Understanding Lipids: Properties and Functions

• Lipids comprise biomolecules with diverse, seemingly unrelated structures
• The common feature among lipids is low water solubility
• The name "lipid" originates from the Greek word "lipos," meaning "fat" or "lard"
• This category includes fats, waxes, vegetable oils, steroids, and fat-soluble vitamins
• They support energy storage, cell structure, hormones and vitamins in the body

Biological functions of lipids:

• Act as an excellent energy source
  • Releases 9 kcal of energy per gram upon oxidation, twice that of carbohydrates
• Primarily stored as triglycerides in adipocytes, serving as concentrated energy reserves
• Form fundamental components
  • Consist of phosphoglycerides, sphingolipids, and steroids make up cell membranes
  • This regulates molecular traffic and cell communication
• Steroid hormones facilitate intercellular communication throughout the body
• Prostaglandins, hormone-like substances, influence both producing and target cells
• Facilitate the uptake of A, D, E, and K
• Act as a carrier for lipid-soluble compounds
• Diets low in fat (less than 20% of calories) can result in deficiency of these vitamins
• Provides padding for vital organs, reserving 4% of total body fat for this purpose
• Subcutaneous fat acts as insulation against cold

Lipid Classification

• Lipids lack repeating monomeric units
• Classified by hydrolysis products and molecular structure similarities
• Simple lipids: yield fatty acids and glycerol and waxes which yield be fatty acids and long chain alcohol
• Compound lipids
  • Phospholipids yield fatty acids, glycerol or sphingosine, phosphoric acid, and amino/amino alcohol acid
  • Glycolipids yield fatty acids, sphingosine, and carbohydrate after hydrolysis
• Terpenes and Steroids lack fatty acids, included due to water insolubility
• Eicosanoids are biologically important lipids with regulatory roles of prostaglandins, thromboxanes, and leukotrienes

Simple Lipids

• Solid consistency at room temperature
  • Typically derived from animal sources and may be identified as "animal fats"
  • Contain a higher proportion of saturated fatty acids with single bonds

Oils

• Liquid state at room temperature
  • Usually have plant origins, known as "vegetable oils"
  • Contain a greater proportion of unsaturated fatty acids

Fats and oils:

• Identified as triglycerides or triacylglycerols (TAG)
• Composed of three fatty acids linked to glycerol, a trihydroxy alcohol
• Naturally occurring triglycerides contain two or three different fatty acids and are called mixed triglycerides

Fatty Acids

• Monocarboxylic acids that are long-chain.
• Usually contain an even number of carbon atoms
  • The 12-18 carbon atoms are the most common in biological systems
• Fatty acids, like lauric, myristic and palmitic acids, commonly called by common names
• Saturated fatty acids
  • Follow the general formula CH3(CH2)nCOOH
• Unsaturated fatty acids
  • Contain at least one carbon-to-carbon double bond
• Cis configuration
  • Double bonds in naturally occurring unsaturated fatty acids

Impact of Carbon Atoms and Saturation

• Saturated fatty acid melting points increases with carbon number
• Saturated fatty acids containing ten or more carbons are solids at room temperature
• Unsaturated fatty acids
  • Melting point of saturated fatty acids surpass their unsaturated counterparts with same chain length
• Saturated fatty acids have straight chain structures leading to close packing
  • Introduction of a cis double bond creates a 30° bend causing lower intermolecular attractions and melting points

Saturated vs Unsaturated

• Unsaturated fatty acid melting points increase with hydrocarbon chain length

Fatty Acid Packing

• The degree of saturation impacts how fatty acids pack into stable aggregates
• Saturated acids have a usual extended conformation
• Double bonds restrict rotation introducing a bend in hydrocarbon tail
• Saturated fatty acids tightly pack into crystalline arrays through hydrophobic interactions
• Fatty acids disrupt packing, resulting in less stable aggregates
• Natural fats/oils mixtures of triglycerides with mixed fatty acid compositions
• Different fatty acid compositions lead to differences in melting points

Fatty Acid examples

• Olive oil
  • Liquid due to high proportion of unsaturated
• Butter
  • Soft solid because C16 and C18 saturated are higher than olive oil
• Beef fat
  • Hard solid due to long-chains saturated are even higher than butter

Triglycerides:

• High hydrophobicity which results in coalescence as fat droplets in cells
• Most adipose tissue in mammals occurs immediately under skin and in the abdominal cavity
• Serves as energy storage, thermal padding, especially important in aquatic mammals

Omega-3 Fatty Acids

• Eating two servings "oily" fish weekly is recommended by American Heart Association
• Fish like salmon, albacore tuna, sardines, lake trout, and mackerel contain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)
• Additionally recommends ALA (alpha-linolenic acid), found in flax seed, soybeans and canola
• Methyl group position determines name; omega-3 has a double bond at the third carbon
• Shown to reduce cardiovascular disease risk through decreasing blood clot formation and atherosclerotic plaque
• EPA creates prostacyclin which inhibits clumping thus reducing clot formation
• DHA is prevalent in sperm, brain cells, and retina and reduces triglyceride levels
• ALA may reduce cardiovascular disease on its own or as precursor for DHA and EPA

Linoleic acid

• Essential for synthesizing arachidonic acid
• First double bond is six carbons from methyl end, termed omega-6 fatty acids

Omega-3 vs Omega-6 Fatty Acids

• Prostaglandins that exhibit anti-inflammatory effects are from omega-3 fatty acids
• Prostaglandins with inflammatory effects from omega-6 fatty acids
• Suggested ratio not exceeding 4-5 times the amount of omega-3
• United States diets often contain 10-30 times more omega-6 fatty acids
• To encourage dietary change, National Institutes of Health provided intakes of
  • EPA/DHA 650 mg/day, ALA 2.22 g/day, Linoleic acid 4.44 g/day.

Physical Lipid Properties

• Pure fats and oils
  • Colorless, tasteless, odorless
  • Colors, odors, and flavors come from absorbed substances soluble in them
  • Yellow color comes from the pigment carotene
• Butter taste
  • diacetyl, CH3CO-COCH3 and 3-hydroxy-2-butanone, CH3C=OCHOHCH3
• Fats and oils are lighter than water, densities of about 0.8 g/cm3, poor conductors of heat and electricity, and excellent insulators for the body

Chemical Lipid Properties

• Hydrogenation adds H2 to double bonds of unsaturated fatty acids, converting them to single bonds for saturated fatty acids
• Commercial hydrogenation stops saturation of liquid vegetable oil before completion
• Complete hydrogenation gives a very brittle product
• Partial hydrogenation changes liquid vegetable oil to a soft, semisolid fat
• Saturation = increased melting point
• Can produce soft margarines, solid stick margarines, and solid shortenings by limiting hydrogen amount
• Contain more saturated fatty acids but no cholesterol, unlike some animal products
• Hydrolysis adds water to ester bonds of lipids with strong acids/digestive enzymes (lipases)
• Saponification - Lipids react with a strong base (alkaline hydrolysis) to give glycerol and salts of fatty acids (soaps)
• Solid soaps are produced when using NaOH and can be molded; KOH produces liquid soaps
• Polyunsaturated oils make soft soaps with Names like coconut or avocado based on the sources of oil

Olestra: Fat Substitute

• Artificial fat from food scientists in 1968 as nutrition source for premature babies
• Fats in cottonseed or soybean oil become fatty acids bonded with hydroxyl groups on sucrose instead of glycerol
• Cannot be digested because of large molecule, travels through tract undigested
• Eliminates Vitamins (A, D, E and K) + carotenoids
• FDA requires fortification with those vitamins
• May cause diarrhea/cramps for some
• Snack foods with olestra are in supermarkets

Cleaning Action of Soap

• Soap includes salt of long-chain fatty acid, two ends of soap molecule have different polarities
• Nonpolar hydrophobic carbon chain is soluble in grease/oil but insoluble in water
• Polar hydrophilic carboxylate salt end is soluble in H2O but not in grease/oil
• Soap cleans grease by dissolving nonpolar carbons in fats/oils, polar carboxylate dissolves in the water
• Results in soap molecules coating oil/grease, forming micelles clusters because outer surfaces micelles are hydrophilic while oils/fats small globules dispersed in layer which can be washed away
• Soap's polar carboxylates react with hard water ions which make water insoluble
• Detergents don't form insoluble products with hard water
• Consist of nonpolar carbons connected to sulfate groups calcium/magnesium soluble salts and break down which reduces pollution as soaps do

Oxidation of Lipids

• A fat or oil becomes rancid when its double bonds are oxidized by oxygen and microorganisms
• The oxidation products are short chain fatty acids and aldehydes
• Oxidation of oils happens on skin surfaces during heavy exercise
  • Warm temperatures+microorganisms allow fast oxidation when exposed to H2O/O
  • Results in short chained aldehydes+acids

Rancidity in Lipids

• Rancid fats/oils develop bad odor
• 2 Reactions for rancidity:
  • Hydrolysis + Oxidation

Rancidity by Hydrolysis

• Common due to having low molar mass acids, all which have offensive odors
• Moisture/warmth result ester linkages degrading liberating volatile acids via enzymes (lipases)
• Can be prevented by covering and refrigerating the butter

Rancidity by Oxidation

• Triglycerides containing unsaturated fatty acids
• Must add antioxidants minimize rancidity
• Suppresses by depriving adsorbed oxygen

Waxes:

• Esters of saturated and long-chain fatty acid alcohol from 14 to 30 carbons
• Plant waxes are surfaces of stems/leaves to prevent dehydration from harmful organisms
• Animal waxes serve as protective coatings to feathers/skin+hair to keep them soft/pliable
• Beeswax for furniture wax
  • Wax and cosmetics
  • Lanolin in lotions for water balance

Phospholipids and Glycolipids (Membrane Lipids)

• Not soluble with aqueous environment to separate aqueous environment in cell
• Similar to detergent molecules in having 2 hydrophobic tails and hydrophilic heads
• Three major types of membrane lipids
  • Glycerophospholipids/phosphoglycerides
    • Phosphate esters in lieu of fatty acid esters

Sphingolipids

• Abundant in the brain/nerve tissues
  • Derivative of amino alcohol sphingosine rather than glycerol
  • Two kinds including: Sphingomyelins and Glycolipids

Lecithin

• Contains major quaternary ammonium salt choline linked to phosphoric acid residue
• Cephalins
  • Ethanolamine or Serine linked to phosphate form
• Used as emulsifiers
  • Coat nonpolar liquids + keep in water suspension
  • Soybean oil is added to many bar brands and other sources of food to act as an emulsifier
  • Occurs in egg yolk which emulsifies oil droplets of mayonnaise

Sphingomyelins

• In body, components of nerve cell membranes
• Found in in myelin sheath to insulate central nervous system; essential cerebral function+nerve transmission

Glycolipids

• Two carbohydrates and and lipids, no phosphorus and contain sugars rather than one to which phosphate esters are bonded
• Glycolipid classification includes cerebrosides + gangliosides on back bone sphingosine

Clinical significance of glycolipids include

• Glucocerebroside
  • macrophage membrane
• Galactocerebroside is found in brain cells

Sphingolipids and Recognition Sites

• Discovered a century ago
• Have 60 types and located in membranes of cells
• On plasma of neurons used as recognition sites of carbohydrate moieties

Antigens and Antibodies

• Differ among the blood groups reflect that exist in the oligosaccharides RBC membranes.
  • A B and O A antigen = B-D-N-acetylgalactosamine end of RBC B antigen = a-D-galactose end in RBC Agglutination occurs due to antigen-antibody reaction, which stimulates antibodies to destroy antigen

Structure of Biological Membranes

• Lipid layers that contain hydrophobic tails that are enclosed by ionic heads through interaction with water
• Has hydrocarbon membrane prevents mixing between molecules on either sides
• Polar heads face water and are highly solvated

Representations of phospholipid include

• Hydrophilic head group + hydrophobic fatty acid tails
• Biologic layers consists primarily of lipids and proteins to carry out functions and can vary with source
• Some membranes contain small amounts of carbohydrates
• Two main lipid types occur in biological membranes: phospholipids and sterols

Cell Fluidity

• Composed of molecules for motion
• Determined by of unsaturated + saturated fatty acid groups in phospholipid.
  • The unsaturated fatty tails that are bent contribute to the fluidity of membranes
• Chol regulates by its shape
  • Fits next to fatty acid chain+lipid bilayer+restricts membrane motion

Selective Permeability

• Hydrophobic chains make barriers to substances
• Specific proteins cause movement into/out cells
• To cross lipid a polar must attach carrier protein +membrane translocation or protein channel

Sealing capabilities

• Breaks leads to water
• Proteins also supports this

Asymmetry

• Lipid makeup in each half is different from each
• Each side leads to different effects
• Occurs during membrane assembly

Lipids/Bile salts

• Lipids that bond to types of molecules, which are lipoproteins
• Transport lipids
• Sparingly soluble in water for movement across organs via lipoproteins
• core hydrophobic+ encircled by proteins/phospholipids/cholesterols

Plasma Lipoprotein Classes

• Four major classes
  • Chylomicrons
    • low density, Triglycerides, small to organ
  • Low density
    • .019 g/mL to adipose and stores storage for liver
  • Low Density-
    • Peripheral regulator/tissue help cholesterol
  • High-Density Lipoproteins (HDLs)
    • liver transfers
• Chylomicrons are aggregates of triglycerides
• synthesized liver with lipoprotein
• high high HDL lowers atherosclerosis because the HDL moves cholesterol to the liver where it's used for bile synthesis
• People with saturated fats have high chol levels but is unclear
• high unsat-decrease chol. +high HDL
• recommend low sat-fat in diet

Liposome Delivery Systems

• Tiny that mixes with clothing
• Liposomes used efficiently to the systems from cancer

Steroids

• Consist of three cyclohexane rings + one cyclopentane ring FUSED
• Includes cholesterol, steroid hormones and bile acids with Vitamin D
• The most prevalent animal steroid is cholesterol body which with acids
• Increase pressure+narrowing, high results

Functions of Steroids

• To control lipids for fluidity
• Body starting material for steroids

Hormones

• Control chemical target event
• Sex hormones (androgens) for secondary
• Sex hormones (estrogen - Control hormone control
• Aldosterone regulates tissue by the salt levels
• Hydrocortisone controls inflammation & arthtis
• Synthetics tricked, and synthe mimic tests building.

Bile salts

• Made intestines for cleaning of cholesterols.
• Vitamin D: Regulates calcium levels