330 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS: GROUP II
in nature in the solution state. Nevertheless, the models provide an important
and fascinating picture of a complex enzyme system. The lessons learned from
these studies can be applied to other members of the P - type ATPase superfam-
ily — including Na + , K + - ATPases (see Section 5.4.1) and H + , Na + - ATPases —
about which less structural information is available.
To introduce details of the SERCA1a protein, we will fi rst look at the large
reorganizations that take place between the Ca 2 E 1 and E 2 states of the enzyme.
Figure 1 of reference 102 shows representations of this Ca 2+ - ATPase with
calcium ions (PDB: 1SU4,^97 the Ca 2 E 1 state) and without calcium ions (PDB:
1IWO, the E 2 state).^102 The fi gure includes a representation of the SR mem-
brane as well as a simplifi ed scheme showing the Ca 2+ - ATPase cycle (see
Figure 6.28 ). The SR membrane representation in Figure 1 of reference 102 is
an insertion of a bilayer of dioleoylphosphatidylcholine (DOPC) generated
through a molecular dynamics calculation by the authors. The PDB: 1SU4 X -
ray crystallographic structure (with calcium ions in their binding sites) is
shown in this text as Figure 6.29. It is represented as E1Ca 2+ on the left in
Figure 1 of reference 102 (see also Figure 6.28 ).
Figure 6.29 C a 2+ - ATPase in the Ca 2 E 1 state as represented by the PDB: 1SU4 X - ray
crystallographic structure. Visualized using CambridgeSoft Chem3D Ultra 10.0 with
notations in ChemDraw Ultra 10.0. (Printed with permission of CambridgeSoft Cor-
poration.) (See color plate)
SR Membrane
M7 - M10
greenM1 - M2 magentaM3 - M6 orangeDomain A, 1-43, 124-235
yellowDomain N, 360-600
redDomain P, 330-359, 601-739
blueCalcium ionsCytoplasmLumen