Essential Cell Biology 5th edition

136 CHAPTER 4 Protein Structure and Function
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 4−29 Fibrous proteins collagen and elastin form very different structures. (A) A collagen molecule is a
triple helix formed by three extended protein chains that wrap around one another. Many rodlike collagen molecules
are cross-linked together in the extracellular space to form collagen fibrils (top), which 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 molecules are cross-linked together by covalent bonds (red ) to form rubberlike, elastic
fibers. Each elastin polypeptide chain uncoils into a more extended conformation when the fiber is stretched, and
recoils spontaneously as soon as the stretching force is relaxed.
ECB5 e4.29/4.29
The most common covalent cross-links in proteins are sulfur–sulfur
bonds. These disulfide bonds (also called S–S bonds) are formed, before
a protein is secreted, by an enzyme in the endoplasmic reticulum that
links together two –SH groups from cysteine side chains that are adjacent
in the folded protein (Figure 4−30). Disulfide bonds do not change a
protein’s conformation, but instead act as a sort of “atomic staple” to reinforce
the protein’s most favored conformation. Lysozyme—an enzyme
in tears, saliva, and other secretions that can disrupt bacterial cell walls—
retains its antibacterial activity for a long time because it is stabilized by
such disulfide cross-links.
Disulfide bonds generally do not form in the cell cytosol, where a high
concentration of reducing agents converts such bonds back to cysteine
–SH groups. Apparently, proteins do not require this type of structural
reinforcement in the relatively mild conditions inside the cell.
cysteine
C
polypeptide 1
C
Figure 4−30 Disulfide bonds help stabilize
a favored protein conformation. This
diagram illustrates how covalent disulfide
bonds form between adjacent cysteine side
chains by the oxidation of their –SH groups.
As indicated, these cross-links can join
either two parts of the same polypeptide
chain or two different polypeptide chains.
Because the energy required to break one
covalent bond is much larger than the
energy required to break even a whole
set of noncovalent bonds (see Table 2−1,
p. 48), a disulfide bond can have a major
stabilizing effect on a protein’s folded
structure (Movie 4.6).
CH 2
SH
SH
CH 2
C
C
CH 2
SH
SH
CH 2
C
polypeptide 2
OXIDATION
REDUCTION
C
CH 2
S
S
CH 2
C
CH 2
S
S
CH 2
C
intrachain
disulfide
bond
interchain
disulfide
bond
ECB5 04.30