� The —NH3+ in the amino acid donates a proton.
+ OH–
H N—CH —COO– H N—CH —COO–
Amino Acids as Acids
13
H3N—CH2—COO– H2N—CH2—COO–
Zwitterion Negative ionat pI pH > pICharge: 0 Charge: 1−
In solutions more acidic than the pI,
� The COO− in the amino acid accepts a proton.
+ H++
H N—CH —COO– H N—CH —COOH
Amino Acids as Bases
14
H3N—CH2—COO– H3N—CH2—COOH
Zwitterion Positive ionat pI pH< pICharge: 0 Charge: 1+
pH and Ionization
H+ OH−
+ +
H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO–
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3 2 3 2 2 2
positive ion zwitterion negative ion
(at low pH) (at pI) (at high pH)
Electrophoresis: Separation of
Amino Acids
In electrophoresis, an electric current is used to separate
a mixture of amino acids, and
� The positively charged amino acids move toward the
negative electrode.
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� The negatively charged amino acids move toward the
positive electrode.
� An amino acid at its pI does not migrate.
� The amino acids are identified as separate bands on the filter paper or thin-layer plate.
Electrophoresis
With an electric current, a mixture of lysine, aspartate,
and valine are separated.
17
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CH3 CH3
+ | |H3N—CH—COOH H2N—CH—COO–
(1) (2)
Learning Check
18
Which structure represents:
A. Alanine at a pH above its pI?
B. Alanine at a pH below its pI?
CH3 CH3
+ | |H3N—CH—COOH H2N—CH—COO–
(1) (2)
Solution
19
Which structure represents:
A. Alanine at a pH above its pI? (2)
B. Alanine at a pH below its pI? (1)
Chapter 19 Amino Acids and
Proteins
19.3
Formation of Peptides
20
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The Peptide Bond
A peptide bond
� Is an amide bond.
� Forms between the carboxyl group of one amino acid and the amino group of the next amino acid.
O CH3 O
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O CH3 O+ || + | ||H3N—CH2—C—O– + H3N—CH—C—O–
O H CH3 O+ || | | ||
H3N—CH2—C—N—CH—C—O– + H2Opeptide bond
Formation of A Dipeptide
22
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Learning Check
Write the dipeptide Ser-Thr.
OH CH3
| |
CH2 O HCOH O
+ | ║ + | ║
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+ | ║ + | ║H3N─CH─C─O – + H3N─CH─C─O–
Ser Thr
Solution
Write the dipeptide Ser-Thr.OH CH3
| |CH2 O HCOH O
+ | ║ + | ║H N─CH─C─O – + H N─CH─C─O–
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H3N─CH─C─O – + H3N─CH─C─O–
Ser peptide ThrOH bond CH3
| |CH2 O H HCOH O
+ | ║ | | ║NH3─CH─C─N ─CH─C─O– + H2O
Ser-Thr
Naming Dipeptides
A dipeptide is named with
� A -yl ending for the N-terminal amino acid.
� The full amino acid name of the free carboxyl group
(COO-) at the C-terminal end.
25
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Write the three-letter abbreviations and names of the
tripeptides that could form from two glycine and one
alanine.
Learning Check
26
Write the names and three-letter abbreviations of the
tripeptides that could form from two glycine and one
alanine.
Glycylglycylalanine Gly-Gly-Ala
Solution
27
Glycylglycylalanine Gly-Gly-Ala
Glycylalanylglycine Gly-Ala-Gly
Alanylglycylglycine Ala-Gly-Gly
Learning Check
What are the possible tripeptides formed from one
each of leucine, glycine, and alanine?
28
Solution
Tripeptides possible from one each of leucine,
glycine, and alanine
Leu-Gly-Ala
Leu-Ala-Gly
Ala-Leu-Gly
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Ala-Leu-Gly
Ala-Gly-Leu
Gly-Ala-Leu
Gly-Leu-Ala
Learning Check
Write the three-letter abbreviation and name for the
following tetrapeptide:
CH3
│CH3 S
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CH3 S
│ │CH–CH3 SH CH2
│ │ │CH3 O H CH2 O H CH2O H CH2 O
+ │ ║ │ │ ║ │ │ ║ │ │ ║H3N–CH–C–N–CH–C–N–CH–C–N–CH–CO–
Solution
Ala-Leu-Cys-Met Alanylleucylcysteylmethionine
CH3
│CH3 S
│ │
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│ │CH–CH3 SH CH2
│ │ │CH3 O H CH O H CH2O H CH2 O
+ │ ║ │ │ ║ │ │ ║ │ │ ║H3N–CH–C–N–CH–C–N–CH–C–N–CH–CO–
Ala Leu Cys Met
Chapter 19 Amino Acids and
Proteins
19.4
Protein Structure: Primary and Secondary Levels
32
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Primary Structure of Proteins
The primary structure of a protein is
� The particular sequence of amino acids.
� The backbone of a peptide chain or protein.
33Ala─Leu─Cys─Met
CH3
SH
CH2
CH3
S
CH2
CH2CH O
O-CCH
H
N
O
CCH
H
N
O
CCH
H
N
O
CCHH3N
CH3
CH3CH
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Primary Structures
The nonapeptides oxytocin and vasopressin
� Have similar primary structures.
� Differ only in the amino acids at positions 3 and 8.
34
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Primary Structure
of Insulin
Insulin
� Was the first protein to
have its primary structure
determined.
� Has a primary structure of
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� Has a primary structure of
two polypeptide chains
linked by disulfide bonds.
� Has a chain A with 21
amino acids and a chain B
with 30 amino acids.
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Secondary Structure – Alpha Helix
The secondary structures of proteins indicate the
three-dimensional spatial arrangements of the
polypeptide chains.
36
An alpha helix has
� A coiled shape held in place by hydrogen bonds
between the amide groups and the carbonyl
groups of the amino acids along the chain.
� Hydrogen bonds between the H of a –N-H group
and the O of C=O of the fourth amino acid down
the chain.
Secondary Structure – Alpha Helix
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Secondary Structure – Beta
Pleated Sheet
A beta-pleated sheet is a secondary structure that
� Consists of polypeptide chains arranged side by
side.
� Has hydrogen bonds between chains.
38
� Has hydrogen bonds between chains.
� Has R groups above and below the sheet.
� Is typical of fibrous proteins such as silk.
Secondary Structure: β-Pleated
Sheet
39
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Secondary Structure: Triple Helix
A triple helix
� Consists of three alpha helix chains woven together.
� Contains large amounts glycine, proline, hydroxy
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� 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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Indicate the type of protein structure as
1) primary 2) alpha helix
3) beta-pleated sheet 4) triple helix
A. Polypeptide chains held side by side by H bonds.
Learning Check
41
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.
Indicate the type of protein 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.
Solution
42
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.
Chapter 19 Amino Acids and
Proteins
19.5
Protein Structure: Tertiary and Quaternary Levels
43
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Essential amino acids
� Must be obtained from the diet.
� Are the ten amino acids not
Essential Amino Acids
TABLE 19.3
44
acids not synthesized by the body.
� Are in meat and
diary products.
� Are missing (one or more) in grains and vegetables.
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The tertiary structure of a protein
� Gives a specific three dimensional shape to the
polypeptide chain.
� Involves interactions and cross links between
Tertiary Structure
45
different parts of the peptide chain.
� Is stabilized by
Hydrophobic and hydrophilic interactions. Salt bridges.
Hydrogen bonds.
Disulfide bonds.
Tertiary Structure
� The
interactions of
the R groups
give a protein
its specific
46
its specific
three-
dimensional
tertiary
structure.
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Tertiary Structure
TABLE 19.5
47
Globular Proteins
Globular proteins
� Have compact, spherical shapes.
� Carry out synthesis, transport, and
Myoglobin
48
transport, and metabolism in the cells.
� Such as myoglobin store and transport oxygen in muscle.
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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
49
� Such as feathers contain beta keratins with large amounts of beta-pleated sheet structures.
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Select the type of tertiary interaction
1) disulfide 2) ionic
3) H bonds 4) hydrophobic
Learning Check
50
A. Leucine and valine
B. Two cysteines
C. Aspartic acid and lysine
D. Serine and threonine
Select the type of tertiary interaction as:
1) disulfide 2) ionic
3) H bonds 4) hydrophobic
A. 4 Leucine and valine
Solution
51
B. 1 Two cysteines
C. 2 Aspartic acid and lysine
D. 3 Serine and threonine
Quaternary Structure
The quaternary structure
� Is the combination of two or
more tertiary units.
� Is stabilized by the same
interactions found in tertiary
hemoglobin
52
interactions found in tertiary
structures.
� Of hemoglobin consists of 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 Protein Structure
TABLE 19.6
53
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Summary of Protein Structures
54
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Identify the level of protein structure as:
1) Primary 2) Secondary
3) Tertiary 4) Quaternary
A. Beta-pleated sheet
Learning Check
55
A. Beta-pleated sheet
B. Order of amino acids in a protein
C. A protein with two or more peptide chains
D. The shape of a globular protein
E. Disulfide bonds between R groups
Identify the level of protein structure
1. Primary 2. Secondary
3. Tertiary 4. Quaternary
A. 2 Beta-pleated sheet.
Solution
56
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.
19.6
Protein Hydrolysis and Denaturation
Chapter 19 Amino Acids and
Proteins
57
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Protein hydrolysis
� Splits the peptide bonds to give smaller peptides
and amino acids.
� Occurs in the digestion of proteins.
Protein Hydrolysis
58
� Occurs in the digestion of proteins.
� Occurs in cells when amino acids are needed to
synthesize new proteins and repair tissues.
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.
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proteins.
+
H3N CH COH
OCH3
+
H2O, H+
++
heat,
CH2
OH
H3N CH C
O
N
H
CH C
O
OH
CH3
CH2
OH
CH C
O
OHH3N
Denaturation involves
� The disruption of bonds in the secondary, tertiary
and quaternary protein structures.
� Heat and organic compounds that break apart H
Denaturation
60
� Heat and organic compounds that break apart H
bonds and disrupt hydrophobic interactions.
� Acids and bases that break H bonds between polar
R groups and disrupt ionic bonds.
� Heavy metal ions that react with S-S bonds to form
solids.
� Agitation such as whipping that stretches peptide
chains until bonds break.
Denaturation of protein occurs
when
� An egg is cooked.
� The skin is wiped with alcohol.
Applications of Denaturation
61
alcohol.
� Heat is used to cauterize blood vessels.
� Instruments are sterilized in autoclaves.
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What are the products of the complete hydrolysis of
the peptide Ala-Ser-Val?
Learning Check
62
The products of the complete hydrolysis of the
peptide Ala-Ser-Val are
alanine
Solution
63
alanine
serine
valine
Tannic acid is used to form a scab on a burn. An
egg is hard boiled by placing it in boiling water.
What is similar about these two events?
Learning Check
64
Acid and heat cause the denaturation of protein.
They both break bonds in the secondary and tertiary
structures of proteins.
Solution
65
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