Chapter Twenty
Proteins
Roles of Proteins
•
•
•
•
•
•
•
Type
Structural
Contractile
Transport
Storage
Hormonal
Enzyme
Protection
Examples
tendons, cartilage, hair, nails
muscles
hemoglobin
milk
insulin, growth hormone
catalyzes reactions in cells
immune response
Ch 20 | 2 of 59
Roles of Proteins
Ch 20 | 3 of 59
Amino Acids
• Proteins are polymers of amino acids
• Contain a carboxylic acid group and an amino group
on the alpha carbon
• Side group R gives unique characteristics
R
I
side chain
H2N— C — COOH
I
H
Ch 20 | 4 of 59
Classifying Amino Acids
• Nonpolar
– An amino acid that contains a nonpolar side chain
– R = H, CH3, alkyl groups, aromatic
• Polar
– An amino acid with a side chain that is polar but neutral
–
O
ll
R = –CH2OH, –CH2SH, –CH2C–NH2,
(polar groups with –O-, -SH, -N-)
Ch 20 | 5 of 59
The 20 Standard Amino Acids, Grouped According
to Side-Chain Polarity.
Ch 20 | 6 of 59
The 20 Standard Amino Acids, Grouped According to SideChain Polarity. (cont’d)
Ch 20 | 7 of 59
Classifying Amino Acids
• Polar/Acidic
– An amino acid that contains a second carboxyl group in its side
chain
– R = –CH2COOH, or -COOH
• Polar/ Basic
– An amino acid that contains a second amino group in its side
chain
– R = –CH2CH2NH2
Ch 20 | 8 of 59
The 20 Standard Amino Acids, Grouped According
to Side-Chain Polarity. (cont’d)
Ch 20 | 9 of 59
Essential Amino Acids
•
•
•
•
•
10 amino acids not synthesized by the body
Arg, His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val
Must obtain from the diet
All in diary products
1 or more missing in grains
and vegetables
Ch 20 | 10 of 59
Fischer Projections of Amino Acids
• All amino acids except glycine are chiral.
• Amino acids have stereoisomers
• In biological systems, only L amino acids are used in
proteins
COOH
H
NH2
CH3
D-Alanine
H2N
H
CH3
L-Alanine
COOH
COOH
COOH
H2N
H
CH2SH
L-Cysteine
H
NH2
CH2SH
D-Cysteine
Ch 20 | 11 of 59
Designation of handedness in standard amino acid structures involves
aligning the carbon chain vertically and looking at the position of the
horizontally aligned NH2 group.
Ch 20 | 12 of 59
Zwitterions
• Ionization of the –NH2 and the –COOH group
– -COOH loses a proton (acid)
– -NH2 gains a proton (base)
• Zwitterion has both a + and – charge
• Zwitterion is neutral overall
+
NH2–CH2–COOH
glycine
H3N–CH2–COO–
Zwitterion of glycine
Ch 20 | 13 of 59
pH and Ionization
In solution, at least three different forms of amino acids can
exist: positive ion, zwitterion, and negative ion
H+
OH–
+
H3N–CH2–COOH
+
H3N–CH2–COO–
H2N–CH2–COO–
Positive ion
zwitterion
Negative ion
Low pH
neutral pH
High pH
Ch 20 | 14 of 59
pH and Ionization
• Acidic amino acids such as aspartic acid have a
second carboxyl group that can donate and accept
protons
– Amino acids with ionizable side chains have 4 forms in
solution
• -Cys, Tyr, Lys, Arg, His, Asp, Glu
• Whether a group is ionized or not depends on its
pKa
– If pH > pKa, the group has been deprotonated
– If pH < pKa, the group is protonated
Ch 20 | 15 of 59
Amino Acid
a-carboxylic acid
a-amino
Alanine
2.35
9.87
Arginine
2.01
9.04
Asparagine
2.02
8.80
Aspartic Acid
2.10
9.82
3.86
Cysteine
2.05
10.25
8.00
Glutamic Acid
2.10
9.47
4.07
Glutamine
2.17
9.13
Glycine
2.35
9.78
Histidine
1.77
9.18
Isoleucine
2.32
9.76
Leucine
2.33
9.74
Lysine
2.18
8.95
Methionine
2.28
9.21
Phenylalanine
2.58
9.24
Proline
2.00
10.60
Serine
2.21
9.15
Threonine
2.09
9.10
Tryptophan
2.38
9.39
Tyrosine
2.20
9.11
Valine
2.29
9.72
Side chain
12.48
6.10
10.53
10.07
16
Step-wise Ionization of Amino Acids
• 1. Draw the amino acid in the fully protonated
form
– Low pH
– All acid groups are protonated (-COOH)
– All amino groups are protonated (-NH3+)
• 2. Identify the protons that will come off (and the
order in which they will come off)
• 3. Take the protons off 1 by 1
• Example: Glutamic Acid
Ch 20 | 17 of 59
Ionization of Glutamic Acid
• Draw the step-wise ionization of glutamic acid
Ch 20 | 18 of 59
HomeWork Assignment
• What overall charge will the following amino acids
have at pH 5.5?
• Lysine
• Phenylalanine
• Glutamic Acid
Ch 20 | 19 of 59
Electrophoresis
• Electrophoresis separates amino acids according
to their charges
– Positively charged amino acids move towards the
negative electrode
– Negatively charged amino acids move toward the
positive electrode
– Neutral amino acids will not move in either direction
• Amino acids are visualized as separate bands on
filter paper or thin layer plate
Ch 20 | 20 of 59
Electrophoresis
Ch 20 | 21 of 59
Peptide Bonds
Amide bond formed by the carboxylate group of an amino acid and
the –amino group of the next amino acid
O
||
+
CH3
+ |
NH3–CH2–COH
+
+
O
||
H3N–CH–COO–
CH3
|
NH3–CH2–C – N–CH–COO–
|
H
peptide bond
Ch 20 | 22 of 59
Peptides
• Peptide
– A sequence of amino acids in which the amino acids are
joined together through amide (peptide) bonds
• Dipeptide
– A peptide consisting of 2 amino acids
• Tripeptide
– A peptide consisting of 3 amino acids
• Polypeptide
– A peptide consisting of many amino acids
Ch 20 | 23 of 59
Peptides
• Amino acids linked by amide (peptide) bonds
Nterminus
Gly
H2Nend
Lys
Phe
Peptide bonds
Arg
Ser
Cterminus
-COOH
end
Name: Glycyllysylphenylarginylserine
Ch 20 | 24 of 59
Protein Structure
• A polypeptide containing 50 or more amino acids
is called a protein
• There are different ways to describe the structure
of a protein:
–
–
–
–
Primary Structure
Secondary Structure
Tertiary Structure
Quaternary Structure
Ch 20 | 25 of 59
Secondary Structure: Alpha Helices
• 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”
Ch 20 | 26 of 59
Tertiary Structure
• Specific overall shape of a protein
• Results from cross-links between R groups of amino acids
in chain
disulfide
–S–S–
+
ionic
–COO–
H3N–
H bonds
C=O
HO–
hydrophobic
–CH3
H3C–
Ch 20 | 27 of 59
Levels of Protein Structure
Ch 20 | 28 of 59
Levels of Protein Structure
A telephone cord has three levels of structure.
Ch 20 | 29 of 59
Primary Structure
Human
Myoglobin
Ch 20 | 30 of 59
Secondary Structure
• Geometrical orientation of polypeptide chains
• Two main kinds of secondary structure:
– Alpha helices
– Beta pleated sheets
Ch 20 | 31 of 59
Secondary Structure
The hydrogen bonding
between the carbonyl
oxygen atom of one
peptide linkage and the
amide hydrogen atom
of another peptide
linkage.
Ch 20 | 32 of 59
Secondary Structure
Two pleated sheet protein structure.
Ch 20 | 33 of 59
Secondary Structure
Four representations of
the helix secondary
structure.
Ch 20 | 34 of 59
Secondary Structure
The secondary
structure of a single
protein.
Ch 20 | 35 of 59
Tertiary Structure
Four types of
interactions
between amino acid
R groups produce
the tertiary structure
of a protein.
Ch 20 | 36 of 59
Tertiary Structure
The tertiary structure of
the single-chain protein
myoglobin.
Ch 20 | 37 of 59
Disulfide Bonding
Disulfide bonds involving cysteine
residues can form in two different ways.
Ch 20 | 38 of 59
Human Insulin
Human insulin, a small
two-chain protein, has
both intrachain and
interchain disulfide
linkages as part of its
tertiary structure.
Ch 20 | 39 of 59
Substitutions in Insulin
Ch 20 | 40 of 59
Disulfides and Hair
Ch 20 | 41 of 59
A schematic diagram showing the tertiary structure of the single-chain
protein myoglobin.
Ch 20 | 42 of 59
Quaternary Structure
• Proteins with two or more chains
– Quaternary structure is the relative organization of multiple chains
to each other
• Example is hemoglobin
Carries oxygen in blood
Four polypeptide chains
Each chain has a heme group to bind oxygen
A conjugated protein (has a prosthetic group)
Ch 20 | 43 of 59
Quaternary Structure
Tertiary and quaternary
structure of the oxygencarrying protein
hemoglobin.
Ch 20 | 44 of 59
Globular and Fibrous Proteins
Globular proteins
“spherical” shape
Water soluble
Multiple Types of 2o structure
Transport, Metabolism, etc.
More numerous
Insulin
Hemoglobin
Enzymes
Antibodies
Fibrous proteins
long, thin fibers
Not water soluble
1 Type of 2o Structure
Strength, Protection
Few in the body
Hair
Wool
Skin
Nails
Ch 20 | 45 of 59
Fibrous Proteins
The tail feathers of a
peacock. Fibrous
Protein is α-keratin
PhotoDisc
Ch 20 | 46 of 59
Fibrous Proteins
The coiled-coil
structure of the fibrous
protein alpha keratin.
Ch 20 | 47 of 59
Fibrous Proteins
Ch 20 | 48 of 59
Collagen
Three helical peptide chains.
Ch 20 | 49 of 59
Collagen
→ Electron
Micrograph
Collagen
fibers
Prof. P.M. Motta & E. Vizza / Photo Researchers
Ch 20 | 50 of 59
Protein Denaturation
Protein denaturation
process.
Ch 20 | 51 of 59
Heat Denaturation
Heat denatures the
protein in egg white.
E.R. Degginger
Ch 20 | 52 of 59
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
Ch 20 | 53 of 59
Denaturation
Ch 20 | 54 of 59
Disulfides and Hair
Ch 20 | 55 of 59
Immunoglobulins
Structure of
immunoglobulin.
Ch 20 | 56 of 59
Immunoglobulins
Immunoglobulinantigen complex
Ch 20 | 57 of 59
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
Ch 20 | 58 of 59
Hydrolysis of a Dipeptide
Ch 20 | 59 of 59