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6.2 Fibrous proteins
Fibrous proteins, unlike globular proteins, contain repetitive amino acid sequences, giving
rise to very regular secondary structures. They can be divided into three main structural
groups two of which, the triple helix of collagen and the coiled coils of keratin, myosin and
cytoskeleton components, are made up of multiple helices wrapped around each other,
Proteins in the third group, which includes spider's silk, are made up of
-
sheets [94]. The structural units of a fibrous protein are micro-fibrils that aggregate so that
the gross structure has a specific strength and elasticity.
The fibrils of collagen consist of three left-handed polyproline II helices running in
parallel to form a right handed triple super-helix (figure 29). Each chain, which contains
about 1000 residues (~ 3000Å) is made up of repeat sequences Gly-X-Y where X is often
proline and Y is often hydroxyproline formed by post-translational modification of proline.
The chains are held together by hydrogen bonding between the proline C
α
-helices and
β
O groups of one
chain and the glycine NH groups of another. The glycine sidechains point towards the
interior of the superhelix where there is not enough space for larger sidechains [95]
.
Coiled coils are left handed super-helices formed from two
α
-helices in which the number
of residues per turn in each
′
-helix is reduced from 3.6 to 3.5 and the sequences tend to be
repeated every seven residues. The first and fifth residues of the repeat in each helix are
hydrophobic and oriented towards the helix axis forming the contact face as the helices coil
around each other [96].
α
a) b)
Figure 29. Backbone trace of a collagen like polypeptide with repeated pro pro gly
sequence. 1a3j, [97]. Proline is colored dark grey, glycine is colored light grey. a)
Side view b) Looking down the right-handed triple helix from the N terminal end
6.3 Membrane Proteins
To enable the transfer of a polypeptide into a membrane bilayer its surface residues must be
predominantly non-polar and the backbone C
′
O and NH groups must be internally
hydrogen bonded. Consequently many transmembrane protein domains are formed from
hydrophobic
-helical bundles connected by hydrophilic loops that project either side of the
bilayer. An example is bacteriorhodopsin (figure 30), which comprises seven
α
-helices that
span the membrane to form a channel. The channel contains covalently bound retinal,
which undergoes isomerisation upon absorbing a photon and thereby changes the
conformation of the protein so that a proton is transferred from the cytosol to the
extracellular side of the membrane.
α
