wavelength anomalous dispersion (SAD) phasing techniques. The SAD tech-
nique collects one dataset at one wavelength. This yields two possible phasing
solutions. The mean is taken of these to calculate an electron density map
suffi ciently detailed to identify the metal ion ’ s location. The MAD technique
collects multiple data sets at different wavelengths. This is useful to identify
different metal ions because the anomalous dispersion value varies strongly
with wavelength.
In the PDB: 1YEW structure, the zinc ion site in pmoC is not identifi ed in
pMMO by other spectroscopic techniques such as ICP – AES. The reference
190 authors believe that this zinc ion is either adventitious (perhaps depositing
from the crystallization buffer) or in a location usually occupied by another
metal ion, possibly another copper ion or an iron ion in vivo.
The reference 190 authors come to the following conclusions for the PDB:
1YEW pMMO structure: (1) At 2.80 - Å resolution and using SAD and MAD
data collection techniques, the metal ions and their coordinating amino acid
ligands can be identifi ed; however, determining meaningful bond distances are
not possible; and (2) at 2.80 - Å resolution, small exogenous ligands and water
(solvent or ligating) cannot be identifi ed or placed with accuracy, although the
dinuclear copper site in pMMO probably has bridging ligands. In general, X -
ray crystallography is an excellent technique for determining metal ion stoi-
chiometry, locating metal ions within a protein fold, and identifying amino acid
ligands. Using anomalous data collection techniques, SAD and especially
MAD, researchers can identify specifi c metal ions in the crystal. Determining
metal ion oxidation states in crystals is problematic, and any determination
should be confi rmed by single - crystal spectroscopy or other experimental
techniques. Bond distances and angles (especially for light atoms), as well as
detection of exogenous ligands, depend critically on the resolution of the
crystal structure. Very few crystal structures are determined at the 1.8 - to 2.0 - Å
resolution needed to achieve these measurements accurately. The authors end
on a hopeful note, stating that the technological advances in synchrotron radia-
tion and in crystallographic hardware and software, plus the use of robotics
for protein expression and crystallization, will result in more and better X - ray
crystallographic structures of biomolecules.
7.10 CONCLUSIONS
Chapter 7 has reported on the importance of iron in biological species. Because
iron is the most abundant transition metal found in biological species, one
would expect a wide variety of iron - containing proteins and metalloenzymes.
Only a few of these have been treated in any detail in this chapter. Little or
no mention has been made of how or why iron ions evolved to be the most
biologically abundant transition metal ions; probably their usefulness in redox
situations and for electron transport has something to do with their popularity.
Iron homeostasis in biological species has not been discussed, although this
CONCLUSIONS 465