Upload
beverly-warren
View
234
Download
0
Tags:
Embed Size (px)
Citation preview
Chapter 11
Lipids and Membranes
Lipids
Biomolecules defined in terms of solubility: Insoluble in water but soluble in nonpolar solvents. Waxy, greasy or oily compounds.
Biological Functions: Energy Storage Structural component of cell membranes Signaling molecules
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Two major classes
Saponifiable Lipids
Saponification: base hydrolysis of esters to produce carboxylic acid salt and alcohol
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• Simple Lipids:– Contain fatty acids and alcohols
• Complex Lipids:– Contain multiple fatty acids,
alcohol, something else
Fatty Acids CA’s with long hydrocarbon chains (12 to 20 or
more carbons, usually even numbers) Numbered from the carboxylate end, and the a-
carbon is adjacent to the carboxylate group Terminal methyl carbon is denoted the omega (w)
carbon Components in triacylglycerols and phospholipids
Figure 11.1 Fatty Acid Structure
Section 11.1: Lipid Classes
Characteristics of Fatty Acids
1. Straight chain (unbranched) carboxylic acids
2. Comprised of 10-20 carbons
3. Usually have even number of carbons
4. Can be saturated or unsaturated
5. Usually no other functional groups
• Unsaturated fatty acids usually contain double bonds in the cis configuration, and can be mono- or poly-unsaturated
• This creates kink or bend in chain that prevents unsaturated fatty acids from packing together closely unlike saturated fatty acids
– Results in weaker intermolecular forces, lower MP’s – Usually liquid at room temp
O
O-
CH3
CH3
O
O-
unsaturated
saturated
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Section 11.1: Lipid Classes
Properties of Unsaturated Fatty Acids
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• The kink or bend in chain of unsaturated fatty acids prevents from packing together closely unlike saturated fatty acids– Results in weaker intermolecular forces, lower MP’s – Increases fluidity of biological membranes
Plants and bacteria can synthesize all fatty acids they require from acetyl-CoA
The human body can synthesize nonessential fatty acids, while essential fatty acids must be acquired from the diet
Essential Fatty Acids Linoleic acid (omega-6 fatty acid) Linolenic acid (omega-3 fatty acid)
Section 11.1: Lipid Classes
Diets rich in omega-3 fatty acids may: decrease serum cholesterol, triglycerides reduce risk of heart disease
Fish Nuts Kidney Beans
Sources of omega-3 fatty acids: Spinach Broccoli and cauliflower Oils
Eicosanoids Omega-3 and Omega-6 fatty acids are the source of Eicosanoids
Hormone-like signaling molecules Include:
Prostaglandins: Involved in inflammation, digestion, and reproduction
Thromboxanes: Involved in platelet aggregation and vasoconstriction following tissue injury
Leukotrienes: White blood cell chemoattractants; involved in vasoconstriction, edema, and bronchoconstriction
Section 11.1: Lipid Classes
Structures of Fats and Oils
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• Fats: Triglycerides from animal (saturated)• Oils: Triglycerides from vegetables (unsaturated)• These are esters (alcohol + acid)
• Alcohol derived from glycerol• Acid from fatty acids
Most common
lipids
triglyceride
Fats are solid at room temperature and have a high saturated fatty acid composition
Oils are liquid at room temperature and have a high unsaturated fatty acid composition
Figure 11.6 Space-Filling and Conformational Models of a Triacylglycerol
Section 11.1: Lipid Classes
Roles in animals: energy storage (also in plants), insulation at low temperatures, and water repellent for some animals’ feathers and fur
Better storage form of energy for two reasons:1. Hydrophobic and coalesce into droplets; store an equivalent amount of energy in about one-eighth the space2. More reduced and thus can release more electrons per molecule when oxidized
Figure 11.5 Triacylglycerol
Section 11.1: Lipid Classes
Reactions of Triglycerides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
hydrolysis
saponification
hydrogenation
triglyceride
Ester of 3 alcohols, 3
acids
Chemical Properties of Fats and Oils
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• Triglycerides exhibit chemical properties of esters and alkenes
Rxn: Hydrolysis
Breakdown of cellular fats to supply energy begins with lipase catalyzed hydrolysis reaction
Chemical Properties of Fats and Oils
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Rxn: Saponification
Soapmaking (up to AD 500), by adding base (lye or aqueous extract of wood ash) to animal fat
Chemical Properties of Fats and Oils
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Rxn: Hydrogenation
Results in partial hydrogenation (partially-hydrogenated vegetable oils) Semi-solids that don’t separate Crisco
Wax Esters Waxes are complex mixtures of nonpolar lipids Protective coatings on the leaves, stems, and
fruits of plants and on the skin and fur of animals Wax esters composed of long-chain fatty acids
and long-chain alcohols are prominent constituents of most waxes
Examples include carnuba (melissyl cerotate) and beeswax
Figure 11.8 The Wax Ester Melissyl Cerotate
Section 11.1: Lipid Classes
Phospholipids Amphipathic (hydrophilic and lipophilic) with a polar head group (phosphate and other polar or charged groups) and hydrophobic fatty acids
Act in membrane formation, emulsification, and as a surfactant (lowers surface tension between liquids)
Spontaneously rearrange into ordered structures when suspended in water
Figure 11.9 Phospholipid Molecules in Aqueous Solution
Section 11.1: Lipid Classes
Two types of phospholipids: phosphoglycerides and sphingomyelins
Sphingomyelins contain sphingosine instead of glycerol (also classified as sphingolipids)
Phosphoglycerides contain a glycerol, fatty acids, phosphate, and an alcohol
Simplest phosphoglyceride is phosphatidic acid composed of glycerol-3-phosphate and two fatty acids
Phosphatidylcholine (lecithin) is alcohol esterified to the phosphate group as choline
Section 11.1: Lipid Classes
Phosphoglycerides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• Complex lipids• Serve as major components of cell membranes• Also known as phospholipids• Structure similar to triglycerides
triglyceride
phosphoglyceride
Phosphoglycerides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• The most common phosphoglycerides have choline, ehanolamine, or serine attached to the phosphate group.
Another phosphoglyceride, phosphatidylinositol, is an important structural component of glycosyl phosphatidylinositol (GPI) anchors
GPI anchors attach certain proteins to the membrane surface
Proteins are attached via an amide linkage
Figure 11.10 GPI Anchor
Section 11.1: Lipid Classes
Phospholipases Hydrolyze ester bonds in glycerophospholipid
molecules Three major functions: membrane remodeling,
signal transduction, and digestion
Figure 11.11 Phospholipases
Section 11.1: Lipid Classes
Toxic Phospholipases—various organisms use membrane-degrading phospholipases as a means of inflicting damage
Bacterial a-toxin (creates pores in cells leading to apoptosis) and necrosis from snake venom (PLA2)
Section 11.1: Lipid Classes
Sphingolipids Complex lipid found in cell membrane Contain sphingosine instead of glycerol
Section 11.1: Lipid Classes
Sphingomyelin is found in most cell membranes, but is most abundant in the myelin sheath of nerve cells
Section 11.1: Lipid Classes
https://www.premedhq.com/myelin-sheath-schwann-cells
The ceramides are also precursors of glycolipids A monosaccharide, disacchaaride, or
oligosaccharide attached to a ceramide through an O-glycosidic bond
Most important classes are cerebrosides, sulfatides, and gangliosides (may bind bacteria and their toxins)
Figure 11.14a Selected Glycolipids
Section 11.1: Lipid Classes
Glycolipids
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• Another type sphingolipid• Contain carbohydrates• AKA cerebrosides due to abundance in brain tissue• No phosphate linkage
Related Diseases
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• Some human diseases are related to abnormal accumulation of sphingomyelins and glycolipids
Isoprenoids Biomolecules containing repeating
five-carbon structural units, or isoprene units
Isoprenoids consist of terpenes and steroids
Terpenes are classified by the number of isoprene units they have
Monoterpenes (used in perfumes), sesquiterpines (e.g., citronella, 3 isoprene units), tetraterpenes (e.g., carotenoids, 8 isoprene units)
Figure 11.15 Isoprene
Section 11.1: Lipid Classes
Carotenoids are the orange pigments found in plants
Mixed terpenoids consist of a nonterpene group attached to the isoprenoid group (prenyl groups)
Include vitamin K and vitamin E
Figure 11.16 Vitamin K, a Mixed Terpenoid
Section 11.1: Lipid Classes
Steroids
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Exhibit feature of other lipids (e.g., soluble in non-polar solvents) Cholesterol is most abundant steroid in human body
Essential component of cell membranes Precursor for other steroids:
Bile salts Sex hormones Vitamin D Adrenocorticoid hormones
Synthesized in liver or obtained from food May contribute to atherosclerosis
CH3
CH3
CH3
CH3
OH
CH3
Steroid Cholesterol
Cholesterol is an important molecule in animal cells that is classified as a sterol, because C-3 is oxidized to a hydroxyl group
Essential in animal membranes; a precursor of all steroid hormones, vitamin D, and bile salts
Usually stored in cells as a fatty acid ester
Section 11.1: Lipid Classes
CH3
CH3
CH3
CH3
OH
CH3
Figure 11.19 Animal Steroids
Section 11.1: Lipid Classes
Bile Salts
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://www.medicinenet.com/gallstones/article.htm
Yellow-brown or green liver secretion stored in gallbladder
Bile salts are released into intestine to separate large globules of lipids into smaller droplets
Bile salts also emulsify (mix immiscibles) cholesterol found in the bile for excretion
Gallstones may occur if cholesterol too high and/or bile salts too low
Steroid Hormones
http://schoolworkhelper.net/2010/07/the-endocrine-system-function-and-structure/
Hormone: Chemical produced in the cell or gland that delivers a message affecting cells in another part of the organism
Steroid hormones derived from cholesterol Two Major Steroid Hormones:
Adrenocorticoid Hormones Male and Female Sex Hormones
Steroid hormones diffuse through the cell membrane and combine with receptor proteins in the cytoplasm
Hormone-receptor complex cause cell to respond to hormone by interacting with cell DNA and stimulating protein synthesis
CH3
CH3
CH3
CH3
OH
CH3
Adrenocorticoid Hormones
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Produced in adrenal glands located at top of kidneys Classified into 2 functional groups: Mineralocorticoids: Regulate concentration of ions (Na+) in body fluids
Aldosterone: most important mineralocorticoidPromotes absorption of Na+ and Cl- in kidney tubules
Glucorticoids: Enhance carbohydrate metabolism Cortisol is major glucocorticoid in human body Increases glucose and glycogen concentration in body Cortisol, cortisone and prednisolone exert anti-inflammatory effects
Used to treat rheumatoid arthritis, bronchial asthma
Male Sex Hormones
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://artofamandanelson.blogspot.com/2011/09/generating-topics-steroids.html
Male sex hormones (androgens) produced in testes Testosterone: Most important
Promotes normal growth of male genitalia and aids in development of secondary sex characteristics
Anabolic steroids banned for use by athletes include testosterone and derivatives
Use of these can lead to:Liver tumorsTesticular atrophyDecreased sperm count
Female Sex Hormones
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Female sex hormones promote development of secondary sex characteristics Increase in voice pitch Increased breast size Inhibition of facial hair
Estrogens: Primary female sex hormones, play important roles in reproduction:
Estradiol Estrone Progesterone
Lipoproteins Transport lipid molecules
through the bloodstream from organ to organ
Protein components (apolipoproteins) for lipoproteins are synthesized in the liver or intestine
Figure 11.21 Plasma Lipoproteins
Section 11.1: Lipid Classes
Lipoproteins are classified according to their density:
Chylomicrons are large lipoproteins of extremely low density that transport triacylglycerol and cholesteryl esters (synthesized in the intestines)
Very low density lipoproteins (VLDL) are synthesized in the liver and transport lipids to the tissues
Low density lipoproteins (LDL) are principle transporters of cholesterol and cholesteryl esters to tissues
High density lipoprotein (HDL) is a protein-rich particle produced in the liver and intestine that seems to be a scavenger of excess cholesterol from membranes
Section 11.1: Lipid Classes
Properties of Unsaturated Fatty Acids
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
• Phospholipids form bilayer in cell membrane
Membrane lipids: Responsible for many membrane properties
Membrane fluidity refers to the viscosity of the lipid bilayer
Lipids may diffuse, or move around, within the membrane
Lipids may also flip from one side of the membrane to the other
Figure 11.25 Lateral Diffusion in Biological Membranes
Section 11.2: Membranes
The movement of molecules from one side of a membrane to the other requires a flipase
Membrane fluidity largely depends on the percentage of unsaturated fatty acids and cholesterol
Cholesterol contributes to stability with its rigid ring system and fluidity with its flexible hydrocarbon tail
Figure 11.24 Diagrammatic View of a Lipid Bilayer
Section 11.2: Membranes
Membrane Properties: Selective permeability is provided by the
hydrophobic chains of the lipid bilayer, which is impermeable to most all molecules (except small nonpolar molecules)
Membrane proteins help regulate the movement of ionic and polar substances
Small nonpolar substances may diffuse down their concentration gradient
Self-sealing is a result of the lateral flow of lipid molecules after a small disruption
Asymmetry of biological membranes is necessary for their function
The lipid composition on each side of the membrane is different
Section 11.2: Membranes
Membrane Proteins—most functions associated with the membrane require membrane proteins
Classified by their relationship with the membrane: peripheral or integral
Figure 11.26 Integral and Peripheral Membrane Proteins
Section 11.2: Membranes
Integral proteins embed in or pass through the membrane
Red blood cell anion exchanger
Peripheral proteins are bound to the membrane primarily through noncovalent interactions
Can be linked covalently through myristic, palmitic, or prenyl groups
Figure 11.27 Red Blood Cell Integral Membrane Proteins
Section 11.2: Membranes
Membrane Microdomains—lipids and proteins in membranes are not uniformly distributed
Specialized microdomains like “lipid rafts” can be found in the external leaflet of the plasma membrane
Figure 11.28 Lipid Rafts
Section 11.2: Membranes
Lipid rafts often include cholesterol, sphingolipids, and certain proteins
Lipid molecules are more ordered (less fluid) than non- raft regions
Lipid rafts have been implicated in a number of processes: exocytosis, endocytosis, and signal transduction
Figure 11.29 The Lipid Raft Environment
Section 11.2: Membranes
Membrane Function There are a vast array of membrane functions,
including transport of polar and charged substances and the relay of signals
Figure 11.30 Transport across Membranes
Section 11.2: Membranes
Membrane Transport Ions and molecules constantly move across the
plasma membrane and membranes of organelles Important for nutrient intake, waste excretion,
and the regulation of ion concentration Biological transport mechanisms are classified
according to whether they require energy
Section 11.2: Membranes
In passive transport, there is no energy input, while in active transport, energy is required
Passive is exemplified by simple diffusion and facilitated diffusion (with the concentration gradient)
Active transport uses energy to transport molecules against a concentration gradient
Figure 11.30 Transport across Membranes
Section 11.2: Membranes
Simple diffusion involves the propulsion of each solute by random molecular motion from an area of high concentration to an area of low concentration
Diffusion of gases O2 and CO2 across membranes is proportional to their concentration gradients
Facilitated diffusion uses channel proteins to move large or charged molecules down their concentration gradient
Examples include chemically gated Na+ channel and voltage-gated K+ channel
Section 11.2: Membranes
Active transport has two forms: primary and secondary
In primary active transport, transmembrane ATP-hydrolyzing enzymes provide the energy to drive the transport of ions or molecules
Na+-K+ ATPase
Figure 11.31 The Na+-K+ ATPase and Glucose Transport
Section 11.2: Membranes
In secondary active transport, concentration gradients formed by primary active transport are used to move other substances across the membrane
Na+-K+ ATPase pump in the kidney drives the movement of D-glucose against its concentration gradient
Figure 11.31 The Na+-K+ ATPase and Glucose Transport
Section 11.2: Membranes
Membrane Receptors provide mechanisms by which cells monitor and respond to changes in their environment
Chemical signals bind to membrane receptors in multicellular organisms for intracellular communication
Other receptors are involved in cell-cell recognition
Binding of ligand to membrane receptor causes a conformational change and programmed response
Section 11.2: Membranes