Lipids and Bilayers Flashcards
Describe lipids?
They are not polymeric bu they do aggregate
Very varied
Largely hydrophobic, therefore insoluble in water
Soluble in organic solvent e.g. chloroform and methanol
They can be separated chromatographically and identified by mass spec
What are the general biological functions of lipids?
- Lipids form lipid bilayers that are essential components of biological membranes
- Lipids containing hydrocarbon chains = energy stores
- Many intra/intercellular signalling events involve lipid molecules
What are the main types of lipids?
Triacylglycerols/triglycerides
Glycerophospholipids
Steroids
Describe fatty acids?
Long hydrocarbon unbranched chains with a carboxylic acid at the end
Usually in between 14-20 carbons in the chain
Most common: palmitic, oleic, linoleic and stearic acids
They can be saturated - only single bonds
OR
They can be unsaturated - contains at least one double bond
What are some features of saturated fatty acids?
Contains only single C-C bonds, fitting closely together in a regular pattern
Have strong interaction between long carbon chains
Have higher melting points
Usually solids at room temperature
What are some features of unsaturated fatty acids?
Double bonds = kinks in the chain Don't pack closely together Fewer attractions between chains Lower melting point Liquids at room temp
Polyunsaturated Fatty acids = more than one C=C bond
Monounsaturated Fatty acid = one C=C bond
What are the types of double bond within unsaturated fatty acids?
Cis and Trans
Cis - two H atoms on the same side of the double bond = rigid 30° kink
This is the most common
Trans - H atoms on opposite sides
Formed when polyunsaturated fatty acids from plants are ‘partially hydrogenated’
Consumption of trans fats causes cardiovascular disease
Describe triacylglycerols?
Glycerol + 3 fatty acids (esterfied)
Fats and oils in plants/animals mainly consists of triacylglycerols
Nonpolar and water insoluble
Function as enery reserves (adipocytes - fat cells)
Fats provide six times the metabolic energy of equal weight of hydrated glycogen
What are the types of triacylglycerols?
Simple - 3 fatty acids esterified to glycerol are the same
Mixed - different fatty acid chains are connected to glycerol
Most contain 2-3 different types
-ate at the end of a fatty acid becomes -oyl in fatty acid ester
What are Glycerophospholipids?
Glycerol-3-phosphate
C1 and C2 are esterified with fatty acids
C3 has a phosphoryl group usually linked to another polar group (X)
X helps determine the name
= Amphiphilic
Non-polar aliphatic tails
Polar phosphoryl-X heads
What are some glycerophospholipid names based on different X groups?
Water - Phosphaditic acid Ethanolamine - Phosphatidylethanolamine Choline - Phosphatidylcholine Serine - Phosphatidylserine Myo-inositol - Phosphatidylinositol
What are some facts of the different glycerophospholipids?
Phosphatidylcholine - most abundant in cell membranes
Phosphatidylethanolamine - 2nd most abundant, helps proteins to be positioned in the membrane
Phosphatidylserine - presence on the outer membrane ofdying cells signalsmacrophages to digest them
Phosphatidylinositol - abundant in thebrain
Give an example where glycerophospholipids are used to prevent a medical condition?
DPPC - major lipid of lung surfactant
Surfaces of alveoli covered in surfactant
Collapse of alveolar space is prevented by surfactant, as tightly packed DPPC (due to saturated, straight chains) resist compression
Premature infants have low surfactant production pre-birth = at risk for respiratory distress syndrome
Treated by exogenous surfactant to the lungs
What are phospholipases?
Phospholipases hydrolyse glycerophospholipids
E.g. Phospholipase A2 removes the fatty acid at C2 leaving a lysophospholipid
Lysophospholipid = powerful detergent (disrupt cell membranes and lyse cells) e.g. bee/snake venom
What are plasmalogens?
Glycerophospholipids, where C1 chain is linked by either and a or b unsaturated ether linkage is the cis configuration (rather than ester linkage)
Function isn’t well understood as the vinyl ether group is easily oxidised
They react with O2 free radicals - preventing free-radical damage to other organelles
What are sphingolipids?
Major membrane components
Derivatives of amino alcohol sphingosine (= bond in trans)
General structure: sphingosine, fatty acid, PO4, amino alcohol
The N-acyl fatty acid derivatives of sphingosine are known as ceramides
Ceramides are the parent compounds of the more abundant sphingolipids
What are some types of sphingolipids?
Sphingomyelins
Cerebrosides
Gangleosides
Describe sphingomyelins?
Can have choline instead of amino alcohol but can have ethanolamine as well
Make up 10-20% of plasma membrane lipids
Rich in the myelin sheath
Describe cerebrosides?
Lack a phosphate and are non-ionic
They are ceramides with a head consisting of a single sugar residue
e.g. galactocerebrosides and glucocerebrosides
Describe gangliosides?
Ceramides with attached oligosaccharides that include at least one sialic acid residue
Primarily components of cell surface membranes and 6% of brain lipids
Receptors for pituitary glycoprotein hormones and the cholera toxin
Likely involved in cell growth and differentiation & carcinogenesis
What is a disease of gangliosides?
Tach Sachs disease
A fatal neurodegenerative disorder
The inability to catabolise gangliosides
What are steroids?
Eukaryotic origin - contains 4 fused rings (A-D)
A-C has 6C and D has 5C
Can be further classified as a sterol due to C3-OH group e.g. in cholesterol
Describe cholesterol?
Forms 30-40% of plasma membrane lipid
Provides integrity and fluidity to the membrane - fluidity allows cells to change shape rapidly
Weak amphiphilic character due to OH group - this interaction with the head of the phospholipids also holds the membrane together
At high temperatures, they attract each other and prevent the membrane from breaking down
Fused ring provides great rigidity
What is cholesterol the precursor of in mammals?
Steroid hormones
They control a variety of physiological functions through regulation of gene expression
They are classified according to the physiological responses the evoke
What are some steroid hormones?
Glucocorticoids - affect carbohydrate, protein and lipid metabolism
e.g. cortisol - responses to stress, infection etc
Aldosterone - regulate the excretion of salt and water by the kidneys
Androgens & estrogens - affect sexual development and function e.g. testosterone and estradiol
What is another sterol derivative?
Vitamin D
This regulates Ca2+ metabolism
Deficiency leads to rickets - stunted growth and deformed bones
What are some common lipids that aren’t membrane components?
Isoprenoids - 5C units, soluble in the lipid bilayer e.g. ubiquinone
Vitamin A (Retinol) - triggers signalling through the optic nerve and stimulates tissue repair
Vitmain K - participates in carboxylation of Glu resididues in proteins involved in blood clotting
Vitmain E - an antioxidant that prevents oxidative damage to membrane proteins and lipids
What are some uncommon lipids that aren’t membrane components?
Eicosanoids - derived from arachidonic acid
20C compounds
The don’t move through the bloodstream but act locally (decompose within secs to mins)
The eicosanoids act at very low concentrations and are involved in the production of pain and fever, and in the regulation of blood pressure, blood coagulation, and reproduction
Why do lipids aggregate?
The hydrophobic effect: minimises unfavourable contacts between water and hydrophobic tails but permit the solvation of the polar heads
Due to amphipathic character
What does the outcome of the aggregation depend on? Forming what?
Length of fatty acid chain
Temperature
Degree of saturation (C=C or C-C)
Micelle, bilayer or liposome
Describe micelles?
Single tailed amphipiles - form spheroidal or ellipsoidal (oval) micelles
The single tail = tapered shape (hydrated head is wider than the tail) forming a cone shaped van der Waal envelope
They can develop water filled spaces
Head on the outside with the water and the tails on the inside
Describe lipid bilayers?
Two hydrocarbon tails on the lipid forms a lipid bilayer
Heads above and below, with tails facing in practically touching
60 Å thick
Sheets of phospholipid bilayers seal their edges - forming a spherical bilayer enclosing an aqueous central compartment
Describe a liposome?
Doughnut shape - heads forming a circle on the very outside and very inside with the tails pointing towards each other
What property do lipid bilayers have?
Fluid-like properties - the membranes are 2D fluids
Lateral diffusion - movement of lipids within its own plane of the bilayer (rapid)
Transverse diffusion (flip-flop) - transfer of a lipid molecule across a bilayer (slow and rare)
Why doesn’t transverse diffusion happen often?
The hydrated polar head has to pass through the anhydrous hydrocarbon core of the bilayer to the other side
Requires flippases to catalyse the movement from one layer of the bilayer to the other
What is significant about the interior of the lipid bilayer?
It is in constant motion due to rotations around the C=C bonds in the tails = low viscosity
What is the fluidity of a bilayer dependent on?
Temperature
Above the transition state: Highly mobile (liquid crystal)
Below the transition state: gel like solid (loses fluidity)
The transition state temperature of the bilayer increases with the chain length and the degree of saturation
Cholesterol decreases fluidity due to rigidity
