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 Chemical





composition
Carbon
Hydrogen
Oxygen
Nitrogen
Sulfur (sometimes)
 Monomer/Building

Block
Amino Acids (20 different amino acids)
Table 5-1
Fig. 5-UN1
carbon
Amino
group
Carboxyl
group
Fig. 5-18
Peptide
bond
(a)
A peptide bond is the
bond joining adjacent
amino acids.
Side chains
Peptide
bond
Backbone
(b)
Amino end
(N-terminus)
Carboxyl end
(C-terminus)
Primary - peptide bond joining adjacent amino
acids
 Secondary - Hydrogen bonding between
nonadjacent amino acids that creates an alpha
helix or pleated sheets
 Tertiary - bond formation between R-groups;
clustering of hydrophobic (nonpolar) or
hydrophilic (polar) R-groups, disulfide bridges,
ionic bonding, grouping based on pH, etc… that
results in 3-dimensional folding
 Quaternary – joining of more than one
polypeptide (not all proteins have this level)

Fig. 5-21
Primary
Structure
Secondary
Structure
pleated sheet
+H
3N
Amino end
Examples of
amino acid
subunits
helix
Tertiary
Structure
Quaternary
Structure
Fig. 5-21b
1
5
+H
3N
Amino end
10
Amino acid
subunits
15
20
25
75
80
90
85
95
105
100
110
115
120
125
Carboxyl end
Fig. 5-21c
Secondary Structure
pleated sheet
Examples of
amino acid
subunits
helix
Fig. 5-21f
Hydrophobic
interactions and
van der Waals
interactions
Polypeptide
backbone
Hydrogen
bond
Disulfide bridge
Ionic bond
Fig. 5-21g
Polypeptide
chain
Chains
Iron
Heme
Chains
Hemoglobin
Collagen
A
slight change in primary structure can
affect a protein’s structure and ability to
function
 Sickle-cell disease, an inherited blood
disorder, results from a single amino acid
substitution in the protein hemoglobin
Fig. 5-22
Normal hemoglobin
Primary
structure
Primary
structure
Val His Leu Thr Pro Glu Glu
1
2
Secondary
and tertiary
structures
3
4
5
6
7
subunit
Secondary
and tertiary
structures
Sickle-cell hemoglobin
Val His Leu Thr Pro Val Glu
1
2
3
Exposed
hydrophobic
region
Quaternary
structure
Normal
hemoglobin
(top view)
Quaternary
structure
Sickle-cell
hemoglobin
Function
Molecules do
not associate
with one
another; each
carries oxygen.
Function
Molecules
interact with
one another and
crystallize into
a fiber; capacity
to carry oxygen
is greatly reduced.
10 µm
Red blood
cell shape
Normal red blood
cells are full of
individual
hemoglobin
moledules, each
carrying oxygen.
4
5
6
7
subunit
10 µm
Red blood
cell shape
Fibers of abnormal
hemoglobin deform
red blood cell into
sickle shape.