Biology Reference
In-Depth Information
Acyl Chain Double Bonds
Double bonds reduce fatty acid T
m
s and so also accumulate in membranes of organisms
exposed to lower temperatures. For example, in cold acclimation the proportion of di-
unsaturated PC and PE in plants increases 50%. In bacteria, double bonds are often replaced
by adding methyl branches or cyclopropyl rings (see Chapter 4). Phytol chains also contain
methyl branches and so can interfere with tighter acyl chain packing associated with cold
temperature.
Phospholipid Head Groups
Phospholipids with identical acyl chains often have widely different T
m
s. Most important
for homeoviscous adaptation are the two major membrane structural phospholipids, PC and
PE. The T
m
s for PEs are usually
20
C or higher than homologous PCs. Therefore another
way to increase membrane 'fluidity' in colder temperatures is to increase the membrane PC/
PE ratio.
>
Sterols
Cholesterol is known to fluidize gel state membranes and reduce the fluidity of fluid state
membranes (Chapter 5 and Chapter 9). Therefore membranes exposed to low temperature
may employ cholesterol or other similar sterols to fluidize their membranes.
Isoprene Lipids
Although far less abundant than membrane sterols, other isoprene lipids, such as vitamin
E and D, could also serve to fluidize membranes at low temperatures.
Anti-freeze Proteins
Anti-freeze proteins (AFPs) were first isolated by Arthur De Vries from Antarctic fish in
1969
[52]
. These proteins have since been found in a wide variety of organisms including
certain vertebrates, plants, fungi, and bacteria that survive in sub-zero environments.
AFPs do not function as ordinary anti-freeze solutes like ethylene glycol since they are effec-
tive at 300
e
500 times lower concentrations. They undoubtedly contribute to homeoviscous
adaptation of membranes.
Divalent Metals
Divalent metals can induce anionic phospholipids to undergo isothermal phase transi-
tions. By binding to the anionic head group, M
2
þ
s impose order to the entire phospholipid,
including the acyl chains. This can result in a substantial increase of the phospholipid's T
m
,
perhaps driving a liquid crystal state lipid into a gel state without any temperature change
(isothermal phase transition).
Table 11.3
shows the effect of Mg
2
þ
and Ca
2
þ
on the anionic
phospholipid, DPPG. While 100 mM NaCl has no effect on the T
m
of DPPG, 5 mM Mg
þ
2
increases the T
m
11
C and 5 mM Ca
2
þ
increases the T
m
a substantial 25
C. Another option
