Ch 10 learning objectives Lipid bilayer & Membrane proteins

Amphipathic lipids

• contain a nonpolar domain (hydrophobic tails)

• has polar domain (hydrophilic heads)

• allows is to associate w/ “like” environments , forming bilayers, micelles, liposomes

Non polar Lipids

• any portions of a lipid that has non polar bonds

  • bonds w/ small EN differences , ex: CH

    • diff: 0 to 0.4 EN

Polar lipids

• any portions of a lipids that have polar bonds

  • bonds w/ larger EN differences , ex: OH

    • diff: 0.41 to 1 EN

what are the main phospholipids in most animal cell membranes

glycerophospholipids yelin= external

what are the 4 most common glycerophospholipids in mammalian membranesyin

• phosphatidylserine (PE)= internal

• phosphatidylserine (PS) = internal, neg. charged

• phosphatidylcholine (PC) = external

• sphingomyelin = extrenal

saturated vs unsaturated lipids

saturated = single bonds

unsaturated = has at least one double bond

Factors affecting membrane fluidity

1. Composition

   • Cholesterol: makes lipids stiff; less fluid

   • saturated fatty acids: less fluid

   • unsaturated fatty acids: more fluid

2. Length

   • short chains are more fluid

3. temperature

   • more fluid at high temperatures

Lateral diffusion

movement within the plane of the leaflet

Flip Flop ( transverse diffusion)

• lipids switch to different leaflet sides

  • needs the help of flippases, scamblases, & phospholipid translocases

  • cholesterol easily flip flops

Flexion

tails can flex and move

rotation

lipids can rotate

Why does asymmetry matter?

1. membrane change:

   • presence of phosphatidylserine (PS) on extracellular side signals cell death

2. cell signaling:

   • PS and phosphatidylinositol (PI) (also found on cytosolic side) bind intracellular signaling proteins

membrane lipid aggregation

• when lipids cluster together in patches / aggregates b/c they attract each other more than others

• These lipid aggregates can affect membrane’s flexibility, permeability, and ability to interact with other molecules.

• This process is essential for cell signaling, membrane trafficking, and maintaining cell structure.

Lipid rafts

• form in areas of greater fluidity (more saturated lipids)

• attracts transmembrane proteins w/ longer hydrophobic transmembrane domains

• important for cell signaling

• composed of

  • cholesterol

  • saturated hydrocarbons

  • glycolipids

    • carbohyrate groups are always on the non-cytosolic side

how does lipid aggregation & rafts affect cell signaling

• serve as platforms or "rafts" where signaling molecules (ex: receptors & enzymes) concentrate ; facilitate the efficient assembly of signaling complexes, allowing for more effective signal transduction

• By clustering specific lipids and associated proteins together, they create microdomains where signaling molecules can interact more readily

Lipid droplet

• storage for excess lipid from where they can be retrieved as building blocks for membrane synthesis or as a food source fueling metabolic energy generation

• surrounded by a phospholipid monolayer

formation of lipid droplet

• through a process involving the accumulation and packaging of neutral lipids, such as triglycerides and cholesterol esters.

• starts with :

  • Lipid Synthesis: Neutral lipids are synthesized within the ER membrane, primarily through enzymatic reactions involving fatty acids and glycerol.

  • Lipid Accumulation: As neutral lipids accumulate within the ER membrane, they start to form small aggregates in the interleaflet space of the membrane.

  • Budding and Sequestration: These lipid aggregates gradually coalesce and bud off from the ER membrane, forming small droplets within the cytoplasm. This process involves specific proteins, such as lipid droplet-associated proteins, which aid in the budding and stabilization of the lipid droplets.

  • Maturation: Once formed, lipid droplets can grow in size by continued incorporation of neutral lipids synthesized within the ER or by lipid uptake from the surrounding environment

single-pass α-helix with lipid anchor

• The α-helical region provides the hydrophobicity necessary for insertion into the lipid bilayer

• while the lipid anchor enhances membrane association and stability.

multipass protein

• spans the lipid bilayer multiple times, in a "snake-like" manner

• facilitating the transport of molecules across membranes, signal transduction, cell adhesion

• has multiple helixes

single pass transmembrane proteins

• they have COOH attached

• it has a helix

beta barrel (rolled up beta sheet)

• this is a bundle of multiple beta strands connect by hydrogen bonds & wraps around to form a close loop

alpha helix inserted in only one leaflet of the lipid bilayer

• this one is hydrophobic which is why its embedded in the hydrophobic region

lipid anchored protein

• attached to the lipid bilayer through covalent attachment to lipid molecules

GPI anchored protein

• are inserted into the outer leaflet of the lipid bilayer during their biosynthesis in the endoplasmic reticulum (ER)

  • first modified with GPI then inserted

• attached to the outer leaflet of the plasma membrane through a GPI moiety.

• This anchor consists of a complex glycolipid structure that is attached to the C-terminus of the protein.

• GPI-anchored proteins play roles in cell signaling, cell adhesion, and immune response.

Peripheral membrane proteins

• associate with the membrane through non-covalent interactions with other membrane-associated proteins or with lipid molecules

• on the cytoplasmic side or the extracellular side.

integral vs lipid anchored vs peripheral proteins

integral = #1,2,3,4,5,6 ← in the bilayer

• canNOT be extracted from the membrane by high salt concentrations or changes in the pH

lipid anchored = #7, 8 ← attached to the outside covalently

• the covalent attachment helps localize water soluble protein to a membrane after its synthesis in the cytosol

peripheral = # 9 , 10 ← attached to the outside non- covalently

• CAN be extracted from the membrane by high salt concentrations or changes in the pH

1. single pas a-helix w/ lipid anchor

2. multipass protein

   5. Beta barrel

• amphipathic

• interact w/ both sides of the plasma membrane

• a typical alpha helical transmembrane domain is composed of 20 -30 amino acids

• beta sheet transmembrane domain is about 10 amino acids in length

proteins that are in membrane are ….

nonpolar amino acids

proteins that interact with cytosol or extracellular fluid are …

polar amino acids

Do normal rules for amino acid composition of spontaneously folding alpha helices apply?

No!

• transmembrane proteins generally inserted by a translocator protein, which stabilize unfavorable helix components

• Chaperones can also stabilize proteins & prevent premature folding

Hydropathy plots

• hydropathy index

  • the hydrophobicity of an amino acid segment (positive hydropathy index = hydrophobic amino acid)

• hydropathy plots: demonstrate presence of membrane spanning alpha helices

• CANNOT identify membrane spanning beta sheets

• high positive values or peaks indicate hydrophobic segments that are likely to form transmembrane helices.

• reegions with negative values on the hydropathy plot correspond to hydrophilic segments that are typically located in the aqueous environment inside or outside the cell.

Reducing conditions

• external cell environment: Non- reducing

  • allows formation of disulfide bonds

• internal cell environment: Reducing

  • prevents formation of disulfide bonds

    • carbohydrate groups are found in the extracellular side of the plasma membrane

Lipid linked transmembrane proteins

• attached to cytosolic face by:

  • fatty acid chains

  • prenyl groups (ex: farnesyl)

• can be attached to the exoplasmic face by glycophosphatidylinosital (GPI) anchors

Detergents

amphipathic molecules used to solubilize integral membrane proteins

• ionic detergents (ex: SDS)

• Non-ionic detergents (ex: triton X-100 & Beta- octylgucoside)

• break apart and dissolve lipids and membrane proteins by interacting w/ hydrophobic/ hydrophilic domains

• “Cone shaped”

  • at low concentrations they exist as monomers

  • at high conc. they form micelles in water (Critical micelle concentration)

Critical micelle concentration (CMC)

• when detergent molecules in a solution begin to aggregate and form micelles.

• Micelles are spherical assemblies of detergent molecules arranged with their hydrophobic tails pointing inward and their hydrophilic heads facing outward

Chimeric cell studies

involve fusing two different types of cells, each labeled with different fluorescent markers. By observing the movement of these markers within the fused cell, researchers can infer the lateral diffusion of proteins or lipids in membranes. If proteins or lipids are free to move laterally within the membrane, then both markers should mix evenly throughout the fused cell over time. Conversely, if movement is restricted, distinct regions of the cell with different marker distributions will persist.

• Chimeric cell studies compare the mixing of different markers between fused cells to assess lateral diffusion

FRAP shows lateral diffusion by …

involve bleaching a small region of fluorescently labeled proteins or lipids within a cell membrane and then observing how quickly the fluorescence returns to the bleached area. If lateral diffusion is occurring, unbleached molecules from surrounding areas will move into the bleached region, resulting in a recovery of fluorescence over time. The rate of fluorescence recovery provides insights into the speed and extent of lateral diffusion in the membrane.

• directly observe the recovery of fluorescence in a bleached area to measure the speed of lateral movement.

Ways diffusion is restricted:

• aggregation of proteins

• binding of molecules to extracellular matrix

• binding to intracellular molecules (ex: cyto skeleton)

• binding to molecules on other cells (ex: cell junctions)

  • tight junctions = connect cells and prevent molecules from moving between cells

membrane curvature

• membrane protein & lipid composition can influence curvature

• important for intracellular organelles that need to change shape to create transport vesicles

  • factors that cause curvature

    • insertion in one leaflet causes bending

    • curved structure causes bending

    • large polar lipid heads ( phosphatidylinositol)