Biology Reference
In-Depth Information
(e.g. palmitic and stearic acid) or with fatty acyl chains that have double bonds at position 9
(e.g. oleic acid, 18:1
D
9
) or farther (e.g. linoleic acid, 18:2
D
9,12
) down the chain. Therefore
cholesterol was said to fit into a '
D
9 pocket'. Cholesterol avoids fatty acyl chains with double
9 (e.g. arachidonic acid, 20:4
D
5,8,11,14
; eicosapentaenoic acid,
20:5
D
5,8,11,14,17
; and docosahexaenoic acid, 22:6
D
4,7,10,13,16,19
). Hydrophobic match between
cholesterol and a saturated fatty acid is required for the formation of lamellar liquid ordered
(l
o
) phase, the phase of lipid rafts.
Hydrophobic match is far more complex than it would initially appear. There are many
possible ways in which membranes can deal with a mismatch. Hydrophobic match is
a tool to regulate local bilayer thickness and hence membrane enzyme activity and
membrane trafficking.
bonds before position
D
E. LIPID INTERDIGITATION
Another unusual example of lipid polymorphism involves lipid interdigitation, where one or
both acyl chains extendbeyond the bilayermid-plane
[73
75]
.When this occurs, the penetrating
chain takes up residence in bothmembrane leaflets, profoundly affecting basicmembrane struc-
ture and hence function. Therefore interdigitation is yet another way to vary structural proper-
ties of the bilayer. Interdigitation normally occurswhen one chain is significantly longer than the
other (
Figure 10.24
), but can also be induced in symmetrical chain phospholipids. The most
common membrane lipids that normally exhibit substantial differences in their acyl chain
lengths are the large family of sphingolipids. Sphingolipids have a relatively short, permanent
sphingosine chain and a very long (often 24-carbons) variable chain (see Chapter 5). Therefore it
has been proposed that membrane domains that are enriched in sphingolipids (i.e. lipid rafts)
may exhibit considerable interdigitation into the opposite leaflet.
There are three basic interdigitated states termed partially interdigitated, mixed interdig-
itated, and fully interdigitated (
Figure 10.24
). Each interdigitated state is characterized, in
part, by the number of acyl chains subtended directly under the polar head group; 2 for
the partially interdigitated, 3 for the mixed interdigitated and 4 for the fully interdigitated
state. It is clear from the illustration in
Figure 10.24
that each state is characterized
by a different bilayer thickness. The thickest membrane would be associated with the non-
interdigitated state, while the thinnest bilayer would characterize the fully interdigitated
state. Membrane thickness is most accurately assessed by X-ray and neutron diffraction
(Chapter 9), although other methodologies including DSC, Raman and FTIR, NMR, electron
microscopy, ESR, and fluorescence have been employed. These other methodologies,
however, must be 'calibrated' with respect to X-ray diffraction.
e
Interdigitated-Lipid Shape
Most membrane lipids that have a tendency to interdigitate do so in the gel state and have
a large discrepancy in the length of the two acyl chains
[75]
. This discrepancy can be quan-
tified as the 'chain inequivalence'
[74]
:
Chain inequivalence
ΒΌ D
C
=
CL
