Chapter 2
Chemistry Comes Alive
49
2
Primary structure:
The sequence of
amino acids forms the
polypeptide chain.
Secondary structure:
The primary chain forms
spirals (
α
-helices) and
sheets (
β
-sheets).
Tertiary structure:
Superimposed on secondary structure.
α
-Helices and/or
β
-sheets are folded up
to form a compact globular molecule
held together by intramolecular bonds.
Quaternary structure:
Two or more polypeptide chains, each
with its own tertiary structure, combine
to form a functional protein.
Tertiary structure of prealbumin
(transthyretin), a protein that
transports the thyroid hormone
thyroxine in blood and cerebro-
spinal fluid.
Quaternary structure of a
functional prealbumin molecule.
Two identical prealbumin subunits
join head to tail to form the dimer.
Amino acid
H
H
C
C
C
C
C
N
N
H
H
H
H
H
H
H
H
O
O
O
C
C
C
C
C
N
N
N
R
R
O
R
R
R
O
Amino acid
Amino acid
Amino acid
Amino acid
α
-Helix:
The primary chain is coiled
to form a spiral structure, which is
stabilized by hydrogen bonds.
β
-Sheet:
The primary chain “zig-zags” back
and forth forming a “pleated” sheet. Adjacent
strands are held together by hydrogen bonds.
(a)
(b)
(c)
(d)
Figure 2.19
Levels of protein structure.
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