Biology Reference
In-Depth Information
POPC is 16:0,18:1 PC and PSM is sphingomyelin where the variable chain is palmitic
acid. Assembling this complex phase diagram required a combination of various tech-
niques from several sources. The diagram gives the composition and boundaries of
the lipid rafts. Rafts are present in several phases (see
Figure 10.15
legend for details).
Raft size varies: insert A has large rafts (
>
75
100 nm); B has intermediate size rafts
e
(~20 nm to ~75
20 nm).
Since three-component phase diagrams are very hard to construct and interpret, it is hard
to imagine what a biologically relevant phase diagram with hundreds of lipids would look
like! Phase diagrams do have significant practical limitations.
100 nm), and C has small rafts (
<
e
D. LIPID
PROTEIN INTERACTIONS
e
1. Annular Lipids
One of the first, and most basic questions asked about membrane structure concerned the
possible existence of a single layer of special lipids tightly associated with membrane
proteins
[39]
. It is obvious that an integral protein must be solvated by lipids in the
surrounding bilayer. This layer came to be known as 'annular' or 'boundary' lipids. But
does this layer have the same lipid composition as the surrounding bilayer, or is it somehow
different? The name 'annular' or 'boundary' implies a static lipid layer with some perma-
nence surrounding the protein. This would require a significant attractive interaction
between the annular lipid and the integral protein. But does such a unique layer actually exist
and is it of different composition than the surrounding bulk membrane? An early attempt to
investigate the annular lipid ring compared the rate of nearest neighbor exchange between an
annular lipid and an adjacent bulk bilayer lipid to the lateral exchange rate between two bulk
bilayer lipids (
Figure 10.16
).
EXCHANGE
EXCHANGE
A - B
B - B
INTEGRAL
PROTEIN
ANNULAR
LIPID
BULK
LIPID
FIGURE 10.16
Nearest neighbor exchange between annular (A) and bulk (B) bilayer lipids (A
e
B exchange)
and between two bulk bilayer lipids (B
B exchange).
e
