Biology Reference
In-Depth Information
100
50
10
20
30
40
50
60
70
80
Mol phospholipid per mol ATPase
FIGURE 10.18
Activity of the Ca
2
þ
ATPase isolated from the sarcoplasmic reticulum as a function of phos-
pholipid content. It required ~30 phospholipids to support full activity (open circles). This provided an estimate of
the number of lipids required to completely surround the Ca
2
þ
ATPase once (the size of the annular lipid
boundary). Since in mixed PC/cholesterol membranes Ca
2
þ
ATPase activity exactly followed PC content and was
independent of cholesterol (filled circles), it was proposed that cholesterol was excluded from the annulus
[43]
.
then produced a second, narrow component to the spectra, indicating formation of the bulk
bilayer.
These and numerous other related reports call into question the classic definition of
a biological membrane being 'proteins floating like icebergs in a sea of phospholipids'.
Clearly the lipid bilayer 'sea' must be limited and very, very crowded.
Figure 10.19
shows
a typical example of a membrane cartoon drawing. Similar drawings can be found in most
biology, biochemistry, physiology, and cell biology textbooks. In this depiction a protein,
contrary to what is actually found, is surrounded by ~11 lipid bilayer rings! In the mito-
chondrial Cristae, a very busy membrane having ~75% protein by weight, a simple calcu-
lation indicates there is not enough lipid to totally surround the resident proteins even
once. Many of the mitochondrial proteins must actually be touching. On average, a plasma
membrane protein would have at best enough lipid to form ~2 concentric layers around
a protein. And one of these rings must form the annulus. An additional plasma membrane
problem is related to its very high cholesterol content, ~50 mol% (or more) of the polar
lipids. A report by Warren et al.
[43]
indicates that cholesterol is excluded from the
annulus. If this is in fact true (see discussion below), the second lipid ring, that
surrounding the annulus, must be composed almost entirely of cholesterol. But cholesterol
doesn't form bilayers! In the membrane with the lowest protein content, the myelin
sheath, the proteins would be surrounded by several lipid rings that could keep the
proteins ~11
˚
apart. So how large and crowded is the bilayer 'sea', and where does all
that cholesterol reside?
2. Co-Factor (Non-Annular) Lipids
So far we have seen that most of the annular lipids, if distinct annular lipids even exist at
all, are transient and weakly bound to integral proteins and the bilayer 'sea' is very small and
