Membrane Structure and Composition Flashcards
Functions of Membranes
Define boundaries of a cell and organelles and act as permeability barriers.
Serve as sites for biological functions such as electron transport.
Membrane proteins carry out and regulate the transport of substances across the membrane.
Contain protein molecules that act as receptors to detect external signals.
Provide mechanisms for cell-to-cell contact, adhesion, and communication.
Fluid Mosaic Model
Describes the fluid lipid bilayer with a mosaic of proteins attached to or embedded in the bilayer
Three classes of membrane proteins:
Integral membrane proteins
Peripheral proteins
Lipid-anchored proteins
Fluidity of Bilayer
Lipids in bilayer are in constant motion; proteins are able to move laterally through membrane (except those anchored to internal structures)
Transmembrane Proteins
Contain hydrophobic segments which span the lipid bilayer; anchors protein to membrane
Membrane Lipids
Non-homogenous; diversity in lipid content
Classes of Membrane Lipids:
Phospholipids, glycolipids, sterols
Phospholipids
Most abundant lipids in membrane; include phosphoglycerides and sphingolipids; variation in content among types of membranes; polar head and hydrophobic tail
Location of Lipid Synthesis:
Smooth ER
Glycolipids
Sugar/carbohydrates attached to lipids; cerebrosides and gangliosides prominent in neurons
Sterols
Lipid molecules derived from cholesterol with characteristic four-ring steroid structure; contain a polar group to embed in membrane
Thin-Layer Chromatography
Used to separate different kinds of lipids based on their relative polarities
Membrane Asymmetry
Lipids are distributed unequally between the two monolayers; established during the synthesis of the membrane; most glycolipids are present in the exoplasmic layer
Lipids are mobile within their monolayer
Fast lateral diffusion - can move as fast as several um per second; slow transverse diffusion (flip-flop)
Lateral diffusion can be demonstrated by:
Fluorescence Recovery After Photobleaching (FRAP)
Why fluidity is important for membrane function
Membrane permeability, mobility of membrane proteins, proper organization of membranes, cell movement; membranes function properly only in the fluid state
Tm
Membrane fluidity changes with temperature; transition temperature measures the temperature at which a membrane changes phases from a ordered gel phase to liquid cystalline
How can Tm be measured?
Differential scanning calorimetry - uptake of heat is measured as temperature is increased
Affects of fatty acid saturation on Tm
Unsaturated fatty acids lowers Tm
Affects of fatty acid chain length on Tm
Increased fatty acid chain length increases Tm
Sterols influences on membrane fluidity
Cholesterol (and other sterols) affect membrane fluidity by stacking in between phospholipids.
Lipid Rafts
Localized regions of membrane lipids (and proteins) involved in cell signaling (non-random)
Freeze-fracture Experiment
Membranes are frozen and broken; images revealed evidence of the fluid-mosaic model
Types of membrane proteins:
Integral, peripheral, and lipid-anchored
Integral Membrane Proteins
Contain hydrophobic regions embedded in the interior; difficult to isolate; can cross the membrane once or multiple times (single-pass or multi-pass); most common structures are alpha helices; can form a hydrophilic core with multiple alpha helices; beta barrels can also form channels/porins
Peripheral Membrane Proteins
Bound to the membrane via weak electrostatic forces and hydrogen bonds
Lipid-anchored Membrane Proteins
Covalently bound to lipid molecules embedded in the bilayer; acylation and prenylation used for cytoplasmic proteins; GPI-modifications used for exoplasmic proteins
Topology of Membrane Proteins
Membrane proteins have a specific orientation with respect to the membrane face; topology maintained during membrane budding and fusion; radioactive labeling can be used to determine topology of membrane proteins
Glyoproteins
Membrane proteins with carbohydrate chains covalently linked to amino acid side chains - glycosylation; occurs in the ER and Golgi; in plasma membrane proteins, carbohydrates present on exoplasmic domains;
Glycosylation of Proteins
N-linked glycosylation occurs co- and post-translationally in the rough ER and Golgi
O-linked glycosylation occurs strictly post-translationally and only in Golgi
Mobility of Membrane Proteins
Some proteins move freely, whereas others are constrained because they are anchored or associated in complexes; studied using FRAP and cell membrane fusion experiments (fluorescent antibody tags)
How do we study membrane proteins?
SDS-PAGE
X-ray Crystallography
Molecular Biology
Isolation of Membrane Proteins
Integral Membrane Proteins - use of detergents to disrupt lipid bilayer
Peripheral and Lipid-anchored Membrane Proteins - changes in pH or ionic strength or chelating agents
SDS - Polyacrylamide Gel Electrophoresis
Separates proteins by size
Hydropathy Plot
Hydrophobicity Plot; number and location of transmembrane segments in a membrane protein can be predicted
