Biology Reference
In-Depth Information
Fig. 8.4
The three-dimensional Ca
++
ATPase of muscle sarcoplasmic reticulum determined by
X-ray crystallography at 2.6
˚
resolution (Toyoshima et al. 2000). (
Left
) The enzyme has four
structural domains - (1) the nucleotide-binding domain denoted by
N
, (2) the phosphorylation site-
containing domain,
P
, (3) the actuator domain,
A
, and (4) the calcium-binding
M
domain. Blue
indicates the N terminus and red the C terminus. Transmembrane helix M5 is parallel to the plane
of the paper. The model on the
right
is rotated by 50
around M5 (
Right
Reproduced by permission
pump shown in the
left panel
using the program
MolMol
)
b
Phosphorylation
(
Occlusion
)
[
E
1
.
ATP.2Ca
++
][
E
2
]
[
E
1
.
P][
E
2
.2Ca
++
]
a
Ca
++
binding
c
Ca
++
release
[
E
1
][
E
2
] + Pi
[
E
1
.P][
E
2
] + 2Ca
++
d
Dephosphorylation
Fig. 8.5
The conformon-based mechanism of action of the sarcoplasmic/endoplasmic reticulum
Ca
++
ion pump. The ion pump proteins are written in bold letters. This mechanism has many
features that have been adopted from the models of Ca
++
ion pump proposed by MacLennan and
Green (2000) and by Myung and Jencks (1995) but is distinct from these models in several
important ways as explained in the text
related biochemical and kinetic data summarized by Myung and Jencks (1995) and
by MacLennan and Green (2000), can be integrated with the theoretical concept of
the conformon (Sects.
8.1
and
8.2
) to construct a detailed and molecularly realistic
mechanism of the action of the Ca
++
ion pump as shown in Fig.
8.5
.
