Biology Reference
In-Depth Information
21
C and CaCl
2
levels down to
50
C, accounting for the failure of the
of water down to
Don Juan Pond to freeze.
What is most surprising is the fact that the Don Juan Pond supports a variety of life! Micro-
flora of yeasts, blue-green algae, fungi, and bacteria have all been shown to inhabit the pond,
although they may lack extensive capability for continuous carbon reduction. In a 1979 article
in Nature, Siegel et al.
[55]
reported an extensive, irregular pellicle or mat-like structure
2
e
5 mm thick inhabiting the pond. Regardless of how abundant these Don Juan Pond organ-
isms are, they certainly are a prime example of homeoviscous adaptation.
SUMMARY
Membrane properties must support resident protein activity. One measurable property is
the 'packing free volume' that quantifies the required breathing space for enzyme activity.
Several important membrane properties, including area/molecule, collapse point, 'lipid
condensation', surface elasticity, and molecular 'squeeze out' have been assessed with lipid
monolayers using a Langmuir Trough. It was demonstrated that lipids vary in their lateral
compressibility. Cholesterol and di-saturated phospholipids are poorly compressible, while
polyunsaturated phospholipids are highly compressible. The lateral pressure of a biological
membrane is estimated to be ~30
e
35 mN/m. Freeze fracture EM has supplied direct
evidence that proteins indeed reside inside membranes and, without cytoskeleton involve-
ment, there is no long-range order. Observed order is only a few tenths of a micron and
the membrane bilayer sea is very crowded. Membrane domains can be roughly divided
into large, stable macrodomains (e.g. gap junctions, clathrin-coated pits, caveolae) and small,
unstable lipid microdomains (e.g. lipid rafts). In general, membrane properties are main-
tained by homeoviscous adaptation.
Chapter 12 will discuss the methodologies used to homogenize a cell and then isolate,
purify, and analyze the various cell membrane fractions.
References
[1] Slater SJ, Kelly MB, Taddio FJ, Ho C, Rubin E, Stubbs CD. The modulation of protein kinase C activity by
membrane lipid bilayer structure. J Biol Chem 1994;269:4868
e
71.
[2] Epand RM. Lipid polymorphism and protein
e
lipid interactions. Biochim Biophys Acta 1998;1376:353
e
68.
[3] Straume M, Litman BJ. Influence of cholesterol on equilibrium and dynamic bilayer structure of unsaturated
acyl chain phosphatidylcholine vesicles as determined from higher order analysis of fluorescence anisotropy
decay. Biochemistry 1987;26:5121
e
6.
[4] Mitchell DC, Straume M, Litman BJ. Role of sn-1-saturated, sn-2-polyunsaturated phospholipids in control of
membrane receptor conformational equilibrium: Effect of cholesterol and acyl chain unsaturation on the
metarhodopsin I
metarhodopsin II equilibrium. Biochemistry 1992;31:662
e
70.
[5] Litman BJ, Mitchell DC. A role for phospholipid polyunsaturation in modulating membrane protein function.
Lipids 1996;31:S193
e
7.
[6] Mitchell DC, Gawrisch K, Litman BJ, Salem Jr N. Why is docosahexaenoic acid essential for nervous system
function? Biochem Soc Trans 1998;26:365
e
70.
[7] Mitchell DC, Litman BJ. Molecular order and dynamics in bilayers consisting of highly polyunsaturated
phospholipids. Biophys J 1998;74:879
e
91.
[8] Mitchell DC, Litman BJ. Effect of cholesterol on molecular order and dynamics in highly polyunsaturated
phospholipids bilayers. Biophys J 1998;75:896
e
908.
4
