The Structure of Biological Membranes

Funcons of Cellular Membranes 1. Plasma membrane acts as a selecvely permeable barrier to the environment • Uptake of nutrients • Waste disposal • Maintains intracellular ionic milieau 2. Plasma membrane facilitates communicaon • With the environment • With other cells • Protein secreon 3. Intracellular membranes allow compartmentalizaon and separaon of different chemical reacon pathways • Increased efficiency through proximity • Prevent fule cycling through separaon Composion of Animal Cell Membranes

• Hydrated, proteinaceous lipid bilayers • By weight: 20% water, 80% solids • Solids: Lipid Protein ~90% Carbohydrate (~10%) • Phospholipids responsible for basic membrane bilayer structure and physical properes • Membranes are 2-dimensional fluids into which proteins are dissolved or embedded The Most Common Class of Phospholipid is Formed from a Gycerol-3-P Backbone Saturated Fay Acid

•Palmitate and stearate most common •14-26 carbons •Even # of carbons

Unsaturated Fay Acid Structure of Phosphoglycerides

All Membrane Lipids are Amphipathic Figure 10-2 Molecular Biology of the Cell (© Garland Science 2008) Phosphoglycerides are Classified by their Head Groups

Phosphadylethanolamine

Phosphadylcholine

Phosphadylserine

Phosphadylinositol Ether Bond at C1

PS and PI bear a net negave charge at neutral pH Sphingolipids are the Second Major Class of Phospholipid in Animal Cells Sphingosine

Ceramides contain sugar moies in ether linkage to sphingosine Glycolipids are Abundant in Brain Cells

Figure 10-18 Molecular Biology of the Cell (© Garland Science 2008) Membranes are formed by the tail to tail associaon of two lipid leaflets

Exoplasmic Leaflet Cytoplasmic Leaflet Bilayers are Thermodynamically Stable Structures Formed from Amphathic Lipids

(Hydrophobic)

(Hydrophilic)

Figure 10-11a Molecular Biology of the Cell (© Garland Science 2008) Lipid Bilayer Formaon is Driven by the Hydrophobic Effect • HE causes hydrophobic surfaces such as fay acyl chains to aggregate in water • Water molecules squeeze hydrophobic molecules into as compact a surface area as possible in order to the minimize the free energy (ΔG) of the system by maximizing the entropy (ΔS) or degree of disorder of the water molecules • ΔG = ΔΗ ‒ ΤΔS

Lipid Bilayer Formaon is a Spontaneous Process

Gently mix PL and water

Vigorous mixing

Figure 1-12 Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008) The Formaon of Cell-Like Spherical Water-Filled Bilayers is Energecally Favorable

Figure 10-8 Molecular Biology of the Cell (© Garland Science 2008) Phospholipids are Mesomorphic

Figure 10-7 Molecular Biology of the Cell (© Garland Science 2008) PhosphoLipid Movements within Bilayers (µM/sec)

(1012-1013/sec) (108-109/sec)

Figure 10-11b Molecular Biology of the Cell (© Garland Science 2008) Pure Phospholipid Bilayers Undergo Phase Transion

Gauche Isomer Formaon

All-Trans

Below Lipid Phase Transion Temp Above Lipid Phase Transion Temp Phosphoglyceride Biosynthesis Occurs at the Cytoplasmic Face of the ER

Figure 12-57 Molecular Biology of the Cell (© Garland Science 2008) A “Scramblase” Enzyme Catalyzes Symmetric Growth of Both Leaflets in the ER

Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008) The Two Plasma Membrane Leaflets Possess Different Lipid Composions

Enriched in PC, SM, Glycolipids

Enriched in PE, PS, PI

Figure 10-16 Molecular Biology of the Cell (© Garland Science 2008) A “Flippase” Enzyme promotes Lipid Asymmetry in the Plasma Membrane

Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008) Membrane Proteins May Selecvely Interact with Specific Lipids (Lipid Annulus or Halo)

Lipid Asymmetry Also Exists in the Plane of the Bilayer Phospholipids are Involved in Signal Transducon 1. Acvaon of Lipid Kinases

PI-4,5P PI-3,4,5P

Figure 10-17a Molecular Biology of the Cell (© Garland Science 2008) 2. Phospholipases Produce Signaling Molecules via the Degradaon of Phospholipids Phospholipase C Acvaon Produces Two Intracellular Signaling Molecules

Diacylglycerol

PI-3,4,5P

I-3,4,5P

Figure 10-17b Molecular Biology of the Cell (© Garland Science 2008) Amphipathic Lipids Containing Rigid Planar Rings Are Important Components of Biological Membranes

Flexible Hydrocarbon Tail

Hydrophilic End How Cholesterol Integrates into a Phospholipid Bilayer

C12

Figure 10-5 Molecular Biology of the Cell (© Garland Science 2008) Cholesterol Biosynthesis Occurs in the Cytosol and at the ER Membrane Through Isoprenoid Intermediates

(Rate-Liming Step in ER) Lipid Ras are Microdomains Enriched in Cholesterol and SM that may be Involved in Cell Signaling Processes

Insoluble in Triton X-100

Figure 10-14b Molecular Biology of the Cell (© Garland Science 2008) Electron Force Microscopic Visualizaon of Lipid Ras in Arfical Bilayers

Yellow spikes are GPI-anchored protein

Figure 10-14a Molecular Biology of the Cell (© Garland Science 2008) 3 Ways in which Lipids May be Transferred Between Different Intracellular Compartments

Direct Protein-Mediated Soluble Lipid Vesicle Fusion Transfer Binding Proteins

The 3 Basic Categories of Membrane Protein

GPI Anchor

Single-Pass Mul-pass β-Strands

Transmembrane Helix

Linker Domain Fay acyl anchor

Integral Lipid- Peripheral Anchored (can also interact via PL headgroups)

Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008) 3 Types of Lipid Anchors

Figure 10-20 Molecular Biology of the Cell (© Garland Science 2008) Membrane Domains are “Inside-Out”

Right-Side Out Soluble Protein

Figure 3-5 Molecular Biology of the Cell (© Garland Science 2008) Funconal Characterizaon of Integral Membrane Proteins Requires Solubilizaon and Subsequent Reconstuon into a Lipid Bilayer

Figure 10-31 Molecular Biology of the Cell (© Garland Science 2008) Detergents are Crical for the Study of Integral Membrane Proteins

(Used to denature proteins)

(Used to purify Integral Membrane Proteins) Detergents Exist in Two Different States in Soluon

(Crical Micelle Concentraon)

Figure 10-29b Molecular Biology of the Cell (© Garland Science 2008) Detergent Solubilizaon of Membrane Proteins

Desirable for Purificaon of Integral Membrane Proteins Beta Sheet Secondary Structure Polypepde backbone is maximally extended (rise of 3.3 Å/residue)

H-Bond An-parallel Strands

Side chains alternately extend into opposite sides of the sheet β-Barrel Structure of the OmpX Porin Protein in the Outer Membrane of E. coli

Aromac Side Chain anchors

Alternang Aliphac Side Chains Transmembrane α-Helices 1. Right-handed 2. Stability in bilayer results from maximum hydrogen bonding of pepde backbone 3. Usually > 20 residues in length (rise of 1.5 Å/residue) H-Bond 4. Exhibit various degrees of lt with respect to the membrane and can bend due to helix breaking residues 5. Mostly hydrophobic side-chains in single-pass proteins 6. Mul-pass proteins can possess hydrophobic, polar, or amphipathic transmembrane helices Transmembrane Domains Can Oen be Accurately Idenfied by Hydrophobicity Analysis

Figure 10-22b Molecular Biology of the Cell (© Garland Science 2008) Bacteriorhodopsin of Halobacterium

Figure 10-32 Molecular Biology of the Cell (© Garland Science 2008) Structure of Bacteriorhodopsin

Figure 10-33 Molecular Biology of the Cell (© Garland Science 2008) Structure of the Glycophorin A Homodimer

Coiled-Coil Domains in Van Der Waals Contact

Arg and Lys Side Chain Anchors

23 residue transmembrane helices Structure of the Photosynthec Reacon Center of Rhodopseudomonas Viridis

Figure 10-34 Molecular Biology of the Cell (© Garland Science 2008) 4 Ways that Protein Mobility is Restricted in Biological Membranes

Intramembrane Protein- Interacon with the Protein Interacons cytoskeleton

Interacon with the Intercellular Protein- extracellular maxtrix Protein Interacons Figure 10-39 Molecular Biology of the Cell (© Garland Science 2008)