Week 6 (Lipids) Flashcards
What is the general structure of lipids?
How do they associate?
- Lipids are small molecules having a strong tendency to associate through non covalent forces
- a second stabilising effect arises from Van der Waals interactions between the hydrocarbon regions
- polar, hydrophililic “head”
- nonpolar hydrophobic hydrocarbon “tail”
- in aqueous solution the nonpolar tails associate via an entropy-driven hydrophobic effect
Consequences of lipid structure: What do they commonly form?
- surface monolayers
- bilayers (esp membranes)
- micelles
- vesicles
Describe the structure and properties of fatty acids:
- head: A hydrophilic carboxylate group (a functional group consisting of a carbonyl group (C=O) with a hydroxyl group (O-H) attached to the same carbon atom)
- tail: A hydrocarbon chain
- weak acids (pKa 4.5) ionised at physiological pH
- Often incorporated into triacylglycerols
- If double bonds are present they are usually cis
- each cis C=C inserts a bend in the chain which alters the overall properties
What is the difference between the stearate ion and the oeate ion?
The oleate ion has a C=C double bond in the centre of the molecule
What do many bacterial fatty acids contain?
Branched or cyclic structures
What are the structures of most naturally occurring fatty acids?
They have an even number of carbon atoms and are linear
What are the properties of fully saturated acids?
- they pack together better
- have higher melting points compared with the correspond unsaturated fatty acid and lipids
Are cis fatty acids linear?
No there is a kink in the chain at the site of the double bond
Fatty acid nomenclature: what are these variables? X:Y(c or t ) delta Z
X= total number of carbon atoms
Y= Number of C=C
c= cis; t=trans
Delta Z= starting atom of C=C
What happens in order to maintain the correct fluidity properties of cell membranes?
The percentage of unsaturated fatty acid groups varies inversely with the temperature of the environment
Describe the cell membranes of bacteria growing in the Arctic Ocean
- High levels of unsaturation
- Membranes remain fluid even in the intense cold
Describe the cell membranes of bacteria in the hot springs of the Yellowstone National Park (almost boiling water)
- Membranes with very high levels of saturated fatty acids
- membranes remains fluid even at these high temperatures
What would be the effect of the absence of C=C double bonds in fatty acid chains of cell membranes?
Membrane lipids pack together well
High melting point of the membrane
Describe the properties of reindeer cell membranes relative to their environment
- there is a temperature gradient along the leg
- the hooves function at a lower temperature than the rest of the body
- the percentage of unsaturation in the cell membrane lipids varies along the length of the leg> more unsaturation closer to the hoof
What are triacylglycerols?
Triesters of fatty acids and glycerol
- polar head head (much less polar than the fatty acids)
- non polar tails
- esterification: RCOOH + R’OH—> RCOOR’
ACID+ALCOHOL-> ESTER
Do triacylglycerols dissolve in water?
No
What are triacylglycerols found in cytoplasm as?
Oily droplets because they associate together
Do triacylglycerols have hydrogen bond acceptors or donors?
They only have hydrogen bond acceptors
Why are triacylglycerols efficient for energy storage?
They have long hydrocarbon chains of fatty acids
Many organisms use lipids for cell membranes and for
Energy stores
What are fatty acids stored as?
Triacylglycerols or fats
Are triacylglycerols water insoluble?
Yes
Do triacylglycerols form micelles effectively?
No
What are the major lipid components of biological membranes?
- glycerophospholipids
- sphingolipids
- glycosphingolipids
- glycoglycerolipids
What are glycerophospholipids (phosphoglycerides)?
These are the major class of naturally occurring phospholipids, membrane lipids with phosphate containing head groups
- derivatives of L-glycerol-3-phosphate
- chiral centre at C2
- very polar head groups
- usually the acyl side chains are derived from fatty acids
- often one saturated and the other unsaturated
What are the variable (R) groups on glycerophospholipids?
- phosphatidic acid
- phosphatidylethanolamine
- phophophatidylcholine
- phosphatidylserine
- phosphatidyl inositol
The effect of shape on the aggregation properties of the molecules: what does a double tail yield?
A double tail yields a roughly cylindrical molecule.
Such cylindrical molecules can easily pack in parallel or form extended sheets of bilayer membranes with the hydrophilic head groups facing outward into aqueous regions either side
The effect of shape on the aggregation properties of the molecules: formation of micelles
Fatty acids are wedges shaped and tend fo form spherical micelles
The effect of shape on the aggregation properties of the molecules: bilayers
Phospholipids are more cylindrical and pack together better in a lipid bilayer structure
What are sphingolipids?
- a second major class of membrane lipid
- built around sphingosine
What is the subclass called when a fatty acid is attached to the NH2 via an amine bond?
Ceramides
How do modifications of ceramides lead to other sphingolipids? Give an example
- Addition of groups to the hydroxyls of sphingosine
- Phosphochloine group attached to the C-3 hydroxyl= sphingomyelin
What are lipids containing saccharide groups?
Glycolipids
What is the third major class of membrane lipids?
Glycosphingolipids
What does the glycosphingolipid class include?
Gangliosides: (Anionic glycosphingolipids containing at least one sialic acid residue)
Sphingosine + fatty acid =
Ceramide
Sphingosine + fatty acid + sugar=
Glycosphingolipid
Describe the Tay- Sachs ganglioside (Ganglioside GM2)
- Accumulates in the neural tissue of infants with Tay-Sachs disease
- The defect responsible for this inherited condition is the lack of an enzyme (beta-hexosaminidase A) that normally cleaves the terminal GaINAc
- Results in the destruction of nerve cells in the brain and spinal cord
What is cholesterol?
- a member of the steroid group
- the biosynthetic precursor for steroid
- precursor of bile acids
- weakly ampiphatic
- a component of many animal membranes, influences membrane fluidity by its bulky structure
How does cholesterol influence membrane fluidity?
- cholesterol molecules align themselves with the hydroxyl groups close to the hydrophilic heads of phospholipids and forms hydrogen bonds
- It hydrogen bonds from its single hydoxyl group to two adjacent phospholipids
- the steroid ring structure interacts with the regions of the hydrocarbon chain closest to the phospholipid head leaving the rest of the tail flexible
- stiffens the membranes and helps to prevent in from acting as a true liquid
Does cholesterol reduce the mobility of neighbouring fatty acid chains?
Yes
What are bile salts?
- derived from cholesterol (bile acid synthesis is the major metabolic end product of cholesterol)
- have detergent properties which aid in fat digestion and absorption
- cholic acid and chendeoxycholic acid are the predominant bile acids in humans
- conjugates of cholic acid with glycine and taurine form salts called glycocholate and taurocholate respectively
Describe the structure of cholic acid
- The tail has been oxidised
- The 4 ring structure is considered to be almost flat with two faces
- The 3 extra OH groups have a dashed bond (so are pointing away and are on one side of the molecule
- One side is polar (because of the OH groups)
- The other one is not
How are bile salts suited to solubilise fat?
- The hydrocarbons point up in one direction
- The OH groups point down in the other direction
- The fat that we eat is not water soluble so we produce taurocholate or cholic acid which comes in contact with the fat and like goes with like
- The hydrocarbon side aligns with the fat and the OH side and the polar chain points away and interacts with water moles
- This allows it to act as a detergent and solubilise the fat
Describe the structure of glycholate and taurocholate
Glycocholate:
- Cholic acid where the terminal caroxyl group has been coupled onto the amino acid glycine
Taurocholate:
- cholic acid where the terminal carboxyl group has been couple onto the amino acid taurine via an amide bond
Describe how the structure of taurocholate and other bile salts makes them suited for their function of solubilising fat
- The methyl groups point up in one direction
- The OH groups point down in the other direction
- The fat molecules we eat are not soluble so we produce taurocholate, cholic acid and other bile acids
- The fats align with the hydrocabon side of the molecule
- The other side (OH and polar chain) of the cholic acid is pointing away from the fat and interacting with water molecule
- This allows them to act as detergents and solubilise fat
Antioxidants provide protection against: Oxidative damage from reactive oxygen species such as
- superoxides (O2-)
- hydroxyl radicals (OH where the O has an unpaired electron)
- hydrogen peroxide (H2O2)
Give examples of antioxidants
- vitamin A, C and E
- Uric acid
- glutathione
- enzymes such as superoxide dismutase, catalase, glutathionine peroxidase
- many antioxidants act by scavenging reactive oxygen molecules or by chemically reducing oxidised compounds
Summary: Antioxidants are compounds that inhibit oxidation. Oxidation is a chemical reaction that can produce free radicals, thereby leading to chain reactions that may damage the cells of organisms. Antioxidants such as thiols, glutationine or ascorbic acid (vitamin C) terminate these chain reactions.
What are the functions of antioxidants?
They provide protection against oxidative damage to our cells
What Vitamin A derived from?
A fat soluble vitamin derived from beta-carotene
What is Vitamin C?
A water soluble potent antioxidant
What is vitamin E effective at preventing?
A fat soluble antioxidant particularly effective in preventing the attack of peroxides on unsaturated fatty acids in membrane lipids
What is Uric acid?
An antioxidant
Properties of glutathione as an antioxidant?
- The free thiol group provides protection against oxidative stress
- E.g. helps to keep cysteine thiol groups in proteins in the reduced state
- If two thiol groups become oxidised, they can be reduced nonenzymatically by glutathione
