Essential Cell Biology 5th edition

128 CHAPTER 4 Protein Structure and Function
hydrophobic amino
acid side chain
hydrogen bond
Figure 4−15 Many membrane-bound proteins cross the lipid
bilayer as an α helix. The hydrophobic side chains of the amino acids
that form the α helix make contact with the hydrophobic hydrocarbon
tails of the phospholipid molecules, while the hydrophilic parts of the
polypeptide backbone form hydrogen bonds with one another along
the interior of the helix. About 20 amino acids are required to span a
membrane in this way. Note that, despite the appearance of a space
along the interior of the helix in this schematic diagram, the helix is not
a channel: no ions or small molecules can pass through it.
phospholipid
α helix
An α helix is generated when a single polypeptide chain turns around itself
to form a structurally rigid cylinder. A hydrogen bond is made between
every fourth amino acid, linking the C=O of one peptide bond to the N–H
of another (see Figure 4−12A). This pattern gives rise to a regular righthanded
helix with a complete turn every 3.6 amino acids (Movie 4.2).
Short regions of α helix are especially abundant in proteins that are
embedded in cell membranes, such as transport proteins and receptors.
We see in Chapter 11 that the portions of a transmembrane protein that
cross the lipid bilayer usually form an α helix, composed largely of amino
acids with nonpolar side chains. The polypeptide backbone, which is
hydrophilic, is hydrogen-bonded to itself inside the α helix, where it is
shielded from the hydrophobic lipid environment of the membrane by the
protruding nonpolar side chains (Figure 4−15).
ECB5 e4.15/4.15
Sometimes two (or three) α helices will wrap around one another to form
a particularly stable structure called a coiled-coil. This structure forms
when the α helices have most of their nonpolar (hydrophobic) side chains
along one side, so they can twist around each other with their hydrophobic
side chains facing inward—minimizing contact with the aqueous
cytosol (Figure 4−16). Long, rodlike coiled-coils form the structural
framework for many elongated proteins, including the α-keratin found in
hair and the outer layer of the skin, as well as myosin, the motor protein
responsible for muscle contraction (discussed in Chapter 17).
g
NH 2
“a” and “d”
d
c
Figure 4−16 Intertwined α helices can
form a stiff coiled-coil. (A) A single α helix
is shown, with successive amino acid side
chains labeled in a sevenfold repeating
sequence “abcdefg.” Amino acids “a” and
“d” in such a sequence lie close together
on the cylinder surface, forming a stripe
(shaded in green) that winds slowly around
the α helix. Proteins that form coiledcoils
typically have nonpolar amino acids
at positions “a” and “d.” Consequently,
as shown in (B), two α helices can wrap
around each other, with the nonpolar side
chains of one α helix interacting with the
nonpolar side chains of the other, while
the more hydrophilic amino acid side
chains (shaded in red ) are left exposed to
the aqueous environment. (C) A portion
of the atomic structure of a coiled-coil
made by two α helices, as determined by
x-ray crystallography. In this structure, the
backbones of the helices are shown in red ,
the interacting, nonpolar side chains are
green, and the remaining side chains are
light gray. Coiled-coils can also form from
three α helices (Movie 4.3).
a
e
a
e
a
d
a
d
d
COOH
g
g
d
0.5 nm
g
g
c
stripe of
hydrophobic
amino acids
11 nm
helices wrap around each other to minimize
exposure of hydrophobic amino acid
side chains to aqueous environment
NH 2 NH 2
HOOC
(A) (B) (C)
COOH