Biology Reference
In-Depth Information
O
H
R
1
C
O
O
H
C
R
2
C
H
C
O
O
Alcohol
O
O
H
C
P
H
OH
O
O
Alcohol
O
P
O
-
Phospholipid Name
Abbreviation
Alcohol
Phosphatidic Acid
PA
None
Phosphatidylethanolamine
PE
Ethanolamine
Phosphatidylcholine
PC
Choline
Phosphatidylserine
PS
Serine
Phosphatidylinositol
PI
Inositol
Phosphatidylglycerol
PG
Glycerol
Cardiolipin (Diphosphatidylglycerol)
CL
Phosphatidylglycerol
FIGURE 5.6
Simplified structure for phospholipids. Also included are the names and associated alcohol for
each of the 7 major phospholipid classes in mammalian membranes.
Phosphate
Phosphoric acid is a tri acid with 3 different dissociable groups having pKas of 12.8,
7.2 and 2.1 (
Figure 5.9
). As a first alcohol (glycerol) is esterified to phosphate, the highest
pKa, 12.8, is eliminated. Attachment of a second alcohol removes the next highest pKa,
7.2, leaving only the lowest pKa of 2.1. Since 6 of the 7 common phospholipids have
2 phosphate oxygens esterified, the only dissociable phosphate group left has a pKa of
2.1. At physiological pH this group is almost totally dissociated (approximately
99.999% dissociated) resulting in a full negative charge on the phosphate.
All 7 classes of phospholipids are amphipathic molecules that contribute to lipid bilayer
structure. They vary from no net charge (they are zwitterions) to anions. None have a net
positive charge at physiological pH. Therefore one function of most phospholipids is to
provide negative charge density to membranes. In addition, each phospholipid class
undoubtedly provides other, perhaps unique functions to membranes. However, it is clear
that phospholipids have no inherent catalytic activity. Catalysis is provided by resident
