Lipids in Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 31

Chapter 3
Lipids in Membranes
3.1 Geometry and Nature of Lipids
The most important components that make up cell membranes are various types
of lipids. By cataloging lipid structures (referred to as lipidomics), eukaryotic cells
have been found to invest substantial resources in generating various types of lipids
[ 28 ]. Cells use about 5% of their genes to encode for the synthesis of these lipids.
Lipids perform a few general functions [ 32 ]. First of all, lipids are used for energy
storage, principally as triacylglycerol and steryl esters, in lipid droplets. The matrix
of cellular membranes is formed by polar lipids, which consist of a hydrophobic
and a hydrophilic portion. Furthermore, lipids act as first and second messengers in
signal transduction and molecular recognition processes.
A very comprehensive review of the lipid composition has been presented in a
recent paper [ 32 ]. In eukaryoticmembranes, themajor structural lipid components are
glycerophospholipids; that is, phosphatidylcholine (PC), phosphatidylethanolamine
(PE), phosphatidylserine (PS), phosphatidylinositol (PI), and phosphatidic acid (PA).
Their hydrophobic portion is a diacylglycerol (DAG), which contains saturated or cis -
unsaturated fatty acyl chains of varying lengths. Figure 3.1 shows simplified model
structures of these lipids [ 32 ].
PC appears to be the main lipid component (comprising more than 50% of the
cellular phospholipids) in most eukaryotic membranes. Most importantly, it is con-
sidered to be the neutral-type lipid, which is responsible for creating planar bilayers.
Inclusion of PE, which has a conical molecular geometry in a PC bilayer, raises cur-
vature stress in the membrane. The presence of lipids PE, PS, etc., and cardiolipin
(CL) in a planar bilayer formed by PC causes regulatory effects on membrane pro-
teins (MPs), antimicrobial peptides (AMPs), and ion channels/pores, etc. [ 4 ]. The
membrane regulation of integralMPs/AMPs functionswill be addressed in later chap-
ters of this topic. Organization of lipids in different bilayer phases is also different
[ 10 , 32 ]. Environmental parameters such as temperature, pH, ionic strengths, hydra-
tion, etc., of biological membrane regions determine how lipids assemble inside
membranes. Many publications have addressed these issues from various view points