BE.342/442 Tuesday, September 27, 2005
Topic: Beta Sheets
Administrative announcements: Take-home exam at the end of next week.
Introduction to Beta-sheets:
Beta sheets can appear in a variety of arrangements. Last time, we saw that alpha-helices can
organize into a higher-order structure: coiled coils. Beta sheets, too, pack together into
macrostructures that can be seen with the naked eye! A common one is the silk fiber: a silk scarf
is composed of 80% beta sheets! A single thread of silk is an extended arrangement of beta-
sheets, so stretched that it has little elasticity. Spider’s silk, in constast, is only 50% beta-sheets,
which provide strength. The remaining 50% of loops and alpha-helical coils provide elasticity.
Thus, the content drastically changes the properties of this material.
Demonstration: insect cocoon. Its fibers have a high content of beta-sheets.
The major configurations of beta sheets are:
parallel: adjacent strands are aligned with the C-termini and the N-termini on the same side
anti-parallel: adjacent strands are aligned with C-termini aligned to adjacent N-termini
Higher-order structures of these arrangements include Greek keys and jelly rolls. Another motif
discovered in 1993 is the Bet-helix.
The sort of packing observed in beta sheets is seen in other structures, including:
Architecture, such as tiles on the Cathedrial of Siena, Italy.
Block copolymers, which can pack similarly to proteins
Spider’s silk is composed of stacks of antiparallel beta sheets. In its soluble, gelatinous form
(inside the spider), the silk proteins group into four 20-nm antiparallel strands. When expelled,
they compress into insoluble higher-order structures.
A silk fibroin arranges orients into anti-parallel beta-sheets with alanine staggered on one side,
and glysine on the other. Each fibroin contains 3,000 amino acids. Since each residue on a beta-
sheet spans 3.4 angstroms, meaning each fibroin spans only 1 micron. As the silk is spun into a
cocoon, it forms a single fiber that, stretched out, would span a mile! In contrast, cotton fibers
are composed of relatively short segments that are overlapped and joined together. Hair, too, is
made of short proteins joined in an overlapping arrangement.
What gives silk proteins this remarkable property?