Biology Reference
In-Depth Information
The mechanism in Fig.
8.5
postulates that the ion pump can be divided into two
structural domains, denoted as
E
1
and
E
2
, both enclosed in a square bracket
marking their boundaries.
E
1
has a high affinity for Ca
++
and is accessible only
from the cytosplasmic side of the membrane, and
E
2
has a low Ca
++
affinity and is
accessible only from the luminal side. In step
a
, two calcium ions and one molecule
of ATP bind to
E
1
from the cytoplasmic side. In step
b
,
E
1
is phosphorylated causing
it to be occluded from the cytoplasmic side and the two Ca
++
ions are postulated to be
translocated from
E
1
to
E
2
domains (probably involving a decrease in the Ca
++
-
binding affinity of
E
1
and an increase in that of
E
2
, driven by appropriate
conformons). In step
c
,
E
2
opens toward the luminal side and the Ca
++-
binding
affinity of
E
2
decreases (again presumed to be driven by conformons), thus releasing
Ca
++
ions into the lumen. In step
d
,
E
1
is dephosphorylated to regenerate the original
E
1
and
E
2
. It should be pointed out that the [
E
1
.P][
E
2
.2Ca
++
] state is thought to be
ADP-sensitive (i.e., this complex can transfer the phosphoryl group to ADP added
from the cytoplasmic side, leading to the formation of ATP), which is consistent with
the observations made by Myung and Jencks [1995], but the [
E
1
.P][
E
2
] state is not.
There are three main features that are unique to the mechanism proposed in
Fig.
8.5
:
1. The Ca
++
-binding affinity of the
E
2
domain is not constant but depends on the
structural state of the ion pump as a whole (including the
E
1
domain). That is, the
model assumes that the binding affinity of the Ca
++
-binding sites in the
E
2
domain undergoes transitions among three states - high (H), intermediate (I),
and low (L).
2. The accessibility of the Ca
++
-binding sites in the
E
2
domain is also not constant
but depends on the structural state of the ion pump as a whole. There are three
possible accessibilities - open to the cytoplasmic (C) side only, occluded from
both the cytoplasmic and luminal (L) sides, and open to the luminal side only.
3. The two calcium-binding sites are postulated to be positioned vertically relative
to the plane of the membrane, separated by less than 40-50
˚
, the thickness of
the membrane. The X-ray crystal structure of Toyoshima et al. (2000) indicates
that the two Ca
++
ion binding sites are located side by side horizontally in the
interior of the membrane contrary to what is postulated here. The reasons for this
discrepancy is not clear but may include the possibility that the horizontal
arrangement of the Ca
++
ions seen by Toyoshima et al. (2000) is an artifact of
protein crystallization.
Characteristic features (1) and (2) are combined and represented as in
Scheme
8.18
:
H
(Open to C side)
$
I
(Occluded)
$
L
(Open to L side)
(8.18)
All of the three characteristics of the proposed mechanisms of the Ca
++
ion pump
can be visualized as shown in Fig.
8.6
. The formation of the occluded state is
probably coincident with the phosphorylation of the C domain.
