Lipid Structure and Functions
Classification
- Heterogeneous
- Marginally soluble in water but readily soluble in organic solvent - Classifies by common solubility
- Two main groups o Complex lipids
Long and open-chain compounds with polar head groups and non-polar tail
Fatty acids, triglycerols, glycerophospholipids and sphingolipids, eiconasoids, waxes o Simple lipids
Fused-ring steroids
Representative group: cholesterol, sex hormones, vitamin D
Complex Lipids
Fatty Acids
- Carboxylic acids with long hydrocarbon chains - They are amphipathic compounds
o Head – COO- group o Tail – Hydrocarbon chain - Ionised under proper condition - The hydrocarbon chains are
o Usually unbranched
o Nearly all even number of carbon atoms o Either saturated (no C=C bond) of unsaturated
Cis double bond causes a kink on the tail (Majority)
Trans will not cause the kink on the tail o Commonly contains 12 to 24 carbon atoms
- If the fatty acid is more unsaturated, that means it has lower melting point due to the molecular weight decrease ( unsaturated fatty acids has less H atoms)
- Shorthand:
o 20: 2(∆9,12)
o 20 Carbon fatty acid, 2 double bonds at C9-C10 and C12-C13 respectively o The FIRST carbon is the Carboxyl carbon
- For different type of fatty acids, see LSM1401 (Lipid Slide 12)
Triacylglycerol
- Ester of fatty acid and glycerol - Non-polar, insoluble in water
- Accumulate in adipose tissue and provide the functions: o Fatty acid storage (Energy storage)
o Insulation
- Naming of triacylglycerol:
o More carbon groupsunsaturatedless carbon o 1-Stearoyl, 2-linoleoyl,3-palmitoyl glycerol - Plant oils (rich unsaturated) – liquid in room temperature
o Richer in unsaturated fatty acids
- Animal fats (rich saturated) – greasy solid in room temperature Saponification
- In nature, triacylglycerol is hydrolysed by an enzyme called lipase.
- When a base is used to hydrolyse the triacylglycerol instead, the process is called saponification o Sodium salt of fatty acid is produced (soap)
Glycerophospholipids
- Glycerol esterified with two fatty acids and the third carbon is linked to HIGHLY POLAR or CHARGED via phosphodiester bond
- X is a head-group substituent which a leads to a series of glycerolphospholipids
- When X=H, the compound is known as phosphatidic acid
- Found almost exclusively in plant and animal membranes
Sphingolipids
- Derived from long-chain amino alcohol known as sphingosine - They do not contain glycerol
- Found in both plants and animals - Fatty acid linked to sphingosine by
o Amide bond to form ceramide
- Sphingomyelin, X=phosphocholine - Found in myelin sheath
- Resemble glycerophospholipids ( has similar structure with phophotidylcholone
Phosphoacylglycerol
sphingosides
Different types of Sphingolipids Glycolipids
- When attached to glycolipids
- A compound in which a carbohydrate is bound to OH group of lipid - Many derived from ceramides (sphingosine + Fatty acid)
Gangliosides
- Most complex sphingolipids - It has three or more sugars - At least one sialic acid
Waxes
- Ester of long alcohol chain and fatty-acids chain - HIGHLY INSOLUBLE
- Since they have many chains, their melting point must be high - The nature of waxes can be used as a repellant in animals
o Skins
o Bird feathers o Plant leaves
Eiconasoids
- Derived from arachidonic acid, 20: 4(∆5,8,11,14) - 3 classes:
o Prostaglandins (prostate gland) o Leukotrienes (Leukocytes) o Thromboxanes (Blood Clotting)
- Non-steroidal anti-inflammatory drugs (NSAIDs)
o Inhibit prostaglandin H2 synthase which will then block the synthesis of prostaglandin and thromboxanes from arachidonate
o Example: aspirin, ibuprofen, acetaminophen (paracetamol), meclofenamate - Contain 5-C ring
- Elevate body temperature and causes pain and fever - Synthesis inhibited by aspirin
- Contains 3 conjugated double bonds
- Induce contraction of smooth muscles in lungs - Causes asthma when over-produced
- Contain 6-membered ring with an ether - Formation of blood clots (platelet aggregation) - Stimulate smooth muscle contraction
Steroid
- Class of lipids that has NO FATTY ACIDS - Mostly in plant and animal cells
- Main types:
o Cholesterol o Hormones o Vitamin D
Cholesterol
- Weak amphiphilic character
o There is only one OH group at C3 o Highly hydrophobic
- Major component in animals
- Cholesterol stabilizes the structure of lipids
- Cholesterol can act as a precursor for other steroids and vitamin D3. - If excess, it will lead to atherosclerosis
Steroid Hormones
- Oestrogens
o Female sex hormones o Synthesized in ovaries
o Develop female secondary sex characteristics and control menstrual cycle - Andorgens (Progesterone, Testosterone)
o Male sex hormones o Synthesized in testes
o Develop male secondary sex characteristics
Vitamin D
- Group of structurally related compounds that play a role in regulation of calcium and phosphorus metabolism
- Vitamin D3 increase the synthesis of Ca2+ binding protein and promotes the Ca2+ absorption in intestines
- Synthesized from cholesterol under UV from sunlight
Lipid Aggregates
- 3 types
o Micelles
Spherical structures with hydrophobic tails in core and polar heads at surface
Single-tailed amphiphiles – free fatty acids o Lipid bilayers
Occurs when the head has the same cross sectional area as the tail
As a result, the lipids are double-tailed
Glycerolphospholipids, Sphingolipids o Liposomes
Bilayers fold back to form a sphere-like structure
Due to unfavourable exposure of bilayers edges to water
Form hollow spheres with inner layer more tightly packed
Biological Membranes
- In biological membranes, more bulky molecules are situated outside the membrane
- Arrangement of hydrocarbon tails in interior can be rigid or fluid depend on the saturated and unsaturated structure of fatty acids
- If a fatty acids has more unsaturated structure, it will be more fluid
- Plant membranes has higher percentage of unsaturated fatty acids, so it is more fluid than animal membranes
- Animal membranes also rich in cholesterol, this increases the rigidity - Biological membranes are usually in constant motion
due to the rotation around C-C of tails - Transition temperature
o If the membrane is more rigid, the transition temperature is higher
o If less rigid, the transition temperature is lower
o Eg, Polar bear has less rigid membrane, lower transition temperature so that it can withstand the low temperature
- The phospholipids are constantly in motion
Membrane Proteins
Functions:- Transport substances across membranes - Act as receptor sites
- Sites for enzyme catalysis 3 types:
- Peripheral protein - Integral proteins - Lipid-linked proteins
Peripheral Proteins
- Bound to the membrane by electrostatic / polar interactions - Easily removed by mild procedures such as changing pH
Integral Proteins
- Tightly bound to interior of membranes by hydrophobic interactions
- Can be removed by using detergents or ultrasonication, but usually it will end up denaturing them because it will disrupt the structure of the protein
Lipid-linked Proteins
- Covalently linked to lipids that insert into bilayer
- Covalent linkage via Cys (disulphide bond) or free amino groups on protein
- Common anchors: myristoyl and palmitoyl group - Examples:
o Human erythrocytes glycophorin A
N terminal with attached sugar groups are extracellular
C terminal protrudes into cell
One α-helix segment
Asymmetric orientation o Porins
Proteins in outer membrane of Gram- Negative bacteria
Allow polar solutes pass through membrane
Consists of anti-parallel β-sheet that forms β-barrel
Polar amino acids in the cavity of the barrel
Fluid Mosaic Model
- Fluid - free lateral movement of phospholipids as most interactions are non-covalent - Mosaic – Side-by-side existence of two or more separate components
- Some membrane proteins will be anchored(cannot move) to the internal structure of the protein to prevent free diffusion
- This can be helped to secure the protein when certain reaction is needed
