Transcript Slides

Chapter 10: Lipids
A brief introduction to lipids
Adapted from Wikipedia:
• Lipids are a broad group of naturally-occurring molecules that include: (i.) fatty acids,
(ii.) triacylglycerides, (iii.) phospholipids, (iv.) sphingolipids, (v.) waxes, (vi.) sterols,
(vii.) terpenes, and more.
• The main biological functions of lipids include energy storage, as structural components
of cell membranes, and as important signaling molecules.
• Lipids may be broadly defined as hydrophobic or amphiphilic small molecules; the
amphiphilic nature of some lipids allows them to form structures such as vesicles,
liposomes, or membranes in an aqueous environment.
• Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of
lipids called triacylglycerides.
• Although humans and other mammals use various biosynthetic pathways to both break
down and synthesize lipids, some essential lipids cannot be made this way and must be
obtained from the diet.
• Of the classes of biological molecules that we have covered in this class (proteins,
carbohydrates, nucleic acids, and now lipids), lipids are the most structurally
diverse…yet in some ways (i.e., they all contain nonpolar moieties), they are also quite
uniform.
(i.) Fatty acids
Adapted from Wikipedia:
• A fatty acid is a carboxylic acid often with a long
unbranched aliphatic tail (chain), which is either
saturated or unsaturated.
• The most abundant natural fatty acids have an even
number of carbon atoms because their biosynthesis
involves acetyl-CoA, a coenzyme carrying a two-carbonatom group (see fatty acid synthesis).
• Fatty acids are produced by the hydrolysis of the ester
linkages in a fat or biological oil (both of which are
triglycerides), with the removal of glycerol.
• It is proposed that the blend of fatty acids exuded by
mammalian skin, together with lactate and pyruvate, are
distinctive and enable animals with a keen sense of smell
to differentiate individuals.
Fatty acid structure
Saturated vs. unsaturated fatty acids
Fatty acid nomenclature
Some common fatty acids
Cis vs. trans unsaturated fatty acids
(ii.) Triacylglycerides (TAGs)
Adapted from Wikipedia:
• Glycerol + 3 FA = TAG.
• TAGs are the main constituent of vegetable oil and
animal fats.
• Chain lengths of the fatty acids in naturally occurring
triglycerides can be of varying lengths, but 16, 18 and 20
carbons are the most common.
• Most natural fats contain a complex mixture of individual
TAGs; consequently, they typically melt over a broad
range of temperatures.
• Cocoa butter is unusual in that it is composed of only a
few TAGs, which gives rise to a fairly sharp melting
point, causing chocolate to melt in the mouth without
feeling greasy.
Fat is stored as TAGs in lipocytes,
which are also known as adipocytes
Saponification = alkali hydrolysis of esters
(iii.) Phospholipids
Adapted from Wikipedia:
• Glycerol + 2 FA + phosphate + alcohol = phospholipid.
• Phospholipids are a class of lipids, which are the major lipid
component of all cell membranes as they can form bilayers.
• The 'head' of a phospholipid is hydrophilic (attracted to
water), while the hydrophobic 'tails' repel water.
o The hydrophillic head contains the negatively charged
phosphate group, and may contain other polar groups.
o The hydrophobic tail usually consists of long fatty acid
hydrocarbon chains.
• When placed in water, phospholipids form a variety of
structures depending on the specific properties of the
phospholipid.
Phospholipid structure and aggregation
Phospholipids are classified based
on their alcohol head group
Phospholipase is an enzyme that hydrolyzes phospholipids
into fatty acids and other lipophilic substances
(iv.) Sphingolipids
Adapted from Wikipedia:
• Sphingolipids are a class of lipids derived from the aliphatic
amino alcohol sphingosine.
• Ceramide is the fundamental structural unit common to all
sphingolipids…it consists of a fatty acid chain attached
through an amide linkage to sphingosine.
• There are three main types of sphingolipids, differing in
their head groups:
o Sphingomyelins
have
a
phosphorylcholine
phosphoroethanolamine head group.
or
o Glycosphingolipids are ceramides with one or more
sugar residues joined in a β-glycosidic linkage at the 1hydroxyl position.
 Cerebrosides have a single glucose or galactose at
the 1-hydroxy position.
 Gangliosides have at least three sugars, one of
which must be sialic acid.
Sphingolipid structure
Phosphatidylcholine vs. sphingomyelin
Myelin sheath
Dendrite
Axon terminal
Soma
Node of Ranvier
Schwann cell
Myelin sheath
Nucleus
Axon
Myelin sheath
Adapted from Wikipedia:
• Myelin is a dielectric (electrically insulating) material that
forms a layer, the myelin sheath, usually around only the axon
of a neuron.
• It is essential for the proper functioning of the nervous
system. Myelin is an outgrowth of a glial cell…Schwann cells
supply the myelin for peripheral neurons.
• Myelin is composed of about 80% lipid and about 20% protein.
o Myelin is made up primarily of a sphingolipid called
galactocerebroside.
o The intertwining of the hydrocarbon chains
sphingomyelin serve to strengthen the myelin sheath.
of
• Demyelination is the loss of the myelin sheath insulating the
nerves, and is the hallmark of some neurodegenerative
autoimmune diseases, including multiple sclerosis.
(v.) Waxes
Adapted from Wikipedia:
• Wax refers to beeswax or another substance with similar
properties...the traditional meaning, beeswax, refers to a
substance secreted by bees and used by them in constructing
their honeycombs.
• The term has come to refer more generally to a class of
substances with properties similar to beeswax: (a) plastic
(malleable) at normal ambient temperatures, (b) a melting
point above approximately 45 °C (113 °F) (which
differentiates waxes from fats and oils), (c) a relatively low
viscosity when melted (unlike many plastics), (d) insoluble in
water, and (e) hydrophobic.
• Chemically, a wax is a type of lipid that may contain a wide
variety of long-chain alkanes, esters, polyesters, etc.
o They are usually distinguished from fats by the lack of
triglyceride esters.
o Many biological waxes are mixtures of several different
types of lipids.
Beeswax is an ester of
palmitic acid with the alcohol triacontanol
(vi.) sterols
Adapted from Wikipedia:
• Sterols are
molecules.
an
important
class
of
organic
• They occur naturally in plants, animals and fungi,
with the most familiar type of animal sterol being
cholesterol.
• Cholesterol is vital to cellular function, and a
precursor to fat-soluble vitamins and steroid
hormones.
Cholesterol
Lipoprotein
(aka chylomicron)
Steroids derived from cholesterol
Steroids derived from cholesterol
(vii.) Terpenes = molecules constructed of isoprene
Adapted from Wikipedia:
• Terpenes are a large and varied class of hydrocarbons,
produced primarily by a wide variety of plants, particularly
conifers.
• They are the major components of resin and of turpentine,
which is produced from resin. In fact, the name "terpene" is
derived from the word "turpentine".
• In addition to their roles as end-products in many
organisms, terpenes are major biosynthetic building blocks
within nearly every living creature. Steroids, for example,
are derivatives of the triterpene squalene.
• When terpenes are modified chemically, such as by
oxidation or rearrangement of the carbon skeleton, the
resulting compounds are generally referred to as
terpenoids. Some authors will use the term terpene to
include all terpenoids. Terpenoids are also known as
Isoprenoids.
Some common terpenes
Conjugated terpenes are commonly used
pigments in plants and bird feathers
Many vitamins are terpenes