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- Lipids include any biological compounds that are
insoluble in water and can dissolve in nonpolar
organic solvents.
CHAPTER 13: Lipids: Structure and Function LIPIDS ARE A DIVERSE CLASS OF MOLECULES Triacylglycerides Phospholipids Eisosanoids
- Lipids are highly diverse in both
function & structure:
1. Triglycerides (energy storage) 2. Phospholipids & glycolipids (membrane structure) 3. Sterols (cholesterol & steroids) 4. Eicosanoids (inflammation & pain) Sterols
- Pigments Ø Color of tomatoes, carrots, pumpkins, some birds
- Waxes (from algae & insects) Ø Waxes are lipids used to store energy (“bug fat”)
- Secreted oils (water repellents) Ø Prevents excessive wetting of skin (wet climate) Ø Prevents excessive evaporation (dry climate)
- Fat-soluble vitamins Ø Antioxidants (Vitamin E) Ø Blood clotting (Vitamin K) Ø Electron transfer (Coenzyme Q)
Lipids serve important functional roles at the tissue
and organism levels:
CHAPTER 13: Lipids: Structure and Function OTHER BIOLOGICALLY IMPORTANT LIPIDS
- Lipids are typically classified according to
differences in their structure & biological function:
- Two major structural classes of lipids:
- Lipids that contain or are derived from fatty acids a) Energy storage lipids (triglycerides) b) Structural lipids (membrane lipids) c) Signaling lipids (eicosanoids)
- Lipids that are not associated with fatty acids: a) Sterols (cholesterol & steroid hormones) b) Vitamins CHAPTER 13: Lipids: Structure and Function CLASSIFICATION OF LIPIDS Functional subclasses : Functional subclasses :
- 13.1 Energy Storage Lipids: Triglycerides
- 13.3 Fatty Acid Catabolism
- 13.2 Membrane Lipids: Phospholipids and
Glycolipids
- 13.4 Cholesterol and Other Steroid Hormones CHAPTER 13: Lipids: Structure and Function OUTLINE
- Fatty acids are long-chain hydrocarbons that end
with a carboxylic acid:
- Fatty acids can be saturated or unsaturated: Ø Saturated fatty acids are alkanes (all C-C bonds) Ø Unsaturated fatty acids are alkenes (have 1 or more C=C double bonds) Ø Polyunsaturated fatty acids have multiple C=C bonds CHAPTER 13: Lipids: Structure and Function FATTY ACID STRUCTURES Ø Typically 12-20 carbons in length Ø Naturally occurring fatty acids have an even number of carbons (due to their synthetic route)
- Two nomenclature systems are used to identify the length & double bonds in fatty acids:
- The delta system (blue) Ø The carbonyl carbon is designated as #1 carbon Ø double bonds specified with a ∆ symbol followed by the superscripted number of the first carbon in the double bond CHAPTER 13: Lipids: Structure and Function FATTY ACID CARBON NUMBERING Example: Linoleic acid Delta system: 18:2 ∆9, Omega system: 18:2 ω- 6
- The omega system (pink) Ø begins numbering with the carbon at the end of the chain (ω-1) Ø double bonds are specified relative to end carbon
General, Organic, and Biochemistry Chapter 6 FATTY ACID COMPOSITION IN TRIGLYCERIDES
- Triglycerides may contain a variety of fatty acids: Ø Animal fats are semi-solid, composed mostly of saturated fatty acids (eg. palmitic ) Ø Plant oils are liquid, and contain mostly unsaturated fatty acids (eg. linolenic acid ) CHAPTER 13: Lipids: Structure and Function THE SAT FAT GRAPH
- The graph below indicates the composition of fatty
acids in a wide variety of common triglycerides
- Which of these contain more energy per fatty acid...and why?
- Which of these are “healthier” for you to use in cooking?
- Arachidonic acid is an important polyunsaturated fatty acid (20:4Δ5,8,11,14) for production of eicosanoids :
- Non-steroidal anti-inflammatory drugs ( NSAIDs ) inhibit the enzymes that produce some eicosanoids: Ø Ibuprofen, aspirin, and naproxen are all in this class Ø Used to decrease aspects of inflammation such as fever, pain, itching, and swelling CHAPTER 13: Lipids: Structure and Function FATTY ACIDS AND INFLAMMATION Ø Eicosanoids are lipids involved in the regulation of inflammation, pain, blood clotting & muscle contraction
1. Which vegetable oils are high in saturated fat and
should, therefore, be included in limited amounts
in the diet?
2. Are most unsaturated fats obtained from
vegetable or animal sources?
3. Which product is healthier: coconut oil or canola
oil? Explain.
4. Why does arachidonic acid have a lower melting
point than linolenic acid, even though it has two
more carbon atoms in its chemical formula?
CHAPTER 13: Lipids: Structure and Function PRACTICE PROBLEMS
- For some tissues (liver and heart),
fatty acids provide most of the energy
needs for cells
- Saturated fats provide more energy
density than carbohydrates:
- Fatty acids carry more energy per carbon because they are more reduced
- Fatty acids carry less water per gram because they are nonpolar (higher density)
Fats are more efficient energy
storage molecules
CHAPTER 13: Lipids: Structure and Function WHY STORE ENERGY IN FAT? D-glucose 3.7 Cal/gram 9.6 Cal/gram Stearic acid
1. Stage 1 : Triglycerides are
hydrolyzed into glycerol and
fatty acids.
2. Stage 2 : Fatty acids are
oxidized to acetyl CoA.
3. Stage 3 : Acetyl CoA is
converted into ATP energy
and CO 2 via citric acid cycle
and oxidative phosphorylation.
CHAPTER 13: Lipids: Structure and Function STAGES IN LIPID CATABOLISM
- Fatty acid oxidation takes place in the
mitochondrial matrix :
- In several steps, the oxidation of fatty acids
produces acetyl CoA, NADH, and FADH 2.
Ø Steps occur in cycles removing 2 carbon units per cycle CHAPTER 13: Lipids: Structure and Function STAGE 2: FATTY ACID OXIDATION Ø Same location as the citric acid cycle Ø Products feed into the citric acid cycle & electron transport chain
- Import of fatty acids into the mitochondria prior to
catabolism requires that they first be “ activated ”
- Activation involves the formation of an acyl CoA , a
thioester similar to acetyl CoA:
Ø Enzymatic condensation reaction ( acyl CoA synthase ) Ø Requires energy from ATP to make thioester CHAPTER 13: Lipids: Structure and Function FATTY ACID ACTIVATION To mitochondrial matrix Note the cost : - 2 ATP equivalents are “invested”
- b - oxidation is a cycle of four reactions that are
repeated many times to break down the fatty acid
down by two carbons at a time:
CHAPTER 13: Lipids: Structure and Function b-OXIDATION
Ø The oxidation steps
occur at the third carbon
(the b-carbon)
Ø Each cycle produces 2
carbon units as acetyl-
CoA at each cycle
Acyl-CoA
1. The alkane acyl chain is
oxidized to an alkene
intermediate using FAD
2. The C=C double bond is
hydrated to a secondary
alcohol
3. The secondary alcohol is
oxidized to a ketone using
NAD+
4. Acyl chain transfer to a
new CoA-SH, while acetyl-
CoA is removed
CHAPTER 13: Lipids: Structure and Function SUMMARY OF b-OXIDATION
The products of b - oxidation are energy-rich!!!
- A saturated, 16-carbon fatty acid ( palmitate )
produces the following in 7 cycles:
- 8 two-carbon acetyl CoA units à to citric acid cycle
- 7 FADH 2 molecules
- 7 NADH molecules CHAPTER 13: Lipids: Structure and Function ENERGY YIELD IN b-OXIDATION The number of b - oxidation cycles is half the total number of carbon atoms minus one, because the last step produces two acetyl CoA molecules (a four-carbon chain split into two). to electron transport chain
The ATP yield from one palmitic acid (C-16:0) is:
- Initial activation of palmitic acid = - 2 ATP
- 8 acetyl CoA molecules x 10 ATP = 80 ATP
- 7 FADH 2 molecules x 1.5 ATP = 10.5 ATP
- 7 NADH molecules x 2.5 ATP = 17.5 ATP Total 106 ATP Note: this number differs from textbook due to estimated yields from FADH 2 & NADH CHAPTER 13: Lipids: Structure and Function ENERGY ACCOUNTING IN b-OXIDATION Activated palmitoyl-CoA
CHAPTER 13: Lipids: Structure and Function TYPES OF MEMBRANE LIPIDS Glycerophospholipids Sphingomyelins Sphingolipids Fatty-acid containing lipids Sterols (no fatty acids)
- Membrane lipids can be linked to fatty acids by two
different types of “ backbones ”:
Ø Glycerol = simple 3-carbon, tri-alcohol Ø Sphingosine = more complex hydrocarbon: § Two alcohols § Amine § Long-chain alkene CHAPTER 13: Lipids: Structure and Function TWO BACKBONES FOR MEMBRANE LIPIDS Note the cartoon designations for each backbone CHAPTER 13: Lipids: Structure and Function GLYCEROPHOSPHOLIPID STRUCTURES Cartoon Structure Components:
- Glycerol backbone (like triglycerides)
- Two fatty acid chains
- Phosphoryl group (phosphodiester)
- Amino alcohol head group Fatty acids are attached by ester bonds
CHAPTER 13: Lipids: Structure and Function SPHINGOLIPID STRUCTURES Cartoon Structure Components:
- Sphingosine backbone
- One fatty acid chain
- Phosphoryl group (phosphodiester)
- Amino alcohol head group Fatty acid are attached by an amide bond
- The amino alcohol “head group” associated with
phospholipids varies in structure & charge:
Ø ethanolamine
Ø choline
Ø serine
CHAPTER 13: Lipids: Structure and Function AMINO ALCOHOL HEAD GROUPS
in terms of their charge
at physiological pH
CHAPTER 13: Lipids: Structure and Function GLYCOLIPID STRUCTURES Cartoon Structure Components:
- Sphingosine backbone
- One fatty acid chain
- Carbohydrate head group Fatty acid are attached by an amide bond
- Bile acids are amphipathic molecules that are
used in the digestion of dietary lipids:
Ø Made in the liver and stored in the gall bladder Ø Released into the intestine after a meal rich in fats CHAPTER 13: Lipids: Structure and Function BILE ACIDS Cholesterol Cholate (a bile salt) What are the key chemical differences between these molecules?
- Vitamin D 3 is formed by the action of sunlight on
cholesterol derivatives in the skin:
Ø The ultraviolet (UV) wavelengths cause ring opening Ø Vitamin D 3 is a hormone that is required for the uptake of dietary calcium ions CHAPTER 13: Lipids: Structure and Function VITAMIN D 3 Cholesterol (^) Vitamin D 3 UV light
- Cholesterol is the starting point for the
biosynthesis of the 5 steroid hormone classes:
- Adrenal steroids : Ø Include the glucocorticoids and mineralocorticoids Ø Regulate metabolism, immune system activity & ion balance
- Sex steroids (gonadal hormones): Ø Include the progestins, estrogens, and androgens Ø Regulate reproductive processes CHAPTER 13: Lipids: Structure and Function STEROID HORMONE FUNCTION
