Lipids and Membranes
Moshtaghi-Kashanian
Pathological Biochemist
Kerman University of Medical Sciences
Lipids
• Lipids: a heterogeneous class of naturally occurring
organic compounds classified together on the basis of
common solubility properties
• They are insoluble in water, but soluble in aprotic
organic solvents, including diethyl ether, chloroform,
methylene chloride, and acetone
• Lipids include
• triacylglycerols, phosphodiacylglycerols, sphingolipids,
glycolipids, lipid-soluble vitamins, and prostaglandins
• cholesterol, steroid hormones, and bile acids
Fatty Acids
• Fatty acid: a long, un-branched chain carboxylic acid,
most commonly of 12 - 20 carbons, derived from
hydrolysis of animal fats, vegetable oils, or
phosphodiacylglycerols of biological membranes
• In the shorthand notation for fatty acids
• the number of carbons and the number of double bonds in the
chain are shown by two numbers, separated by a colon
Fatty Acids
Fatty Acids
Among the fatty acids most abundant in plants
and animals
• Nearly all have an even number of carbon atoms, most
between 12 and 20, in an un-branched chain
• The three most abundant are palmitic (16:0), stearic acid
(18:0), and oleic acid (18:1)
• In most unsaturated fatty acids, the cis isomer predominates;
the trans isomer is rare
• unsaturated fatty acids have lower melting points than their
saturated counterparts; the greater the degree of unsaturation,
the lower the melting point
Triacylglycerols
• Triacylglycerol (triglyceride): an ester of
glycerol with three fatty acids
• natural soaps are prepared by boiling
triglycerides (animal fats or vegetable oils)
with NaOH, in a reaction called
saponification (Latin, sapo, soap
Soaps
• Soaps form water-insoluble salts when
used in water containing Ca(II), Mg(II),
and Fe(III) ions (hard water)
Phosphoacylglycerols
• Phosphoacylglycerols (phosphoglycerides) are the second most
abundant group of naturally occurring lipids
• found almost exclusively in plant and animal membranes,
which typically consist of 40% -50% phosphoacylglycerols and
50% - 60% proteins
• the most abundant phosphoacylglycerols are derived from
phosphatidic acid, a molecule in which glycerol is esterified
with two molecules of fatty acid and one of phosphoric acid
• the three most abundant fatty acids in phosphatidic acids are
palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1)
Phosphoacylglycerols
• A phosphatidic acid
• further esterification with a low-molecular weight
alcohol gives a phosphoacylglycerol
• among the most common of these low-molecular
Phosphoacylglycerols
Phosphoacylglycerols
Phosphoacylglycerols
• A lecithin
Waxe
• Esters of long-chain fatty acids and alcohols
• from the Old English word weax = honeycom
Sphingolipids
Glycolipids
• Glycolipid: a compound in which a
carbohydrate is bound to an -OH of the
lipid
many glycolipids are derived from
ceramides
Steroids
• Steroids: a group of plant and animal lipids
that have this tetracyclic ring structure
• The features common to the ring system of
most naturally occurring steroids are
illustrated on the next screen
Steroids
Cholesterol
Androgens
• Androgens - male sex hormones
• synthesized in the testes
• responsible for the development of male
secondary sex characteristics
Androgens
Estrogens
• Estrogens - female sex hormones
• synthesized in the ovaries
• responsible for the development of female
secondary sex characteristics and control of
the menstrual cycle
Estrogens
Biological Membranes
• In aqueous solution, phosphoglycerides
spontaneously form into a lipid bilayer, with a backto-back arrangement of lipid monolayers
• polar head are in contact with the aqueous
environment
• nonpolar tails are buried within the bilayer
• the major force driving the formation of lipid
bilayers is hydrophobic interaction
• the arrangement of hydrocarbon tails in the interior
can be rigid (if rich in saturated fatty acids) or fluid
(if rich in unsaturated fatty acids)
Biological Membranes
• the presence of cholesterol increases rigidity
• with heat, membranes become more disordered; the
transition temperature is higher for more rigid
membranes; it is lower for less rigid membranes
Membrane Proteins
• Functions: transport substances across membranes,
receptor sites, and sites of enzyme catalysis
• Peripheral proteins
• bound by electrostatic interactions
• can be removed by raising the ionic strength
• Integral proteins
• bound tightly to the interior of the membrane
• removed by treatment with detergents or ultrasonification
• removal generally denatures them
Fluid Mosaic Model
• Fluid: there is lateral motion of components in the membrane;
• proteins, for example, “float” in the membrane and can
move along its plane
• Mosaic:components in the membrane exist side-by-side as
separate entities
• the basic structure is that of a lipid bilayer with proteins,
glycolipids, and steroids such as cholesterol embedded in it
• no complexes, as for example, lipid-protein complexes, are
formed
Membrane Transport
• Passive transport
• driven by a concentration gradient
• simple diffusion: a molecule or ion moves through an opening
created by a channel protein
• facilitated diffusion: molecule or ion is carried across a
membrane by a carrier protein
• Active transport
• a molecule or ion is moved against a concentration gradient
• see the Na+/K+ ion pump
Membrane Receptors
• Membrane receptors
• generally oligomeric proteins
• binding of a biologically active substance to a
receptor initiates an action within the cell
Lipid-Soluble Vitamins
• Vitamins are divided into two broad
classes on the basis of their solubility
. Those that are water-soluble
. Those that are lipid-soluble (and hence
classified as lipids)
The lipid-soluble vitamins include A, D, E, and K
Prostaglandins
• Prostaglandins: a family of compounds
that have the 20-carbon skeleton of
prostanoic acid
Prostaglandins
• Prostaglandins are not stored in tissues as
such, but are synthesized from membranebound 20-carbon polyunsaturated fatty acids
in response to specific physiological triggers
• one such polyunsaturated fatty acid is
arachidonic acid
Prostaglandins
• among those synthesized from
arachidonic acid are
Prostaglandins
• Research on the involvement of PGs in
reproductive physiology has produced
several clinically useful derivatives
• 15-Methyl-PGF2a is used as a therapeutic
abortifacient
Prostaglandins
• the PGE1 analog, misoprostol, is used for
prevention of ulceration associated with the
use of aspirin-like non-steroidal
antiinflammatory drugs (NSAIDs)
Leukotrienes
• Leukotrienes: derived from arachidonic acid
• found in white blood cells (leukocytes)
• an important property is constriction of smooth muscles,
especially in the lungs
Thromboxanes
• derived from arachidonic acid
• contain a four-membered cyclic ether within
a six-membered ring
induce platelet aggregation and smooth
muscle contraction