Amino Acids, Proteins, and
Enzymes
Primary and Secondary Structure
Tertiary and Quaternary Structure
Protein Hydrolysis and Denaturation
1
Primary Structure of Proteins
The particular sequence of amino acids
that is the backbone of a peptide chain or
protein
CH3
CH3
CH3 O
+
S
CH CH3
SH
CH2
CH O
CH2 O
CH2 O
H3N CH C N CH C N CH C N CH C O
H
H
Ala-Leu-Cys-Met
H
2
Secondary Structure – Alpha
Helix
• Three-dimensional arrangement of amino
acids with the polypeptide chain in a
corkscrew shape
• Held by H bonds between the H of –N-H group
and the –O of C=O of the fourth amino acid
along the chain
• Looks like a coiled “telephone cord”
3
Secondary Structure – Beta
Pleated Sheet
• Polypeptide chains are
arranged side by side
• Hydrogen bonds form
between chains
• R groups of extend above and
below the sheet
• Typical of fibrous proteins
such as silk
4
Secondary Structure – Triple
Helix
• Three polypeptide chains
woven together
• Glycine, proline, hydroxy
proline and hydroxylysine
• H bonding between –OH
groups gives a strong
structure
• Typical of collagen, connective
tissue, skin, tendons, and
cartilage
5
Learning Check P1
Indicate the type of structure as
(1) primary
(2) alpha helix
(3) beta pleated sheet (4) triple helix
A.
B.
C.
D.
Polypeptide chain held side by side by H bonds
Sequence of amino acids in a polypeptide chain
Corkscrew shape with H bonds between amino
acids
Three peptide chains woven like a rope
6
Solution P1
Indicate the type of structure as
(1) primary
(2) alpha helix
(3) beta pleated sheet (4) triple helix
A.
B.
C.
D.
3 Polypeptide chain held side by side by H bonds
1 Sequence of amino acids in a polypeptide chain
2 Corkscrew shape with H bonds between amino
acids
4 Three peptide chains woven like a rope
7
Tertiary Structure
• Specific overall shape of a protein
• Cross links between R groups of amino
acids in chain
disulfide
–S–S–
+
ionic
–COO–
H3N–
H bonds
C=O
HO–
hydrophobic
–CH3 H3C–
8
Learning Check P2
Select the type of tertiary interaction as
(1) disulfide
(2) ionic
(3) H bonds
(4) hydrophobic
A.
B.
C.
D.
Leucine and valine
Two cysteines
Aspartic acid and lysine
Serine and threonine
9
Solution P2
Select the type of tertiary interaction as
(1) disulfide
(2) ionic
(3) H bonds
(4) hydrophobic
A.
B.
C.
D.
4
1
2
3
Leucine and valine
Two cysteines
Aspartic acid and lysine
Serine and threonine
10
Globular and Fibrous Proteins
Globular proteins
“spherical” shape
Insulin
Hemoglobin
Enzymes
Antibodies
Fibrous proteins
long, thin fibers
Hair
Wool
Skin
Nails
11
Quaternary Structure
• Proteins with two or more chains
• Example is hemoglobin
Carries oxygen in blood
Four polypeptide chains
Each chain has a heme group to
bind oxygen
12
Learning Check P3
Identify the level of protein structure
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
13
Solution P3
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
14
Protein Hydrolysis
• Break down of peptide bonds
• Requires acid or base, water and
heat
• Gives smaller peptides and
amino acids
• Similar to digestion of proteins
using enzymes
• Occurs in cells to provide amino
acids to synthesize other
proteins and tissues
15
Hydrolysis of a Dipeptide
OH
CH3 O
CH2 O
+
H3N CH C N CH C OH
H2O, H+
heat
H
OH
CH2 O
CH3 O
+
H3N CH COH
+
+
H3N CH C OH
16
Denaturation
Disruption of secondary, tertiary and quaternary protein
structure by
heat/organics
Break apart H bonds and disrupt hydrophobic
attractions
acids/ bases
Break H bonds between polar R groups and
ionic bonds
heavy metal ions
React with S-S bonds to form solids
agitation
17
Stretches chains until bonds break
Applications of Denaturation
• Hard boiling an egg
• Wiping the skin with alcohol swab for
injection
• Cooking food to destroy E. coli.
• Heat used to cauterize blood vessels
• Autoclave sterilizes instruments
• Milk is heated to make yogurt
18
Learning Check P4
What are the products of the complete
hydrolysis of Ala-Ser-Val?
19
Solution P4
The products of the complete hydrolysis
of Ala-Ser-Val are
alanine
serine
valine
20
Learning Check P5
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?
21
Solution P5
Acid and heat cause a denaturation of
protein. They both break bonds in the
secondary and tertiary structure of
protein.
22