67
CHAPTER TWO
Chemical Basis of Life
Pleated
structure
Coiled
structure
Three-dimensional
folding
Amino acids
(a)
Primary structure—Each
oblong shape in this
polypeptide chain represents
an amino acid molecule. The
whole chain represents a
portion of a protein molecule.
Secondary structure—The
polypeptide chain of a protein
molecule is often either pleated
or twisted to form a coil. Dotted
lines represent hydrogen bonds.
R groups (see fig. 2.17)
are indicated in bold.
(b)
(c)
Tertiary structure—
The pleated and coiled
polypeptide chain of a
protein molecule folds
into a unique three-
dimensional structure.
(d)
Quaternary structure—Two or more
polypeptide chains may be connected
to form a single protein molecule.
N
N
N
N
N
H
H
H
H
H
C
C
C
C
O
O
O
C
C
C
C
C
C
O
C
O
N
N
H
H
C
O
C
C
O
C
H
N
N
H
O
O
C
C
C
C
N
N
N
NH
H
HOC
C
C
O
O
C
C
C
HO
C
C
C
N
C
N
H
O
C
C
HO
C
C
N
N
N
NH
H
HOC
O
O
C
C
C
HO
C
H
R
H
R
H
R
H
R
H
R
H
R
H
R
H
R
H
R
H
H
R
H
H
R
H
R
H
R
H
H
R
R
H
H
R
R
C
H
C
H
FIGURE 2.19
The primary and secondary levels
of protein structure determine the overall three-
dimensional conformation, vital to the protein’s
function.
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