Transcript CHAPTER 6

Reginald H. Garrett
Charles M. Grisham
www.cengage.com/chemistry/garrett
Chapter 8
Lipids
Reginald Garrett & Charles Grisham • University of Virginia
Outline
• What are the structures and chemistry of fatty acids?
• What are the structures and chemistry of triacylglycerols?
• What are the structures and chemistry of
glycerophospholipids?
• What are sphingolipids, and how are they important for
higher animals?
• What are waxes, and how are they used?
• What are terpenes, and what is their relevance to
biological systems?
• What are steroids, and what are their cellular functions?
• How do lipids and their metabolites act as biological
signals?
• What can lipidomics tell us about cell, tissue, and organ
physiology?
Classes of Lipids
All biological lipids are amphipathic
•
•
•
•
•
•
Fatty acids
Triacylglycerols
Glycerophospholipids
Sphingolipids
Waxes
Isoprene-based lipids (including steroids)
8.1 What Are the Structures and Chemistry
of Fatty acids?
8.1 What Are the Structures and Chemistry
of Fatty acids?
•Fatty acids are comprised of
alkyl chains terminated by
carboxylic acid groups.
•Shown here is palmitic acid, a
16-carbon saturated fatty acid.
•The term “saturated” indicates
that the acyl chain is fully
reduced, i.e., saturated with
hydrogens and electrons.
8.1 What Are the Structures and Chemistry
of Fatty acids?
Structural consequences of unsaturation
• Saturated chains pack tightly and form more
rigid, organized aggregates (i.e., membranes)
• Unsaturated chains bend and pack in a less
ordered way, with greater potential for motion
Fats in the human diet vary widely in their
composition
Diets high in trans fatty acids raise plasma
LDL cholesterol levels
Trans fatty acids are
present at low levels in
dairy and meat
products from ruminant
animals. “Partially
hydrogenated” fats in
foods contain much
higher amounts.
8.2 What Are the Structures and Chemistry
of Triacylglycerols?
Triacylglycerols are also called triglycerides
• They are a major energy source for many
organisms
• Why?
• Most reduced form of carbon in nature
• No solvation needed
• Efficient packing
8.2 What Are the Structures and Chemistry
of Triacylglycerols?
Most of the fatty acids
in plants and animals
exist in the form of
triacylglycerols. If all
three fatty acids are
the same, the
molecule is called a
simple triacylglycerol.
8.2 What Are the Structures and Chemistry
of Triacylglycerols?
Mixed triacylglycerols
contain two or three
different fatty acids.
Polar Bears Prefer Nonpolar Food
Polar bears face an ironic dilemma. They are
surrounded by water they cannot use. Ice and
snow are too cold and seawater is too salty. They
produce all the water they need from metabolism
of fat: (CH2) + 1.5O2 → CO2 + H2O
Interestingly, adult polar bears consume only fat
(from seals they catch). By not consuming
protein (and merely recycling their own proteins
into new ones), they have no need to urinate or
defecate and go for months without doing so, thus
saving precious body water.
8.3 What Are the Structures and Chemistry
of Glycerophospholipids
Glycerophospholipids are essential components of cell
membranes and are also found in other parts of cells.
8.3 What Are the Structures and Chemistry
of Glycerophospholipids
The structure of phosphatidylcholine.
8.3 What Are the Structures and Chemistry
of Glycerophospholipids
Phosphatidyl-β-D-glucopyranoside
• Phosphatidyl-β-D-glucopyranoside is a
glycophospholipid
• Identified in several cell types, it plays a role in
formation of plasma membrane signaling
microdomains involved in cellular differentiation and
maturation.
Ether Glycerophospholipids Include PAF
and Plasmalogens
Ether glycerophospholipids possess an ether
linkage instead of an acyl group at the C-1
position of glycerol.
• Plasmalogens are ether glycerophospholipids
in which the alkyl chain is unsaturated
Ether Glycerophospholipids Include PAF
and Plasmalogens
8.4 What Are Sphingolipids and How Are
They Important for Higher Animals?
• Sphingolipids represent another class of
lipids found frequently in biological membranes
• Sphingosine, an 18-carbon alcohol, forms the
backbone of these lipids rather than glycerol
• A fatty acid joined to sphingosine in amide
linkage forms a ceramide
• Glycosphingolipids are ceramides with one
or more sugars in beta-glycosidic linkage at
the 1-hydroxyl group
8.4 What Are Sphingolipids and How Are
They Important for Higher Animals?
• Glycosphingolipids with one sugar are
cerebrosides
• Ceramides with 3 or more sugars, one of
which is a sialic acid, are gangliosides
8.4 What Are Sphingolipids and How Are
They Important for Higher Animals?
Sphingolipids are based on the
structure of sphingosine.
Sphingosine is an 18-carbon alcohol.
Fatty acids joined in amide linkage at
the highlighted nitrogen form
ceramides.
8.4 What Are Sphingolipids and How Are
They Important for Higher Animals?
Figure 8.10 A ceramide is formed by
joining a fatty acid in amide linkage to
a sphingosine.
8.4 What Are Sphingolipids and How Are
They Important for Higher Animals?
A ceramide with a
phosphocholine head group
is a choline sphingomyelin.
8.4 What Are Sphingolipids and How Are
They Important for Higher Animals?
Gangliosides are
important
components of
muscle and
nerve
membranes.
8.5 What Are Waxes, and How Are They
Used?
•
•
•
•
•
Waxes are esters of long-chain alcohols with
long-chain fatty acids
Waxes are insoluble in water, due to their
mostly hydrocarbon composition
Animal skin and fur are wax-coated and are
water-repellent
Leaves of many plants and bird feathers are
similarly water-repellent
Carnauba wax, from a palm tree in Brazil, is a
hard wax used for high-gloss finishes for
automobiles, boats, floors, and shoes
Lanolin is a wool wax used in cosmetics; Oil of
Olay is named for its lanolin content
8.5 What Are Waxes, and How Are They
Used?
Waxes consist of long-chain alcohols esterified to long-chain
fatty acids. Triacontanol palmitate is the principal component of
beeswax.
8.6 What Are Terpenes, and What is Their
Relevance to Biological Systems?
Terpenes are a class of lipids formed from
combinations of isoprene units
•
•
•
•
•
“Isoprene” is 2-methyl-1,3-butadiene
Monoterpenes consist of two isoprene units
Sesquiterpenes consist of three isoprenes
A diterpene consists of four isoprene units
All steroids (including cholesterol and the
steroid hormones) are terpene-based
molecules
8.6 What Are Terpenes, and What is Their
Relevance to Biological Systems?
•Note the two possible linkage modes:
•“head-to-tail”
•“tail-to-tail”
The structure of isoprene (2-methyl-1,3-butadiene) and the
structure of head-to-tail and tail-to-tail linkages. Isoprene itself
can be formed by distillation of natural rubber, a linear head-totail polymer of isoprene units.
8.6 What Are Terpenes, and What is Their
Relevance to Biological Systems?
Many monoterpenes are readily recognized by their
characteristic flavors or odors (limonene in lemons; citronellal in
roses and perfumes; menthol used in cough drops).
8.6 What Are Terpenes, and What is Their
Relevance to Biological Systems?
The diterpenes
include retinal (the
visual pigment in
rhodopsin), and
phytol (found in
chlorophyll.
Gibberellic acid is a
plant hormone.
8.6 What Are Terpenes, and What is Their
Relevance to Biological Systems?
The triterpene
lanosterol is a
constituent of
wool fat and is
also a precursor
to cholesterol and
the other steroids.
Lycopene is a
carotenoid found
in ripe fruit,
especially
tomatoes.
8.7 What Are Steroids, and What Are Their
Cellular Functions?
• Steroids are polyprenyl (isoprene-based)
molecules built on a core structure of three 6membered rings and one 5-membered ring, all
fused together
• Cholesterol is the most common steroid in
animals and precursor for all other steroids in
animals
• Steroid hormones serve many functions in
animals - including salt balance, metabolic
function and sexual function
8.7 What Are Steroids, and What Are Their
Cellular Functions?
The structure of cholesterol, shown
with steroid ring designations and
carbon numbering.
8.7 What Are Steroids, and What Are Their
Cellular Functions?
Cortisol provides control of carbohydrate, protein, and lipid
metabolism
Testosterone is the primary male sex steroid hormone
Estradiol is the primary female sex steroid hormone
Progesterone is a precursor of testosterone and estradiol
8.8 How Do Lipids and Their Metabolites
Act as Biological Signals?
• Glycerophospholipids and sphingolipids play
important roles as chemical signals in and on cells
• Lipid signals act locally, either within the cell where
they are made or on nearby cells
• These signals typically initiate a cascade of reactions
with multiple effects
• The lifetimes of these signals are usually very short
• The creation and breakdown of lipid signals is
carefully regulated and timed
• Some of the reactions that produce these signals are
shown on the next slide
8.8 How Do Lipids and Their Metabolites
Act as Biological Signals?
Phospholipases A1 and A2
cleave fatty acids from a
glycerophospholipid,
producing
lysophospholipids.
Phospholipases C and D
hydrolyze on either side of
the phosphate in the polar
head group.
8.9 What Can Lipidomics Tell Us about
Cell, Tissue, and Organ Physiology?
• Many human diseases involve the disruption of lipid
metabolic enzymes and pathways.
• New techniques have made possible the global
analysis of lipids and their interacting protein partners
in organs, cells, and organelles – an approach termed
lipidomics
• Typical cells contain over a thousand different lipids
• Complete understanding of lipid function will require
the determination of which lipids are present and in
what concentrations
• Cellular lipidomics provides a framework for
understanding the myriad roles of lipids
End of Chapter Problems
• All problems can be done.