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fusion of opposing vesicles. In cells, SNAREs provide direction and speciicity
and bring opposing bilayers closer to within a distance of 2-3 Å,
21,22
enabling
Ca
then leads to the expulsion
of water between the bilayers at the bridging site, leading to lipid mixing
and membrane fusion. Hence, SNAREs, besides bringing opposing bilayers
closer, dictate the site and size of the fusion area during cell secretion. The
size of the t-/v-SNARE complex is dictated by the curvature of the opposing
membranes
2+
bridging and membrane fusion. The bound Ca
2+
; hence, smaller the vesicle, the smaller the t-/v-SNARE ring
complex formed.
A unique set of chemical and physical properties of the Ca
2+
ion
makes it ideal for participating in the membrane fusion reaction. Calcium
ion exists in its hydrated state within cells. The properties of hydrated
calcium have been extensively studied using X-ray diffraction and neutron
scattering, in combination with molecular dynamics simulations.
31-34
The
molecular dynamic simulations include three-body corrections compared
with
24
quantum mechanics/molecular mechanics and molecular
dynamics simulations. First-principles molecular dynamics has also been
used to investigate the structural, vibrational and energetic properties
of [Ca(H
2
O)
n
]
2+
clusters and the hydration shell of the calcium ion.
32
These studies demonstrate that hydrated calcium [Ca(H
2
O)
n
]
2+
has more
than one shell around the Ca
2+
, with the irst hydration shell having six
water molecules in an octahedral arrangement.
32
In studies using light
scattering and X-ray diffraction of SNARE-reconstituted liposomes, it has
been demonstrated that fusion proceeds only when Ca
2+
ions are available
between the t-SNARE- and v-SNARE-apposed proteoliposomes.
21,22
Mixing
of t-SNARE and v-SNARE proteoliposomes in the absence of Ca
2+
leads to a
diffuse and asymmetric diffractogram in X-ray diffraction studies, a typical
characteristic of short-range ordering in a liquid system.
33
In contrast,
when t-SNARE and v-SNARE proteoliposomes in the presence of Ca
2+
are
mixed, it leads to a more structured diffractogram, with approximately a
12% increase in X-ray scattering intensity, suggesting an increase in the
number of contacts between opposing bilayers, established presumably
through calcium-phosphate bridges, as previously suggested.
21,22,34
The
ordering effect of Ca
2+
on inter-bilayer contacts observed in X-ray studies
21
is in good agreement with light, AFM and spectroscopic studies, suggesting
close apposition of PO-lipid head groups in the presence of Ca
2+
, followed by
the formation of Ca
2+
-PO bridges between the adjacent bilayers.
21,22,35
X-ray
diffraction studies show that the effect of Ca
2+
on bilayers orientation and
inter-bilayer contacts is most prominent in the area of 3 Å, with additional
appearance of a new peak at position 2.8 Å, both of which are within the
ionic radius of Ca
2+
.
21
These studies further suggest that the ionic radius of
Ca
2+
may make it an ideal player in the membrane fusion reaction. Hydrated
ab initio
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