Part I => CARBS and LIPIDS. 1.5 Biological Membranes 1.5a Artificial Membranes 1.5b Lipid Bilayers

Transcription

1 Part I => CARBS and LIPIDS 1.5 Biological Membranes 1.5a Artificial Membranes 1.5b Lipid Bilayers

2 Section 1.5a: Artificial Membranes

3 Synopsis 1.5a - In water, apolar molecules such as fats and oils (eg triglycerides) aggregate ie they form non-homogeneous droplets due to the hydrophobic effect - In contrast, amphiphilic molecules (eg detergents and phospholipids) associate (but do not solubilize or dissolve!) into monolayers (micelles) or bilayers (bicelles) so as to form a homogenous solution with water - The ability of amphiphilic substances to form micelles or bicelles so as to shield their hydrophobic groups while exposing their hydrophilic groups to water is driven by the hydrophobic effect

4 Amphiphiles: Fatty Acids and Detergents Sodium Dodecyl Sulfate (SDS) Apolar Tail Polar Head - Most biomolecules harbor both hydrophilic and hydrophobic characters ie they possess polar (eg polarized and/or charged) and apolar regions/segments - Such biomolecules with hybrid character such as fatty acids (eg palmitate) and detergents (eg SDS) are said to be amphiphilic or amphipathic : amphi both philia attraction/liking phobia disliking/repulsion pathos to suffer/harbor - Detergents are used as cleaning agents due to their ability to interact with both water (through their charged head) and oily substances or stains (through their nonpolar tail) - Do amphiphiles such as fatty acids and detergents aggregate or solubilize in water?!

5 Lipids: Artificial Membrane Systems - In amphiphiles such as fatty acids, the polar head interacts with water via H- bonding while the apolar tails exclude water on thermodynamic grounds - Accordingly, amphiphiles aggregate in water in an highly ordered manner - Such ordered aggregates of amphiphiles form four distinct type of artificial/synthetic membrane systems: (1) Micelles (monolayers) (2) Bicelles (bilayers) (3) Liposomes (bilayers) (4) Nanodiscs (bilayers) - Formation of such artificial membrane systems is driven by the hydrophobic effect ie the ability to exclude water from their apolar tails!

6 (1) Detergent Micelles (Monolayers) Polar Head Central Cavity Water SDS (detergent) Apolar Tail SDS (amphiphile) - In aqueous solution, single-tailed detergents (amphiphilic) such as sodium dodecyl sulfate (SDS) and fatty acids associate (or aggregate) into higherorder structures called micelles m for monolayer! - Such arrangement allows the non-polar (apolar) tails to avoid contact with water while allowing their polar head groups to interact with the solvent - Formation of such micelles is driven by the fact that it is thermodynamically more favorable for the non-polar detergent tails to exclude water and engage in van der Waals contacts with the neighboring tails in addition to being entropically favorable (T S > 0) - The central cavity may also become filled with water depending on the concentration of detergent molecules Spherical Micelle (monolayer) Micelle (3D model)

7 (2) Lipid Bicelles (Bilayers) Polar Head Water Apolar Tails Phospholipid (amphiphile) Disk-like Bicelle (bilayer) Bicelle (3D model) - In a manner akin to the formation of detergent micelles, double-tailed amphiphilic molecules such as phospholipids associate (or aggregate) into higher-order disc-like structures called bicelles b for bilayer! - Unlike less crowded single-tailed detergents, unfavorable steric clashes between double-tailed lipids require them to pack into what are essentially disk-like bilayers (bicelles) in lieu of monolayers (micelles) albeit with monolayers at the edges! - Within such a bicelle, lipid heads stick out on the surface on each side, while tails align up in a more or less extended conformation against each other internally to generate what is predominantly a lipid bilayer this is the physicochemical basis of the assembly of biological membranes! - The ability of such amphiphilic molecules to exclude water from their non-polar surfaces is of course (!) called the hydrophobic effect it is also the basis of folding of proteins into 3D shapes!

8 (3) Liposomes (Bilayers) Central Cavity Lipid Bilayer Nutrient Delivery - When disrupted by physical treatments such as sonication (agitation with ultrasonic waves with ~ mm), phospholipid bicelles (lipid bilayers) form liposomes 3D spherical structures fully enclosed by a single lipid bilayer with a central aqueous cavity - Such liposomes not only serve as artificial membranes for research but can also be used as a vehicle for the delivery of hydrophilic nutrients and drugs (encapsulated in the central cavity) that do not readily diffuse through the cell membranes - Conversely, hydrophobic nutrients and drugs can also be delivered to specific tissues by virtue of the fact that they readily dissolve into the lipid bilayer of the liposomes - Upon reaching their target site, liposomes fuse with biological membranes, thereby emptying their contents into the inside of the cell

9 (4) Nanodiscs (Bilayers) Membrane scaffold proteins (MSPs) Lipid bilayer - Nanodiscs are comprised of a central phospholipid bilayer wherein the outer boundary/perimeter of apolar tails is shielded by amphipathic proteins called membrane scaffold proteins (MSPs) in a double-belt fashion - MSPs are usually modified apolipoproteins proteins involved in lipid metabolism that harbor amphipathic character - Nanodiscs serve as excellent artificial membranes for research in that they represent a more native system for the stabilization/folding of membrane proteins than lipsomes, bicelles and micelles - Nanodiscs may look like lipid bicelles (!) but nanodsics differ from bicelles in that while the former represent relatively homogeneous structures, the latter are highly heterogenous (poorly defined 3D architectures)

10 Exercise 1.5a - Describe the four types of artificial membranes - Why do phospholipids form bilayers rather than monolayers? - How do liposomes differ from micelles? - How do nanodiscs differ from bicelles?

11 Section 1.5b: Lipid Bilayers

12 Synopsis 1.5b - The cell membranes of living organisms, including intracellular organelles such as the mitochondrion and nucleus, are composed of lipid bilayers - Within such a lipid bilayer, the tails of lipids align up against each other while the heads point either toward the exterior or the interior of the cell in a manner akin to a liposome architecture - In addition to lipids, a wide plethora of so-called integral membrane proteins (IMPs) penetrate the lipid bilayer and laterally swim along the plane of the bilayer - IMPs contain a transmembrane structure consisting of α-helices or a - barrel with a hydrophobic surface - The dynamic arrangement and interactions of membrane lipids and proteins are described by the fluid mosaic model

13 Cell Membrane is a Lipid Bilayer - All living cells are enclosed by a semi-permeable barrier or cell membrane called lipid bilayer - The membranes of intracellular organelles such as the mitochondrion and nucleus are also composed of a lipid bilayer

14 Lipid Bilayer Is a 2D Fluid Lateral Diffusion Transverse Diffusion (Flip-flop) - Owing to the ability of lipids to rapidly exchange with each other within the plane of the same bilayer leaflet (lateral diffusion), biological membranes are highly dynamic and fluid structures - On the other hand, exchange of lipids across the bilayer leaflets (transverse diffusion) is extremely rare due to thermodynamic constraints such flip-flop requires the outer polar head to rotate and become momentarily immersed in the apolar environment of lipid tails at the core of the bilayer - The lipid tails within the core of the bilayer are under constant motion due to the free rotation about the C-C bond, while the motion of head groups is relatively restricted due to steric clashes or unfavorable polarity - Because of the mobility of lipids primarily within the plane of the same bilayer leaflet, the lipid bilayer is often described as a two-dimensional (2D) fluid cf the mobility of terrestrial animals (2D) vs birds/fish (3D)!

15 Lipid Bilayer Experiences Phase Transition Liquid Solution Viscous fluid with high mobility (T > T m ) Liquid Crystal Gel-like solid with an ordered array (T < T m ) - Although a pre-requisite for the living cell, the fluidity of lipid bilayer is in general highly dependent upon temperature (T) and lipid composition - Lipid bilayer undergoes a dramatic phase transition in vitro from being a highly viscous fluid (liquid solution) to a gel-like solid (liquid crystal) as the temperature is lowered to and below a certain threshold value (usually around 25 C) this is called transition temperature (T m ) the temperature at which the bilayer melts (cf water-ice phase transition)! - T m is highly dependent upon both the chain length (x) and degree of saturation (m) of fatty acids of membrane lipids longer the chain length and/or higher the degree of saturation, higher the T m! - Changing the composition of fatty acids of membrane lipids can attune the T m of lipid bilayer (so as to maintain its fluidity at ambient temperature) a necessity for cold-blooded animals such as fish - Fortunately, biological membranes experience little or negligible aforementioned bilayer phase transition how so? Enter cholesterol!

16 Effect of Cholesterol on Phase Transition Cholesterol Phospholipids T / C - By virtue of its ability to snug in between phospholipids, cholesterol serves as a fluidity buffer and either broadens or completely abolishes the phase transition observed in lipid bilayers in vitro in response to changes in temperature! - When T > T m, cholesterol (due to its highly rigid fused ring system) decreases bilayer fluidity by interfering with the motions of lipid tails - When T < T m, cholesterol increases bilayer fluidity by disrupting close packing of lipid tails

17 Lipid Bilayer Harbors Membrane Proteins Peripheral Membrane Protein Integral Membrane Proteins Extracellular Lipid Bilayer Peripheral Membrane Protein monotopic polytopic Cytoplasmic - In addition to lipids, proteins also constitute a major component of biological membranes such proteins are referred to as membrane proteins - Membrane proteins are classified into integral or peripheral depending on the nature of their interactions with the lipid bilayer - Integral membrane proteins (IMPs) traverse through the lipid bilayer once (monotopic) or multiple times (polytopic) IMPs are highly hydrophobic and insoluble (precipitate out) in water - Peripheral membrane proteins (PMPs) adhere to the surface of either the inner or outer leaflet of the bilayer via association with lipid head groups or non-transmembrane regions of IMPs PMPs are water-soluble

18 Extracellular Integral Membrane Proteins (IMPs) Bacteriorhodopsin (polytopic -helical) OmpF (antiparallel -barrel) Lipid Bilayer Cytoplasmic - IMPs are essentially amphiphiles the protein regions (or transmembrane segments) immersed within the milieu of the bilayer are predominantly composed of non-polar amino acid residues, while intervening regions (loops) residing on the extracellular and cytoplasmic faces are by and large dominated by polar and charged residues - Transmembrane segments traversing lipid bilayers (biological membranes) usually adopt two major folds or topologies -helical or -barrel - -barrel transmembrane proteins are essentially comprised of a large multi-stranded -sheet that twists and coils to form a closed hollow channel so as to allow passive diffusion of nutrients, salts and water - -helical transmembrane proteins can either traverse through the bilayer once (monotopic) or multiple times (polytopic) to form the so-called helical bundle and they conduct a multitude of roles from signal transduction (cell surface receptors) to energy generation (proton pumps)

19 Peripheral Membrane Proteins (PMPs) - PMPs adhere to the surface of either the inner or outer leaflet of the bilayer via association with lipid head groups or non-transmembrane regions of IMPs - PMPs are essentially water-soluble proteins that impart structural and functional versatility upon biological membranes eg attachment of spectrin, actin and ankyrin to the inner leaflet of biological membranes not only serves as the membrane skeleton that gives the cell shape but also provides a framework for the smooth integration and operation of signaling networks running from the nucleus to the events occurring at the cell surface

20 Fluid Mosaic Model - Lipid bilayer can be described as a mosaic a composite structure made up of a heterogeneous mixture of constituent components such as lipids and proteins arranged in an orderly manner the latter are further decorated with oligosaccharides to form what are called glycoproteins or proteoglycans - In addition to the ability of lipids to freely exchange within the same leaflet of lipid bilayer, integral membrane proteins also undergo a similar lateral diffusion along the plane of the bilayer they essentially float in a hydrophobic sea of lipid bilayer! - Given such 2D fluid of the bilayer with lipids and proteins constantly on the move, the lipid bilayer is best envisioned as a fluid mosaic such a model accounts for most of its biological properties!

21 Asymmetric Distribution of Lipids Asymmetric Distribution of Lipids in the Cell Membrane of Human Erythrocyte - Protein components of biological membranes are not evenly distributed on both sides eg the oligosaccharide moities of glycoproteins are almost exclusively attached to their extracellular regions (where they play a central role in mediating cell-cell interactions) - In a similar manner, lipids are also asymmetrically distributed on each face of the lipid bilayer eg sphingomyelin and phosphatidylcholine are predominantly located in the extracellular leaflet of erythrocytes, whereas phosphatidylethanolamine and phosphaitdylserine are on the cytoplasmic face - Such asymmetric distribution of lipids is necessary to attune specific cell types for their specific needs and physiological functions

22 Exercise 1.5b - Why do glycerophospholipids and sphingolipids but not fatty acids form bilayers? - Explain why lateral diffusion of membrane lipids is faster than transverse diffusion - What factors influence the fluidity of a bilayer? - What are the two types of secondary structures that occur in transmembrane proteins? - Describe the fluid mosaic model