M3: Lipids

Lipids

A diverse group of organic compounds that play important roles in the body.

  • biomolecules that are soluble in organic solvent and insoluble on water
    • contain many nonpolar C-c and C-H bonds; few polar bonds
  • not defined by a particular functional group
  • have variety of structures and functions

Categories (Classifications):

1. Hydrolyzable
  • can be converted into smaller molecules by hydrolysis
    • waxes (makeup)
    • triacylglycerol (fats, oil)
    • phospholipids (complex/combined lipids)
2. Nonhydrolyzable
  • cannot be cleaved into smaller molecules by aqueous solutions
    • steroids
    • fat-soluble vitamins (Vitamin A)
    • eicosanoids

(by structure) ff:

Simple Lipids (hydrolyzable)
  • triacylglycerol
    • fats
    • oils
Compound/Complex
  • in combination with other kinds of molecules
    • glycolipids (carbohydrates + lipids)
    • lipoproteins (proteins + lipids)
    • Phospholipids (phosphorus atom + lipids)
Derived Lipids (nonhydrolyzable)
  • steroids
  • prostaglandins
Functions of lipids:
  • Main:
    • Energy Storage & Source (in the form of triacylglyceride triacylglycerol)
  • Main Component of Cell Membranes
    • Phospholipids
    • Insulation and Protection
  • Hormone Production (vitamins of hormones)
    • Absorption of Fat-Soluble Vitamins

Fatty Acids

Building Blocks
  • hydrolyzable lipids are derived from these
  • carboxylic acids (RCOOH) w/ long chains of C chains of 12-20 C atoms
    • ex. Palmitic acid: CH3(CH2)14COOH
  • naturally occurring have an even number of C atoms
  • number of bonds increase : melting point decrease
  • geometric isomers or cis-trans isomers
Saturated fatty acids
  • no double bonds
    • ex. solid (in bol
    • strong dispersion forces, larger area of contact
Unsaturated fatty acids
  • 1 or more double bonds (generally cis) in their long hydrocarbon chains
    • ex. liquid
    • may bali, weaker dispersion forces, smaller area of contact
Essential
  • cannot be produced by the body
  • must be consumed
  • Linoleic and;
  • linolenic acids
    • omega-6 acid;
    • position of the first C=C in the nonpolar chain
    • condensed structural formula
    • CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
    • Numeric Formula
      • number of carbons : number of double bonds delta (count from carboxyl)/omega (count from methyl) symbol superscript number of carbon on 1st C bond
      • 18:2 omega symbol^6

hydrolyzable lipids (simple)

Waxes

  • esters form from a fatty acids and a high molecular weight alcohol
  • general structure
  • general formation (esterification, less carbon)

ex.

  • Spermaceti wax
    • isolated from the heads of sperm whales
  • long nonpolar C chains, hydrophobic
  • form protective coatings on bird’s feathers and sheep’s wool and make up beeswax
  • beeswax (myricyl palmitate)
  • hydrolyzed with water in the presence of acid or base to re-form the carboxylic acid and alcohol they came from

Triacylglycerols

Fats and Oils (Triglycerides)
  • three esters formed from glycerols and three molecules of fatty acids
    • block diagram
    • general reaction formation
Simple triacylglycerol
  • three identical fatty acid side chains
Mixed triacylglycerol
  • two or thee different fatty acids
Saturated triacylglycerol
  • saturated fatty acids,
  • make up most animal fat
  • solids at room temperature
Unsaturated triacylglycerol
  • at least one unsaturated fatty acid
  • make up most vegetable oils
  • liquids at room temp.
  • increase double bonds = decrease room temp.
  • the higher mono and poly the healthier
  • Monounsaturated triacylglycerol
    • 1 C=C bond
  • Polyunsaturated triacylglycerol
    • many C=C bonds
Importance in Diet
Fats
  • higher melting points
  • solids at room temp.
  • derived from fatty acids with few double bonds
  • used to build cell membrane, insulation and store energy
  • no more than 20-35%
  • high intake of saturated triacylglycerol linked to heart disease
Oils
  • lower melting point
  • liquids at room temp
  • derived from fatty acids with larger double bond
Saturated fats
  • Increased Risk of Heart Disease
    • stimulates cholesterol synthesis which can lead to cholesterol plaques building up inside arteries
    • results in high blood pressure, heart attack and stroke
Unsaturated triacylglycerol
  • lower risk of heart disease
    • decreases level of cholesterol
Omega-3 fatty Acids
  • helpful in lowering the risk of heart attack
Trans Fats
  • double bond of unsaturated triacylglycerol
  • primarily synthesized not naturally occurring
  • act like saturated fats and increases cholesterol levels
  • ex. margarine
Hydrolysis
  • triacylglycerol are hydrolyzed with water in the presence of acid, base, or enzymes (in the body)
    • enzyme - lipase
  • soaps are metal salts of fatty acids prepared by basic hydrolysis (saponification)
    • soaps are made from lard, tallow, coconut oil, palm oil
    • nonpolar tail dissolve grease and oil and the polar end dissolves in water making dirt soluble in water

Focus on the human body

Metabolism

  • store energy in adipose cells below the surface of the skin, breast, and surrounding internal organs of adipose cells in constant
    • weight gained or lost causes them to swell or shrink, but not decrease/increase in number
  • to metabolize triacylglycerol for energy, esters are hydrolyzed by enzymes-- lipases
  • complete metabolism yields CO2, H2O, great deal of energy

Phospholipids

  • hydrolyzable and compound/complex lipids
  • lipids that contain a P atom
  • 2 common types
    • phosphoacylglycerol
    • sphingomyelin
1. Phosphoacylglycerol
  • main component of most cell membranes
  • structure resemble a triacylglycerol except third fatty acid is a phosphodiester bonded to an alcohol
  • main types:
    • cephalin (phosphatidylethanolamine)
    • ox brain
    • lecithin (phosphatidylcholine)
    • good emulsifying agent
    • abundant in eggs
  • two fatty acid chains from two nonpolar “tails” that are parallel to each other
    • the phosphodiester end of the molecule is a charged or polar “head”
2. Sphingomyelin
  • do not contain a glycerol backbone but a Sphingosine backbone
  • do not contain an ester; amide bond
  • example:
    • myelin sheath
    • coating that surrounds nerve cells
    • rich in sphingomyelins
    • lack of it results in multiple sclerosis

Cell membrane

Structure
  • the basic unit of a living organism is the cell
  • surrounds the cytoplasm
  • acts as a barrier to stop the passage of ions and molecules into and out of the cell
  • allows nutrients in and waste out
  • selectively permeable
  • main component:
    • Phospholipids
    • polar head and nonpolar tails
    • lipid bilayer (when mixed with water)
  • composed of lipid bilayers
    • proteins and cholesterol molecules are embedded in the lipid bilayer membrane
    • Peripheral proteins are embedded within and extend outward one side only
    • integral proteins extend through the entire layer
    • forms glycolipids and glycoproteins (when carbohydrates are attached to the exterior of the cell)
Transport Across a Cell Membrane

  • small molecules like O2 and CO2 can diffuse

    • from higher to lower concentration

  • larger molecules need facilitated transport

    • ions like C- or HCO3- travel through integral protein channels

  • active transport

    • Na-, K+, and Ca^2+ move against the concentration agent-- requires energy input

Health & Medicine

Steroid

  • nonhydrolyzable
  • group of lipids whose carbon skeletons contain several fused rings:
1. Cholesterol
  • most prominent steroid
  • synthesized in the liver
  • found in almost all body tissues
  • Sobtained in the diet from many sources
    • meat, cheese, butter, eggs
  • elevated levels of cholesterol in the bloodstream lead to coronary artery disease, heart attack, etc.
  • insoluble in aqueous medium of blood
  • transported through the bloodstream by lipoproteins, aggregates of phospholipids and proteins
    • Low-density lipoproteins (LDLs)
    • transport cholesterol from the liver to the tissues
    • High-density lipoproteins (HDLs)
    • transport cholesterol from tissues back to the liver
  • recommended level:
    • total serum cholesterol < 200 mg/dL
Bad Cholesterol (LDLs)
  • deposit cholesterol on the walls of arteries when they carry more than is needed to form cell membrane
  • forms plaque, restricts blood flow
  • recommended level:
    • LDL < 100 mg/dL
Good Cholesterol (HDLs)
  • reduce the level of cholesterol in the bloodstream
  • brings excess back to the liver
  • recommended levels:
    • HDL > 40 mg/dL
2. Hormone
  • a molecule that is synthesized in one part of an organism which elicits a response at a different site
  • 2 important classes:
    • sex hormones
    • female sex hormones:
      • estrogens and progestins
    • male sex hormone:
      • androgens
    • adrenal cortical steroids
Female sex hormones
Estrogen
  • estrogens estradiol and estrone
  • control the development of secondary sex characteristics
  • regulate the menstrual cycle
  • made in the ovaries
Progestin
  • progestin progesterone
  • pregnancy hormone
  • responsible for the preparation of the uterus for implantation of a fertilized egg
Male sex hormones
Androgens
  • testosterone and androsterone
  • made in the testes (endocrine system)
  • control development of secondary sex characteristics
Anabolic steroids
  • synthetic androgen analogues
  • promote muscle growth
  • same effect as testosterone but are more stable so they are not metabolized quickly
  • used by athletes and body builders but are not permitted in competitive sports
  • prolonged use can cause physical and psychological problems
Adrenal Cortical Steroids
Aldosterone
  • regulates blood pressure and volume by controlling the concentration of Na+ and K+ in body fluids
Cortisone and Cortisol
  • anti-inflammatory agents
  • also regulate carbohydrate metabolism
  • Prolonged use of these steroids can have undesired side effects
    • bone loss & high blood pressure
  • Prednisone
    • synthetic alternative
    • has similar anti-inflammatory properties

Vitamins

Fat-Soluble

  • organic compounds required in small quantities for normal metabolism and must be obtained from the diet
  • either water soluble or fat soluble
  • four fat soluble vitamins:
    • A, D, E, K
    • found in fruits, vegetables, fish, liver, and dairy products
    • stored in adipose cells to be used when needed
Vitamin A
  • found in liver, fish, and dairy products
  • made from beta-carotene
  • needed for vision and for healthy mucous membranes
  • Vitamin A deficiency causes night blindness and dry eyes and skin
Vitamin D
  • synthesized from cholesterol
  • can be obtained in the diet from many foods, especially milk
  • helps regulate Ca and K metabolism
  • Vitamin D deficiency causes rickets (bone malformation)
Vitamin E
  • an antioxidant
  • protecting unsaturated side chains in fatty acids from unwanted oxidation
  • deficiency causes numerous neurological problem, although rare
Vitamin K
  • regulates the synthesis of clotting proteins (prothrombin)
  • deficiency leads to excessive or fatal bleeding

Eicosanoids

  • very potent compounds
  • not stored in cells but rather synthesized in response to external stimulus
  • unlike hormones, they are local mediators
    • perform their function in the environment in which they are synthesized
  • two types:
    • prostaglandins
    • leukotrienes
1. Prostaglandins
  • carboxylic acids
  • contain a five-membered ring
  • have a wide range of biological activities
  • responsible for inflammation
  • Aspirin and Ibuprofen
    • relieve pain and inflammation by blocking synthesis of these molecules
  • also decrease gastric secretions
  • inhibit blood platelet aggregation
  • stimulate uterine contractions
  • relax smooth muscles
  • 2 different cyclooxygenase enzymes responsible for prostaglandin synthesis**:**
    • COX-1
    • involved in the usual production
    • COX-2
    • responsible for additional prostaglandins in inflammatory diseases like arthritis
  • Nonsteroidal inflammatory drugs (NSAIDs) like aspirin and ibuprofen inactivate COX-1 AND -2, but increase risk for stomach ulcer formation
    • Drugs sold as Vioxx, Bextra, and Celebrex block only the COX-2 enzyme without affecting gastric secretions
2. Leukotrienes
  • molecules that contribute to the asthmatic response by constricting the smooth muscle of the lung
Asthma
  • characterized by chronic inflammation
    • inhaled steroids to reduce this inflammation are commonly used
  • new asthma drugs act by blocking the synthesis of leukotriene C4
    • which treats the disease instead of just the inflammation symptoms