Membrane Lipids and Proteins Flashcards
What is a membrane?
Two Dimensional sea of mobile lipid in which proteins diffuse (or float)
Give 2 main examples of cell membranes?
- plasma membrane
- internal membranes ( e.g. endoplasmic reticulum , Golgi apparatus)
what do membranes contain?
- lipids
- proteins
- carbohydrates ( glyco-proteins , - lipids)
N.B. the ratio of proteins and lipids is based on function - eg myelin sheath - will have a higher proportion of lipids , and mitochondria and chrloroplast will have a higher proportion of proteins.
What main property do membrane lipids have?
they are responsible for barrier properties
major membrane lipids include? (4)
- phospholipid
- steroids
- neutral fats
- glycolipid
what are the properties of phospholipids?
- amphipathic - have hydorphillic head and hydrophobic tails)
- polar head group ( charge varies), the head group may have a specific function ( eg intracellular signaling )
- fatty acid ( R) chains vary in length (C12 to C22) and degree pf unsaturation ( number of double bonds ).
each C=C bond introduces a rigid kink in the R tail.
give an example of a steroid and its properties?
- cholesterol and its esters
- amphipathic
- relatively large C-H portion
give an example of a neutral fat?
- glycerides
what are some properties of a glycolipid?
- minor
- includes some antigens (e.g blood groups - A and B)
Discuss the membrane as a bilayer?
- amphpathic lipids at oil- water interface:-
- hydrophilic ‘Polar’ head - water
- hydrophobic tail - oil
- phospholipids spontaneously form bilayers in aqueus solutions
- hydrophobic interactions cause tails to aggregate and excluse water from a core
- structure stablised by tail-tail and head- head interactions
what are the three types of membrane proteins?
- integral ( intrinsic)
- peripheral ( exofacial/ endofacial )
- transmembrane ( which includes single span , multispan and multimeric protein complexes )
what is an integral ( intrinsic ) protein?
- one or more regions embedded in lipid bilayer . most are infact TRANSMEMBRANE proteins!!!
What is a peripheral ( exofacial / endofacial ) protein?
- it is attached to membranes through integral membrane proteins or membrane lipids
- on surface
what is a transmembrane protein ? and what are the three types?
Transmembrane proteins arranged asymmetrically in membrane orientation established during biosynthesis.
The three types are :
- single span - anchoring , receptor - transducer function
- multiple spans - may form an aqueus ‘pore’ in the membrane ( channels , transporters)
- multimeric protein complexes - with central ‘pores’
What are the building blocks of proteins?
Amino acids
Do Amino acids have the same charge?
No
- some are neutral (Zwitterionic)- ‘polar’ depending on side groups ( e.g OH , C=O)
- Cationic (i.e lysine , arginine , ‘basic’
- anionic (i.e. glutamate, aspartate , ‘acidic’
What is a Hydropathy index?
the number on an amino acid representing the hydrophobic or hydrophillic properties of its sidechain.
( it reflects its charge)
What does electrical Charge distribution on a protein depend on?
primary structure
- high charge- (lots of +/- residues)- hydrophillic ( surface of membrane )
- low charge - Hydrophobic ( inside membrane )
(transmembrane domains likely to regions rich in hydrophobic AA residues anchored by interactions with phosopholipid tails)
How do you predict protein structure using a ‘hydropathy plot’?
it shows hydrophobicity/ hydrophiilicity of amino acid/protein.
why is Lactose Permease important ?
- symporter
- membrane protein
- found in bacteria
- uses proton gradient ( in direction of cell) to bring in lactose into the cell
- alpha helix’s ( shield H-bonds within helix)- tend to be polar
- Beta sheets- form outwith membrane - tend to be charged.
Evidence for Fluid mosaic Model ( structure ) ?
- Lipid Area ( unimolecular film ) = 2x membrane area
- Xray and neutron diffraction ( regular variation in densty of ( eg myelin sheath ) )
—— phosphate and cholestrol scatter xrays /// esters scatter neutrons !!
- transmission electron microscopy ( trilaminar ‘railroad’) ( you see the double membrane !!!)
- freeze etching ( reveals embedded proteins in fracture faces) ( electricity and freezing exposes the particles and shows proteins can move laterally!
Evidence for Fluid mosaic model ( dynamics)?
- Electron Spin resonance (ESR) ; spectroscopy 9 use spin- labelled phosopholipids )
- phase - transition ( membrane ‘melts’ from gel ( crystalline) to sol (fluid ) as temperature increases)- reducing melting point because fatty acids favor fluid condiitons…..
- protein tagging ( fluoresent label/ antibody) - these observe specific protein movement (e.g. cell fusion, electric field , FRAP- fluoresence recovery after photobleaching )
N.B. some proteins are immobile !
What is another function of the cell membrane?
- a barrier and exchange surface between cell and the environment
what mechanisms can substances cross the cell membrane ?
- diffusion , osmosis, transport - facilitative /active
What is Diffusion?
Downhill ( down a concentration ) gradient - simple diffusion is across a bilayer or channel.
the process of diffucion can be sped up by having transport proteins - where the protein has a confromational change as solute is transported across the membrane.
What is active primary transport?
transport against a concentration gradient - energy gives it the ability to do so , e,g, Na/Glucose transport , and calcium pump
