that in turn interact with M4 and other membrane helices that contain Ca 2+
ligands. The overall effect is to move carboxylate Ca 2+ ligands and decrease
affi nity for calcium ions, allowing them to be released to the lumen.
An overall scheme for Ca 2+ - ATPase activity, based on structures of fi ve
different states in the Ca 2+ - ATPase cycle, has been proposed by Toyoshima
et al.^100 :
- Ca 2+ binds to the E 2 ground state, breaking the closed confi guration of
the A, N, and P domains and exposing the ATP - binding site. Ca 2+ ions
bind to sites in transmembrane helices M4, M5, M6, and M8. The cyto-
plasmic gate is open and Ca 2+ ions interchange with the cytoplasm.
- ATP binds, crosslinking the N and P domains. The ATP ’ s γ - phosphate
and its bound Mg 2+ ion bend the P domain. The N - domain makes contact
with the A domain in a strained conformation. Part of the M1 helix is
pulled up and forms a barrier to Ca 2+ ion exchange with the cytoplasm.
Calcium ions are occluded.
- Phosphoryl transfer to asp351 allows dissociation of ADP, and then the
N and P domain interface opens. The A domain rotates so that the TGES
loop closes into the gap left by the loss of ADP. The A domain rotation,
in turn, causes marked rearrangement in transmembrane helices, opening
the lumenal gate and releasing Ca 2+ ions into the lumen.
- The TGES loop fi xes a particular water molecule that catalyzes the
hydrolysis of the aspartylphosphate. Release of inorganic phosphate and
Mg2+ unbends the P domain. Transmembrane helices again rearrange to
close the lumenal gate.
There are differences between Sorensen et al.^95 and Toyoshima et al.^100
interpretations in the E 2 P and the E 2 P i states concerning the question, When
does the lumenal gate close? In Sorensen ’ s PDB: 1XP5 structure, E 2 P, the
lumenal gate has closed and asp351 is still phosphorylated. The Toyoshima
PDB: 1WPG E 2 P i structure appears to show that closing of the lumenal gate
is triggered by phosphate release. Resolution of this difference will come with
additional information on the complex mechanism and movements of the
Ca2+ - ATPase enzyme during its cycle. The reader should look for literature
updates on Ca 2+ - ATPase intermediates to answer these questions.
6.5 Conclusions,
Chapter 6 discussed Group II metal ions in biomolecules, concentrating on
magnesium ions in catalytic RNA and on two calcium - containing biomole-
cules: calmodulin and Ca 2+ - ATPase. Readers interested in the evolutionary
aspects of catalytic RNA as a precursor to the DNA - based life forms that exist
in the present time could begin by consulting the publications listed in
CONCLUSIONS 337
