336 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS: GROUP II
remain relatively static. In addition, the P domain rotates approximately 30 °
relative to the M7 – M10 segment. All these motions are relative to the
Ca 2 E 1 P · ADP model (PDB: 1T5T). The rearrangements and movements desta-
bilize the Ca 2+ binding sites in the membrane but at the same time prevent
Ca2+ movement back to the cytoplasm. The rearrangements have permitted
Ca2+ release into the lumen in exchange for 2 – 3 protons, most likely now pro-
tonating some of the former carboxylate Ca 2+ - binding sites. At the phosphory-
lation site, the AlF 4 − group is located between the asp351 residue (P domain)
and a water molecule hydrogen - bonded to the main chain carbonyl of thr181
and the side - chain carboxyl of glu183. These residues are part of the important
A domain TGES motif as mentioned above. It is believed that the glu183 side
chain provides the general base catalysis, abstracting a proton from the water
molecule for attack on the phosphorylated residue (here represented by AlF 4 −).
In addition, the TGES motif occupies the former position of ADP between
the N and P domains. The associative reaction mechanism is invoked for
dephosphorylation (as it was for phosphorylation) because all participants in
the reaction are again in a linear arrangement. Reference 101 includes a sup-
plemental movie illustration of the Ca 2+ - ATPase enzyme action available at
http://www.sciencemag.org/content/vol306/issue5705/images/data/2251/DC1/
1106289s1.mov. This movie illustrates the following scenario. Calcium ions
enter binding sites within the membrane, are trapped there, and cannot
exchange with external calcium ions. ATP, complexed with magnesium, enters
and phosphorylates the pertinent amino acid residue in the presence of potas-
sium (or sodium) ion. Phosphoryl transfer from ATP to the enzyme closes the
entry hatch for calcium ions. Release of adenosine diphosphate (ADP) opens
the exit hatch and allows calcium to escape into the lumen in exchange for
protons. Two calcium ions leave in exchange with two or three protons. Finally,
release of phosphate and magnesium opens the calcium entry hatch and allows
the exchange of protons for calcium. The proposed sequence of events becomes
clearer if one steps through the movie after watching it at full speed a few
times.
While the Sorensen PDB: 1XP5 structure is thought to be in a E 2 P transi-
tion state, the Toyoshima group PDB: 1WPG structure is projected as the E 2 P i
ground state.^100 This Ca 2+ - ATPase model contains thapsigargin in the Ca 2+ site,
the tetrafl uoromagnesate ion ( MgF 42 −) as the bound phosphate analog, Mg 2+ ,
and Na +. Additionally, ADP may bond to the PDB: 1WPG structure, although
as a model of the E 2 P i state, ADP ’ s site would be occupied by part of a rotated
A domain. In the PDB: 1WPG structure, the N and P domains have dissociated
somewhat because the ADP molecule holding them together has been lost.
Meanwhile the A and P domains have more tightly associated as compared to
the Ca 2 E 1 P · ADP modeled by the PDB: 1WPE structure. In fact, the PDB:
1WPG domain A has rotated approximately 110 ° around an axis perpendicu-
lar to the membrane compared to its position in PDB: 1WPE. Rearrangements
in the cytoplasmic domains A, N, and P cause marked changes in the trans-
membrane helices as well. Rotation of the A domain tilts the M1 – M2 helices