3.2 b-Sheets
* major structural element in globular proteins - 20-28%
- like the a helix , a repeating
secondary structure
* proposed in the 1930's from diffraction
data
* The basic unit of a b-sheet is a b-strand
(=> a helix with n = 2
residues/turn)
* backbone dihedral angles phi = -120 and
psi
= +120
* a translation of 3.2 to 3.4 Angstroms/residue
for residues in antiparallel and parallel strands, respectively.
* no intrasegment H-bonds and van
der Waals interactions between atoms of neighboring residues
=> between aligned strands
* sometimes called the beta "pleated" sheet
=> Two forms designated as "Antiparallel"
or
"Parallel"
based
on the relative directions of two interacting
b strands.
=> The average length of a b-sheet is about
6
residues
=> Side chains from adjacent residues of a
strand in a b-sheet are found on opposite
sides of the sheet => no interaction
=> like a-helices, b-sheet
have the potential for amphiphilicity with one face polar and the other
apolar.
=> unlike a-helices, b-sheet
are formed from strands that are very often from distant portions of the
polypeptide sequence.
=> Hydrogen bonds in b-sheets
are on average 0.1 Angstrom shorter than those
found in a-helices.
Figure 8. The protein thioredoxin (2TRX.PDB) contains a five-stranded
beta sheet comprised of three parallel strands
and three antiparallel strands.
download
2TRX.PDB
=> Half of the backbone H-bond donors and acceptors in strands at the edges
of a b-sheets are
uninvolved in the sheet interactions.
=> No capping residues in b-sheets
have not yet been characterized
Figure 9. Hydrogen bond patterns in beta sheets.
Here a four-stranded beta sheet is drawn schematically which contains three
antiparallel and one parallel strand. Hydrogen bonds are indicated with
red lines (antiparallel strands) and green lines (parallel strands) connecting
the hydrogen and receptor oxygen.