Chemistry Reference
In-Depth Information
reliable data collection on crystals which would have been considered too small for study some years ago); (iv)
better data collection facilities; and (v) the quasi-generalised use of multiple anomalous dispersion (notably by
replacement of methionine residues in the protein by selenomethionine residues) to resolve the 'phase problem'
(
Ealick, 2000
).
Structure determination of proteins requires the availability of an ordered sample in the form of a single
crystal. Since protein crystals typically contain large amounts of water, to prevent them from drying, and thereby
losing their regular ordered structure, they must be kept moist in the presence of the liquid of crystallisation during
data collection. This is usually achieved by mounting the wet crystals in small glass capillaries, which are placed
in a narrow beam of monochromatic X-rays. The crystal is then rotated in order to produce a diffraction
photograph. While we can record the intensities and hence the amplitudes of the X-rays diffracted by the crystal,
we cannot translate them into atomic structure without knowledge of both the amplitudes of the scattered beams
FIGURE 6.7
The structure of Bacillus brevis Dps dodecamer. (a) The BbDps dodecamer. The 12 subunits are shown by the ribbon repre-
sentation of their C
a
traces. The 24 iron ions at the ferroxidase centres inside the protein shell are shown as red spheres. (b) Ribbon diagram of the
BbDps monomer. Helices
a
I
e
a
V are shown in different colours. (c) A BbDps dimer. The iron ions in the two ferroxidase centres at the dimer
interface are shown as red spheres. The helices are coloured as in (b). (From
Ren et al., 2003
.
Copyright 2003 with permission from Elsevier.)
