chapter 10 Flashcards
Lipids:
Structurally Diverse Class
Organic molecules that are characterized by low solubility in water, that is, are relatively hydrophobic.
Biological Functions of Lipids
Storage of energy
Reduced compounds: lots of available energy
Hydrophobic nature: good packing
Insulation from environment
Low thermal conductivity
High heat capacity (can “absorb” heat)
Mechanical protection (can absorb shocks)
Water repellant
Hydrophobic nature: keeps surface of the organism dry
Prevents excessive wetting (birds)
Prevents loss of water via evaporation
Buoyancy control and acoustics in marine mammals
Increased density while diving deep helps sinking (just a hypothesis)
Spermaceti organ may focus sound energy: sound stun gun?
Sperm whale
Sperm whale’s head accounts for over 1/3 of its body weight. About 90% made up of spermaceti organ that is filled with spermaceti oil. The oil helps the animal maintain neutral buoyancy at ocean depths where it hunts its food and is able to change density depending on temperature
More Functions
Membrane structure
Main structure of cell membranes
Cofactors for enzymes
Vitamin K: blood clot formation
Coenzyme Q: ATP synthesis in mitochondria
Signaling molecules
Paracrine hormones (act locally)
Steroid hormones (act body-wide)
Growth factors
Vitamins A and D (hormone precursors)
Pigments
Color of tomatoes, carrots, pumpkins, some birds
Antioxidants
Vitamin E
Classification of Lipids
-Based on the structure and function
–Lipids that do not contain fatty acids: cholesterol, terpenes, …
–Lipids that contain fatty acids (complex lipids)
–can be further separated into:
–storage lipids and membrane lipids
Fatty Acids
-Complete oxidation to
-Carboxylic acids with hydrocarbon chains containing
-Almost all natural fatty acids have
-Most natural fatty acids are
-Saturated:
-Monounsaturated:
Polyunsaturated:
-May contain
-Complete oxidation to CO2 and H2O
-Carboxylic acids with hydrocarbon chains containing between 4 to 36 carbons
-Almost all natural fatty acids have an even number of carbons
-Most natural fatty acids are unbranched
-Saturated: no double bonds between carbons in the chain
-Monounsaturated: one double bond between carbons in the alkyl chain
-Polyunsaturated: more than one double bond in the alkyl chain
-May contain ring structures
Fatty Acid Nomenclature
-Simplified
Chain length : number of double bonds
-Double bond position given by a Δ followed by the number of the carbon which is participating in the double bond. Note that the carboxyl group carbon is C-1
-E.g. 20:3(Δ3,9,12) denotes a carboxylic acid with 20 carbons and 3 double bonds between C-3 and C4; C-9 and C-10; C-12 and C-13
-Most commonly occurring have even numbers with unbranched chains- results from the fact that in biological systems the chains are put together from two Carbon (acetate) units
Fatty Acid Nomenclature and Omega-3
Omega-3 fatty acids are essential nutrients
-Humans need them but cannot synthesize them
-Including ALA, DHA, and EPA
—Although DHA and EPA can be synthesized from ALA
Solubility of Fatty Acids
-decreases as
-Longer fatty acyl chain
-Fewer double bonds
-Carboxylic acid group is
Solubility (largely determined by chain length and degree of saturation of the hydrocarbon chain)
-decreases as the chain length increases
-Longer fatty acyl chain, less soluble
-Fewer double bonds, less soluble
-Compare the solubility of lauric acid (12:0, Mr200) 0.063mg/ml vs glucose (Mr 180) 1,100 mg/ml
-Carboxylic acid group is polar and ionized at pH7 and accounts for the slight solubility of the shorter chain fatty acids in water
Melting point of Fatty acids
-decreases as
-decreases as
-At room temp
-Differences due to
-In fully saturated molecules
-Unsaturated- cis double bond forces a
Melting Point (Influenced by length and degree of unsaturation of the hydrocarbon chain)
-decreases as the chain length decreases
-decreases as the number of double bonds increases
-At room temp the saturated fatty acids from 12:0 to 24:0 are waxy while the corresponding unsaturated fatty acids are oily liquids
-Differences due to the degree of packing of the molecules
-In fully saturated molecules, large degree of free rotation around the C-C backbone and very flexible and able to pack tightly into nearly crystalline arrays with atoms in contact all along their length
-Unsaturated- cis double bond forces a kink in the hydrocarbon chain- result is that cannot pack as tightly as the fully saturated form. Overall van der Waal’s interactions less in unsaturated so less thermal energy to break bonds hence have lower melting points
Conformation of Fatty Acids
The saturated chain tends to adopt extended conformations
The double bonds in natural unsaturated fatty acids are commonly in cis configuration, which kinks the chain
Melting Point and Double Bonds
Saturated fatty acids pack in a fairly orderly way
-extensive favorable interactions
Unsaturated cis fatty acid pack less orderly due to the kink
-less-extensive favorable interactions
It takes less thermal energy to disrupt disordered packing of unsaturated fatty acids:
-unsaturated cis fatty acids have a lower melting point
Trans Fatty Acids
-Trans fatty acids form by
-A trans double bond allows a
-Trans fatty acids can
-Consuming trans fats
Trans fatty acids form by partial hydrogenation of unsaturated fatty acids
-Done to increase shelf life or stability at high temperature of oils used in cooking (especially deep frying)
A trans double bond allows a given fatty acid to adopt an extended conformation
Trans fatty acids can pack more regularly and show higher melting points than cis forms
Consuming trans fats increases risk of cardiovascular disease (increase LDL (bad) cholesterol, decrease HDL (good) cholesterol)
-Avoid deep-frying partially hydrogenated vegetable oils
-Current trend: reduce trans fats in foods (Wendy’s, KFC)
If unsaturated
In trans fats the unsaturated double bond has a trans configuration.
Triacylglycerols (Nonpolar)
-In vertebrates, free fatty acids with a free carboxylate group are
-Majority of fatty acids in biological systems are
-Solid ones are
–The ___ storage form
-Less soluble in
-Less dense
-In vertebrates, free fatty acids with a free carboxylate group are bound to a protein carrier called serum albumin
-Majority of fatty acids in biological systems are found in the form of triacylglycerols
-Solid ones are called fats, liquid ones are called oils
-The primary storage form of lipids (body fat)
-Less soluble in water than fatty acids due to the lack of charged carboxylate group
-Less dense than water: fats and oils float
Triacylglycerols
-_____ linkage due to
-Simple if all
-Polar groups of both glycerol and fatty acid are
-Ester linkage due to formation of a condensation bond
-Simple if all acids are the same, e.g. tristerin
-Most are mixed
-Polar groups of both glycerol and fatty acid are linked so triacylglycerols are nonpolar, hydrophobic and insoluble
-Lower specific gravity- float on water
Fats Provide Efficient Fuel Storage
The advantage of fats over polysaccharides:
The advantage of fats over polysaccharides:
-Fatty acids carry more energy per carbon because they are more reduced
-Fatty acids carry less water per gram because they are nonpolar
Glucose and glycogen are for short-term energy needs, quick delivery
Fats are for long-term (months) energy needs, good storage, slow delivery
Fuel Storage
Advantage/Disadvantage of lipids
Lipids are highly oxidation yields
lipids are highly
lipids are highly
Carbohydrates are
Carbohydrates are
Advantage/Disadvantage of lipids and carbohydrates as source of Fuel
Lipids are highly reduced; their oxidation yields tremendous energy
lipids are highly reduced; they are hydrophobic and are unhydrated
lipids are highly reduced; They are hydrophobic and insoluble, thus difficult to be transported
Carbohydrates are hydrophilic; they are highly soluble and thus quick source of energy
Carbohydrates are hydrophilic; have large quantities of water attached and can’t be stored
Saturation of Oils
-Many vegetable oils e.g. corn and olive oil are composed of
-Hydrogenation reduces some
-Exposure of lipids to air
-Many vegetable oils e.g. corn and olive oil are composed of triacylglycerols with unsaturated fatty acids- liquid at room temp
-Hydrogenation reduces some double bonds to single bonds and can give trans double bonds
-Exposure of lipids to air results in oxidative cleavage of double bonds to produce aldehydes and shorter chain carboxylic acids that are more volatile and smell rancid
Waxes
-Waxes are esters of long-chain saturated and unsaturated fatty acids with long-chain alcohols
-Insoluble and have high melting points (60 ~ 100°C)
-Variety of functions:
Storage of metabolic fuel in plankton
Protection and pliability for hair and skin in vertebrates
Waterproofing of feathers in birds
Protection from evaporation in tropical plants and ivy
Used by people in lotions, ointments, and polishes
Wax: The Material of the Honeycomb
Beeswax is a mixture of a large number of lipids, including esters of triacontanol, and a long-chain alkane hentriacontane
Structural Lipids in Membranes (Polar)
Membranes (5 – 10% of the dry mass of cells) serve as a
-Contain
-Diversification can come from:
-The properties of head groups determine
-Different organisms have different
-Different tissues have different membrane
-Membranes (5 – 10% of the dry mass of cells) serve as a barrier to passage of polar molecules and ions
-Contain polar head groups (OH, sugars or more complex as in the phospholipids) and nonpolar tails (usually attached fatty acids) (membrane structure: bilipid layer)
-Diversification can come from:
–modifying a different backbone
–changing the fatty acids
–modifying the head groups
-The properties of head groups determine the surface properties of membranes
-Different organisms have different membrane lipid head group compositions
-Different tissues have different membrane lipid head group compositions
Glycerophospholipids
-Primary constituents of
-Two fatty acids form
-Head group is
-Unsaturated fatty acids are
-The highly polar phosphate group may be
-Primary constituents of cell membranes
-Two fatty acids form ester linkages with the first and second hydroxyl groups of L-glycerol-3-phosphate
-Head group is charged at physiological pH
-Unsaturated fatty acids are commonly found connected to C2
-The highly polar phosphate group may be further esterified by an alcohol; such substituent groups are called the head groups
Phosphatidylcholine
Phosphatidylcholine is the major component of most eukaryotic cell membranes
Many prokaryotes, including E. coli, cannot synthesize this lipid; their membranes do not contain phosphatidylcholine
Plasmalogen
-Common in
-Also found in
-Function is
Ether Lipids:
-Vinyl ether analog of phosphatidylethanolamine
-Common in vertebrate heart tissue
-Also found in some protozoa and anaerobic bacteria
-Function is not well understood
Resistant to cleavage by common lipases but cleaved by few specific lipases
Increase membrane rigidity?
Sources of signaling lipids?
May be antioxidants?
Alkyl ether lipid- saturated
Plasmalogen- double bond between C-1 and C-2
Platelets-Activating Factor
Ether Lipids:
Aliphatic ether analog of phosphatidylcholine
Acetic acid has esterified position C2
First signaling lipid to be identified
Stimulates aggregation of blood platelets
Plays role in mediation of inflammation
Chloroplasts Contain
Galactolipids and Sulfolipids
Predominate in plant cell membranes.
One or two galactose residues connected by a glycosidic linkage to C-3 of a 1,2 diacylglycerol
Sulfonate group bears a negative charge like a phosphate group
Glycerol dialkyl glycerol tetraether
-Found in archaebacteria- live in extreme conditions of pH and temperature
-Molecule is longer (32C) to make a membrane with a hydrophobic core and hydrophilic ends (glycerol)
-Ether bond linkage more stable to hydrolysis than ester bond
Sphingolipids
-The backbone of sphingolipids is
-The backbone of sphingolipids is a
-A fatty acid is joined to
-A polar head group is connected to
-The sugar-containing glycosphingolipids are found largely in
-The backbone of sphingolipids is NOT glycerol
-The backbone of sphingolipids is a long-chain amino alcohol sphingosine
-A fatty acid is joined to sphingosine via an amide linkage rather than an ester linkage as usually seen in lipids
-A polar head group is connected to sphingosine by a glycosidic or phosphodiester linkage
-The sugar-containing glycosphingolipids are found largely in the outer face of plasma membranes
Sphingomyelin
-Sphingomyelin is present in the
-Sphingomyelin is abundant in
-Ceramide (sphingosine + amide-linked fatty acid) + phosphocholine attached to the alcohol
-Sphingomyelin is present in the plasma membranes of animal cells
-Sphingomyelin is abundant in myelin sheath that surrounds some nerve cells in animals
Sphingomyelin is structurally similar to
phosphatidylcholine
Glycosphingolipids and Blood Groups
-The blood groups are determined in part by the type of
-The structure of sugar is determined by an
-O antigen
-A blood group
-B blood group
-The blood groups are determined in part by the type of sugars located on the head groups in glycosphingolipids.
-The structure of sugar is determined by an expression of specific glycosyltransferases
–Individuals with no active glycosyltransferase will have the O antigen
–Individuals with a glycosyltransferase that transfers an N-acetylgalactosamine group have A blood group
–Individuals with a glycosyltransferase that transfers a galactose group have B blood group
Breakdown of membrane lipids
-Membrane lipids continuously made/broken
-Specific enzymes attack each hydrolyzable bond
-Defects in these enzymes lead to severe consequences (Tay-Sachs)
Defects in the turnover of membrane lipids
to a number of diseases
Sterols and Cholesterol
Sterol
–Steroid nucleus: four fused rings (three 6-C and one 5-C)
–Hydroxyl group (polar head) in the A-ring
–Various nonpolar side chains
–Cholesterol is amphipathic (OH at C-3, steroid nucleus and the hydrocarbon side chain at C-17)
The steroid nucleus is almost planar
Physiological Role of Sterols
-Cholesterol and related sterols are present in the
-Mammals obtain cholesterol from
-Cholesterol, bound to
–Cholesterol in low-density lipoproteins tends to
-Many hormones are derivatives of
-Cholesterol and related sterols are present in the membranes of most eukaryotic cells (stigmasterol in plants, ergosterol in fungi)
–Modulate fluidity and permeability
–Thicken the plasma membrane
–Most bacteria lack sterols
–Precursor of bile salts, Vitamin D and Hormones
-Mammals obtain cholesterol from food or synthesize it de novo in the liver
-Cholesterol, bound to proteins, is transported to tissues via blood vessels
–Cholesterol in low-density lipoproteins tends to deposit and clog arteries
-Many hormones are derivatives of sterols
Steroid Hormones
-Steroids are oxidized derivatives
Steroids have the but lack the
-More polar than
-Steroid hormones are synthesized
-They are carried through the body in the
-Many of the steroid hormones are
-Steroids are oxidized derivatives of sterols
-Steroids have the sterol nucleus, but lack the alkyl chain found in cholesterol
-More polar than cholesterol
-Steroid hormones are synthesized from cholesterol in gonads and adrenal glands
-They are carried through the body in the bloodstream, usually attached to carrier proteins
-Many of the steroid hormones are male and female sex hormones
Biologically Active Lipids
–Are present in much
–Play vital roles as
-Lipid soluble vitamins
-Are present in much smaller amounts than storage or structural lipids
-Play vital roles as signaling molecules between nearby cells
-Lipid soluble vitamins (A, D, E, and K)
Vitamin A
Helps increase the vision
Sources: carrot
Vitamin D
Needed for bone formation
Sources: flesh fatty fish (salmon, tuna)
Vitamin E
Also known as tocopherols are also
biological anioxidants
Sources: Fruits and vegetables
Vitamin K
Helps in blood coagulation process
Sources: Vegetables
Arachidonic Acid Derivatives as Signaling Lipids
Enzymatic oxidation of arachidonic acid yields
-Prostaglandins (inflammation and fever)
-Thromboxanes (formation of blood clots)
-Leukotrienes (smooth muscle contraction in lungs)
Mechanism of prevention of headache and heart attack by aspirin
Vitamin D regulates
calcium uptake
Vitamin A (Retinol)
Involved in
Precursor for
Involved in visual pigment
Precursor for other hormones involved in signaling
Vitamin E- collective name for
-Contain a substituted
-Are
-Aromatic ring
tocopherols
-Contain a substituted aromatic ring and long isoprenoid side chain.
-Are hydrophobic and associate with membranes, lipid deposits and lipoproteins
-Aromatic ring destroys free radicals to protect fatty acids from being oxidized
Vitamin K
–Undergoes
-Prothrombin splits the
-Undergoes oxidation and reduction during active prothrombin formation.
-Prothrombin splits the peptide bonds in fibrinogen to fibrin an insoluble protein that holds blood clots together
Vitamin E, K, and other lipid quinones
are
antioxidants
Polyketides are biologically
active lipids with medicinal uses
