During the cycle, large conformational changes take place within domains
of the enzyme to make sure that calcium ions do not “ leak ” back into the
cytoplasm but are “ occluded ” at binding sites within the SR membrane portion
of the enzyme until such time that the enzyme ’ s “ lumenal gate ” opens to
release the Ca 2+ ions to the lumen.
6.4.2.1 Ca2+-ATPase Protein SERCA1a and the Ca 2+-ATPase Cycle. In the
following paragraphs we will discuss the structural information that has been
gathered on crystallizations of the Ca 2+ - ATPase protein from rabbit fast - twitch
muscle known as SERCA1a. This will be done through descriptions of the
various X - ray crystallographic structures corresponding to stages described in
Figure 6.28. The Ca 2 E 1 state was fi rst described (PDB: 1SU4) by Toyoshima
et al.^97 This was the fi rst atomic - scale model for any P - type ATP - dependent
pump. The Ca 2 E 1 state is compared to the E 2 state (PDB: 1IWO) as described
in reference 98. In the PDB: 1IWO X - ray structure, calcium ions are replaced
by thapsigargin (TG), a Ca 2+ - ATPase cycle inhibitor.
Very large conformational changes take place between the Ca 2 E 1 and E 2
states. (See Figure 6.28 .) Moller and co - workers^95 described the structure of a
Ca 2 E 1 · ATP intermediate in the cycle with Ca 2 E 1 - adensoine ( β – γ - methylene) -
triphosphate, AMPPCP (PDB: 1T5S). AMPPCP is an inactive ATP analog that
occupies the normal ATP site in the enzyme. Later, the Ca 2 E 1 · ATP state is also
modeled in the Toyoshima structure deposited as PDB: 1VFP.^99 This structure
contains the AMPPCP ATP analog along with calcium and magnesium ions.^99
Moller and co - workers^95 described the Ca 2 E 1 P · ADP intermediate with the
structure containing AlF 4 −as a phosphate analog (PDB 1T5T). Toyoshima
et al.^100 have also described a Ca 2 E 1 P · ADP intermediate containing ADP, AlF 3 ,
calcium and magnesium ions (PDB: 1WPE) and an E 2 P intermediate state
containing ADP, MgF 42 −, magnesium and sodium ions, and thapsigargin in the
calcium ion sites (PDB: 1WPG). Toyoshima has discussed the PDB: 1WPG
structure as an example of the E 2 P state, although Moller and co - workers^101
have published the PDB structure PDB: 1XP5 ( AlF 4 −, magnesium, and potas-
sium ions stabilized by thapsigargin in the calcium ion sites) as an example of
the E 2 P state. The differences center on the point at which phosphate ions are
released to the lumen during the cycle. In this discussion, the Toyoshima group
descriptions are followed. Interested readers should consult the more recent
literature for updates on the ongoing structural information leading to more
accurate mechanistic detail for Ca 2+ - ATPases. One structure eagerly sought by
those working in this fi eld would be one in which the asp351 residue is phos-
phorylated. Toyoshima et al.^100 have modeled a phosphorylated aspartic acid
residue from a bacterial response regulator, CheY (PDB: 1QMP), that uses
identical residues for stabilization of the phosphoryl group. The modeled
aspartylphosphate has been inserted into the E 2 P state structure (PDB: 1WPG)
containing ADP, MgF 42 −, magnesium and sodium ions, and thapsigargin in the
calcium ion sites. It is important to remember that all the Ca 2+ - ATPase models
discussed are solid - state equilibrium structures of a dynamic system operating
PHOSPHORYL TRANSFER: P-TYPE ATPASES 329