Structure of Silk:
The secondary structure of silk is an example of the beta
pleated sheet. In this structure, individual protein chains
are aligned side-by-side with every other protein chain aligned
in an opposite direction. The protein chains are held together
by intermolecular hydrogen bonding, that is hydrogen bonding
between amide groups of two separate chains. This intermolecular
hydrogen bonding in the beta-pleated sheet is in contrast to
the intramolecular hydrogen bonding in the alpha-helix.
The hydrogen on the amide of one protein chain is hydrogen
bonded to the amide oxygen of the neighboring protein chain.
The pleated sheet effect arises form the fact that the amide
structure is planar while the "bends" occur at the
carbon containing the side chain.
See the graphic on the left.
Fortunately, the "side" chain R groups in silk are
not very bulky. The basic primary structure of silk consists
of a six amino acid unit that repeats itself. The sequence where
every other unit is glycine in silk is: -gly-ala-gly-ala-gly-ala-.
Although glycine and alanine make up 75-80% of the amino acids
in silk, another 10-15% is serine and the final 10 % contain
bulky side chains such as in tyr, arg, val, asp, and glu.
Silk - Chime
in new window
These amino acids with bulky side chains disrupt the regular
patterns set by the gly-ala-ser. Different species of silkworms
produce different portions of ordered and disordered regions.
The disordered regions provide a small amount of elasticity since
the ordered beta-pleated sheet is already fully extended and
cannot stretch further without breaking.
The beta pleated sheet motif is found in many proteins along
with the alpha helix structure. The chains may run parallel (all
N terminals on one end) or anti-parallel (N terminal and C terminal
ends alternate). See small graphic on left.
|Quiz: Explain the similarity
in hydrogen bonding of the alpha-helix and the beta-pleated sheet
secondary protein structure.
|Explain the difference in hydrogen
bonding of the alpha-helix and the beta-pleated sheet secondary