Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE SHAPE AND STRUCTURE OF PROTEINS
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lefthanded
(E)
righthanded
Figure 3–22 Some properties of a helix.
(A–D) A helix forms when a series of
subunits bind to each other in a regular
way. At the bottom, each of these helices
is viewed from directly above the helix and
seen to have two (A), three (B), and six
(C and D) subunits per helical turn. Note
that the helix in (D) has a wider path
than that in (C), but the same number of
subunits per turn. (E) As discussed in the
text, a helix can be either right-handed or
left-handed. As a reference, it is useful to
remember that standard metal screws,
which insert when turned clockwise, are
right-handed. Note that a helix retains the
same handedness when it is turned upside
down. (PDB code: 2DHB.)
(A) (B) (C) (D)
acid glycine at every third position. This regular structure allows the chains to
wind around one another to generate a long regular triple helix (Figure 3–23A).
Many collagen molecules then MBoC6 bind m3.26/3.22 to one another side-by-side and end-toend
to create long overlapping arrays—thereby generating the extremely tough
collagen fibrils that give connective tissues their tensile strength, as described in
Chapter 19.
Proteins Contain a Surprisingly Large Amount of Intrinsically
Disordered Polypeptide Chain
It has been well known for a long time that, in complete contrast to collagen,
another abundant protein in the extracellular matrix, elastin, is formed as a highly
disordered polypeptide. This disorder is essential for elastin’s function. Its relatively
loose and unstructured polypeptide chains are covalently cross-linked to
50 nm
short section of
collagen fibril
elastic fiber
collagen
molecule
300 nm × 1.5 nm
1.5 nm
collagen
triple
helix
STRETCH
RELAX
single elastin molecule
cross-link
(A)
(B)
Figure 3–23 Collagen and elastin. (A) Collagen is a triple helix formed by three extended protein chains that wrap around one another (bottom).
Many rodlike collagen molecules are cross-linked together in the extracellular space to form unextendable collagen fibrils (top) that have the tensile
strength of steel. The striping on the collagen fibril is caused by the regular repeating arrangement of the collagen molecules within the fibril.
(B) Elastin polypeptide chains are cross-linked together in the extracellular space to form rubberlike, elastic fibers. Each elastin molecule uncoils into a
more extended conformation when the fiber is stretched and recoils spontaneously as soon as the stretching force is relaxed. The cross-linking in the
extracellular space mentioned creates covalent linkages between lysine side chains, but the chemistry is different for collagen and elastin.
MBoC6 m3.27/3.23