Organic chemistry and Biological chemistry for Health Sciences
59-191
Lecture 16
Non-hydrolyzable lipids:
These are compounds lacking groups that react with water.
Example
Steroids (include cholesterol and many sex hormones).
Tarpenes (responsible for pleasant odors of many plant oils).
Fatty acids are among the products when hydrolyzable lipids react with water.
Structural features of the common fatty acids:
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They are usually monocarboxylic acids
R group is usually a long unbranched chain
Number of carbon atoms is almost always even
R group can be saturated, or it can have one or more double bonds, which are cis
The most abundant saturated fatty acids are palmitic acid, and stearic acid which have 16
and 18 carbons respectively.
The unsaturated fatty acids most commonly obtained from lipids are listed in table 19.1.
All have cis double bonds. Oleic acid is most abundant most widely distributed fatty acid
in nature.
Aspirin is known to combat both pain and fever by inhibiting postaglandin synthesis. It
does so by inactivating a key catalyst, an enzyme called PGH2 synthase that is essential
for postaglandin synthesis. Postaglandins are made from arachidonic acid, one of the
essential fatty acids.
The molecules of the most abundant lipids are triacylglycerols or triglycerides, ester
between glycerol and fatty acids.
The three acyl units in the triacylglycerol molecules of a given fat or oil are usually
contributed by two or three different fatty acids. Triacylglycerols are neutral fats. In a
particular fat or oil, certain fatty acids predominate, others either are absent or are present
in trace amounts, and virtually all of the molecules are triacylglycerols.
Oleic acids is the most abundant monounsaturated fatty acid, and its acyl group is very
common among both the fats and oils. Unsaturated fat, like vegetable oil, incorporates
more of the acyl groups of the polyunsaturated fatty acids. The saturated fatty acid units
are far more common in animal fats, which are thus called saturated fats.
The three acyl groups in the triacylglycerol molecules of a given fat or oil are usually
contributed by two or three different fatty acids. So fats and oils are actually mixtures of
different molecules that share common structural features.
Chemical properties of triacylglycerol:
A special enzyme, (a lipase) hydrolyze triacylglycerols stored in our fat tissue completely
to give glycerol and a mixture of fatty acids.
The saponification of the ester links in triacylglycerols by the action of strong base (e.g.
NaOH or KOH) gives glycerol and a mixture of the salts of fatty acids. The salts make up
“old fashioned” soap.
PHOSPHOLIPIDS:
Phospholipids are esters of either glycerol or sphingosine, which is a long-chain, dihydric
amine alcohol with one double bond. All phospholipids have very polar ionic sites in
addition to long hydrocarbon chains. The polar part of the phospholipid is extremely
important for the formation of cell membrane.
Glycerophospholipids:
These lipids have phosphate units and two acyl units. They can be classified in two
different groups: phosphatides and plasmalogens. Both are esters of glycerol.
Phosphatides:
Molecules of phosphatides have two ester bonds to fatty acids plus one ester bond to
phosphoric acid. The phosphoric acid unit, in turn, is joined by a phosphate ester link to a
small alcohol molecule. Without this link, the compound is called phosphatidic acid.
Three particularly important phosphatides are esters between phophatidic acid and either
choline, ethanolamine, or serine, forming respectively phophatidylcholine (lecithin),
phosphatidylethanolamine (cephalin) and phosphatidylserine.
One part of each phosphatide molecule is very polar and the remainder is non-polar. The
polar part carries full electrical charge that makes phosphatide more soluble in water
compared to triacylglycerols.
These three are the most common hydrolyzable lipids used to make animal membranes.
STEROIDS:
Steroids are aliphatic compounds of high formula mass whose molecules include a
characteristic four ring feature called the steroid nucleus. It has three six membered rings
and one five membered ring.
Cholesterol is an unsaturated steroid alcohol that makes up a significant part of the
membranes of animal cells. The membrane of human red blood cell is 25% cholesterol
by mass. So cholesterol is essential to all cell membranes.
Cholesterol is the raw material for making bile salts and steroid hormones like estradiol,
progesterone, testosterone and androsterone.
Cholesterols enter in the body via the diet, but up to 800 mg per day can be synthesized
in the liver from two carbon acetate units. Some cholesterol made in the body is
converted to esters of cholesterol and some is used to make bile salts, like sodium
cholate.
Bile salts are made in the liver and secreted into the intestinal tract, where they act as a
powerful surface-active agent. They aid in both in the digestion of dietary lipids and also
absorption of fat-soluble vitamins and fatty acids from the digestive tract into circulation.
Cell membranes are made of both lipids and proteins. The principal lipids in the cell
membrane are phospholipids and glycolipids, as well as cholesterol.
Phospholipids and glycolipids, with both hydrophilic and hydrophobic groups, are called
amphipathic compounds. The polar and ionic sites of phospholipids and glycolipids are
called hydrophilic groups because they are able to attract water molecules. The non-polar
hydrocarbon sections of membrane lipids are called hydrophobic groups because they are
water avoiding. Amphipathic phospholipids form a lipid biayer, with the non-polar
hydrophobic region facing each other at the core of the bilayer and their polar head group
facing outward. By forming the bilayer the hydrophobic part can get as far away from
water molecules as possible. The hydrophilic heads stick out into the aqueous phase to be
in contact with water as much as possible.
Globular proteins are embedded in lipid bilayer at regular intervals, held by hydrophobic
interactions between lipids and hydrophobic domains of the proteins. They perform
services such as accepting hormone molecules and relaying them or their messanger
inside providing passages-closable molecular channels-for small ions and molecules, and
acting as pumps to move solute across a cell membrane.
The individual lipid and protein subunits in a membrane form a fluid mosaic, but it is free
to change constantly. The membrane mosaic is fluid because the interaction among lipids,
and between lipids and proteins, are non-covalent, leaving individual lipid and protein
molecules free to move laterally in the plane of the membrane.
Cholesterol molecules help to stabilize the membrane. In the lipid bilayer they occur with
their long axes lined up side by side with the hydrocarbon chains of the other lipids. The
cholesterol OH groups are hydrogen bonded to O atoms of ester groups of membrane
lipid molecules. Because cholesterol molecules are relatively rigid, cholesterol molecules
help to keep a membrane from being too fluidlike.