Lipids Flashcards
(31 cards)
What are lipids?
Largely non-polar (hydrophobic) hydrocarbon compounds that have a variety of very different structures and functions
Types of lipids
- Fatty acids (main one)
- Triacyglycerols
- Glycerophospholipids
- Prostaglandins (messenger molecules)
- Terpenoids
- Steroids
Structure of saturated fatty acids
CH3(CH2)nCO2H
Where n = 12,14,16 or 18
These tend to be found in animal fats and dairy products
Structure of unsaturated fatty acids
Have double bonds in increasing numbers which are ALWAYS Cis (never trans)
Staggered not conjugated
These are found in vegetable and plant oils
Example of simple fatty acid esters
Waxes e.g. beeswax or fat in the head of a sperm whale
What are triacylglycerols?
Fatty acid esters that are made up of fatty acids and a glycerol
Structure of triacylglycerols
- Saturated R groups = FATS
- Unsaturated R groups (one or more double bonds) = OILS
- Get hydrolysed by lipase enzymes which have the same mechanism as Serine proteases (Asp, His & Ser catalytic triad)
- Fatty acids are further catabolised (broken down) to acetyl-CoA resulting in energy (ATP)
How do double bonds affect the shape of fatty acids?
- Stearic acid = saturated fatty acid which is linear (no double bonds)
- Oleic acid = has one double bond which produces a small kink in the structure
- Linoleic acid = has 2 double bonds so the kink is bigger
- Alpha-linolenic acid = has 3 double bonds which produce a curve-like structure
How does the shape of fatty acids affect the physical chemistry of triacylglycerols?
Fats have linear chains that are packed closely together = higher melting point and is solid as structure is more stable
Oils have a more disordered structure due to the kinks/curves so chains are unable to pack closely and tightly = lower melting point and is liquid as structure is more mobile and less stable
What are glycerophospholipids?
Form membranes by arranging themselves into a lipid bilayer
With a polar head group, non-polar fatty acid chains and cholesterol molecules between these
Structure of glycerophospholipids
2 fatty acid chains and a polar head group beneath them
Name some other types of lipids
- Lipoproteins (have fatty acid chains attached)
- Glycoproteins (same structure as glycerophospholipids but have a sugar group)
- Sphingolipids
- Prostaglandins
- Terpenes
- Steroids
Describe prostaglandins
- all derived from arachidonic acid
- occur in nearly all mammalian tissue
- very low levels
- have strong physiological effects (act as messenger molecule = regulate BP, pain, inflammation etc)
Describe terpenes
- derived from 5 carbon isoprene units
- biosynthetic precursors to steroids
- an example of of this is squalene which has a 5 carbon repeating unit
Describe steroids
Include cholesterol, cortisone and testosterone (steroid hormones derived from cholesterol)
How is an ester formed?
Through a condensation reaction (esterification) of an alcohol and carboxylic acid
This is a slow reaction (does not occur spontaneously) so needs a catalyst - catalyst can’t be a base e.g. alkoxide
Describe fisher ester synthesis
Uses acid catalysis to activate the electrophile
This causes resonance stabilisation where the positive charge is delocalised onto the oxygen atom, making it a better electrophile
- Protonated carboxylic acid is more susceptible to attack
- Nucleophile attacks electrophilic Carbon
- Positive charge on alcohol group is transferred
- Water becomes a better leaving group
- The positive oxygen charge is neutralised
- Ester and water products formed
Other chemical ester syntheses
- Transesterification = converts one ester into another ester under anhydrous conditions
- Acid chloride = Cl is a good leaving group and alcohol acts as the nucleophile forming a tetrahedral intermediate & then base deprotonates the alcohol to give ester product, no need for a catalyst as it’s a fast reaction on its own
- Anhydride = 2 carboxylic acids joined together and have lost water, alcohol is the nucleophile, one of the carboxylic acid groups leaves (stabilised by resonance) & base deprotonates the alcohol to form ester
Importance of anhydrous conditions in ester synthesis
Prevents the formation of H2O which would bring about a reverse hydrolysis
Gives high yield of the ester product
What is ester hydrolysis?
From ester to a carboxylic acid
Describe acid catalysed ester hydrolysis
Exact reverse of fisher ester synthesis
- Protonation activates the ester carbonyl
- Allows attack by weak nucleophile e.g. water (now much more electrophilic reactive)
- Tetrahedral/Sp3 intermediate formed
- Proton is transferred from the water to the alcohol
- Acid is deprotonated
- Carboxylic acid is formed
Describe base catalysed ester hydrolysis
Involved hydroxide anions = much stronger nucleophile (than water)
- Hydroxide attacks ester carbonyl
- Forms sp3 intermediate
- Alkoxide is the leaving group
Hydrolysis of triacylglycerols using aqueous NaOH is used to produce soap
Describe the structure of alkenes
Double bonds in fatty acids
- Sp2 carbons = bond angle of 120
- Planar
- Nucleophilic due to pi electrons (reactive part of alkenes)
- Always Cis in fatty acids
What mechanism is involved in alkene chemistry?
Electrophilic addition
Involves:
- Addition of various electrophiles to produce anti/syn products
- Reduction to alkanes
- Oxidation leading to cleavage to produce other functional groups
