Figure 3.
Fig. 3. Electron density maps of the CREB bZIP divalent
cation-binding pocket. A, (2F[o] F[c]) omit
electron density map (gray wire mesh) contoured at 1 and
calculated with phases from the final model in which residues
307 to 321 of each monomer, the hexahydrated magnesium ion and
the water molecules involved in DNA binding were deleted and
followed by xyzb refinement until convergence, which removes
phase bias. Labeled are the hexahydrated magnesium ion, residues
Tyr307 and Glu312', and Lys304 and Arg301 from each subunit. B,
(F[o] F[c]) omit
electron densities using coefficients F[o](BaCl[2])
F[o](MgCl[2]) (contoured at 6 and
displayed as red wire mesh) or F[o](Na[2]WO[4])
F[o]((NH[4])[2]SO[4]) (contoured at 4 and
displayed as blue wire mesh) confirming unequivocally the
identity of the density in A to be a divalent cation. The slight
off-centering of the Ba^2+ ion is very likely the result of its
preferred hepta aquo coordination versus the hexa aquo
coordination of a Mg2+ ion. A and B were generated with O (15).
The above figure is reprinted
by permission from the ASBMB:
J Biol Chem
(2000,
275,
35242-35247)
copyright 2000.
F[c]) omit
electron density map (gray wire mesh) contoured at 1 
and
calculated with phases from the final model in which residues
307 to 321 of each monomer, the hexahydrated magnesium ion and
the water molecules involved in DNA binding were deleted and
followed by xyzb refinement until convergence, which removes
phase bias. Labeled are the hexahydrated magnesium ion, residues
Tyr307 and Glu312', and Lys304 and Arg301 from each subunit. B,
(F[o] 
6