The Lipid Bilayer: Composition and Structural Organization PDF

1 THE LIPID BILAYER: COMPOSITION AND STRUCTURAL ORGANIZATION 10/2/2024 Tobias Weinrich, PhD School of Integrative Biological and Chemical Sciences  2 Student Learning Outcomes ▪ Reading guide learning objectives. ▪...

1 THE LIPID BILAYER: COMPOSITION AND STRUCTURAL ORGANIZATION 10/2/2024 Tobias Weinrich, PhD School of Integrative Biological and Chemical Sciences  2 Student Learning Outcomes ▪ Reading guide learning objectives. ▪ List the four primary features of a biological membrane and explain why they are important for cellular function. ▪ Describe the fluid mosaic model of the cell membrane ▪ Explain how the chemical composition of a membrane (including lipids, carbohydrates, and proteins) contributes to its function. ▪ Explain the role of different lipid types in membrane fluidity and function. ▪ Illustrate how proteins are associated with the plasma membrane. ▪ Discuss the concepts of membrane fluidity, asymmetry, and heterogeneity.  3 Lecture Structure 1. Function 4. Membrane properties 2. Structure 4.1. Fluidity 3. Composition A. Restrictions to lipid mobility 3.1. Lipids B. Restrictions to protein mobility A. Phospholipids 4.2. Faces B. Cholesterol 4.3. Asymmetry C. Glycolipids A. Lipids 3.2. Proteins B. Proteins A. Membrane domains C. Carbohydrates B. Classification 4.4. Heterogeneity 3.3. Carbohydrates A. Lipid Rafts A. Glycolipids B. Caveola B. Glycoproteins 4.5. Permeability → Lecture 10 Glycocalyx  4 1. Function ▪ Cell boundary – defines a cell and separates inside from outside ▪ Selective barrier to molecules – impermeable to hydrophilic molecules ▪ Determines composition of cytoplasm ▪ Boundary of intracellular compartments ▪ Organization and localization of function – complexity and size ▪ Enzymatic and transport processes ▪ Signal detection and transmission ▪ Cell adhesion and cell-cell communication Exterior Cytosol  5 2. Structure ▪ Fluid mosaic model – 2D fluid with freely moving phospholipids and proteins ▪ Phospholipids – structural unit ▪ Proteins – specific functions ▪ Dynamic structures Glycolipids Glycoproteins Extracellular leaflet/face Polar head 5 nm Cytosolic leaflet/face Apolar tail  6 3. Composition Plasma Membrane bilayer 1. Lipids ≈ 50% ▪ The “fluid” part of the model ▪ Bilayer – impermeable barrier 2. Proteins ≈ 50% ▪ The “mosaic” part of the model Mitochondrial ▪ Embedded in the bilayer – specific functions membranes 3. Carbohydrates ≈ 2-10% ▪ Attached to lipids (glycolipids) and proteins (glycoproteins)  7 3.1. Lipids ▪ Phospholipid – structural component of bilayer ▪ Amphipathic – hydrophilic head & hydrophobic tails ▪ In aqueous solution ▪ Spontaneous formation of bilayers and liposomes  8 3.1. Lipids A. Phospholipids (+50%) PE ▪ Phosphoglycerides PS (glycerol) PC SM ▪ Sphingolipids (sphingosine) Structure cis- ▪ Hydrophobic tail = fatty acid (12-24 C long) trans- double bonds ethanolamine ▪ Hydrophilic head = glycerol + phosphate + polar molecule choline Serine Function inositol ▪ Structural foundation – barrier ▪ Fluidity & Flexibility ▪ Signaling and recognition ▪ Domains (lipid rafts) trans-double bond  9 3.1. Lipids Lipid composition B. Cholesterol (sterol) (20-25%) ▪ Animal cells only ▪ Structure ▪ Hydrophilic head + rigid planar steroid ring structure + nonpolar tail Rigid planar ring structure ▪ Function ▪ Increases stiffness of membranes → less fluid C. Glycolipids (5%) (discussed in 3.3. Carbohydrates)  10 3.1. Lipids ▪ Viscous consistency with fluid-like properties ▪ Factors that influence membrane fluidity ▪ Cholesterol decrease fluidity ▪ Phospholipids ▪ Shorter fatty acid tail ▪ Unsaturation (i.e. cis double increase fluidity bonds) gel-like → fluid-like ▪ High temperature Short fatty Long fatty acid tails acid tails Homeoviscous adaptation (poikilotherms) – change in lipid composition to maintain appropriate membrane fluidity in response to temperature changes  11 3.1. Lipids Transition or melting temperature (Tm) ▪ Temperature at which a lipid bilayer transitions from a gel-like state (rigid) to a more fluid-like (fluid) state Normal membrane ACTIVITY: predict Tm of membrane rich in saturated phospholipids  12 3.2. Proteins ▪ Membrane protein composition: Inner mitochondrial membrane – 70% ▪ Variable protein % Myelin sheath – 20% ▪ Phospholipid molecules : protein molecules – 50:1 ▪ Frequently linked to carbohydrates – glycoproteins ▪ Function: diverse ▪ Transporters ▪ Receptors ▪ Ion channels ▪ Enzymes ▪ Anchors ▪ Structure: ▪ Protein backbone – peptide bond – polar/hydrophilic  13 3.2. Proteins Structure: ▪ Transmembrane proteins ▪ ⍺ helix: single-pass or multi–pass ▪ Flexible ▪ β barrel: porins ▪ Stiff ▪ Hydropathy plots: distribution of hydrophobic amino acids along the of a peptide sequence hydrophobic hydrophilic  14 3.2. Proteins Classification: Exterior ▪ Transmembrane (integral) membrane proteins α-helix (20-30 hydrophobic aa) or β-sheets Cytosol 3 ▪ Single-pass 1 2 6 ▪ Multi-pass ▪ Lipid-anchored proteins – covalent bond ▪ Cytosolic – covalent bond with lipid (acyl link or prenyl link) ▪ Extracellular – glycosylphosphatidylinositol (GPI) ▪ Membrane-associated proteins (peripheral membrane proteins) 4 5 8 ▪ Cytosolic ▪ Extracellular Exterior Cytosol 7  15 3.3. Carbohydrates ▪ Asymmetric distribution: only found in extracellular face of membranes ▪ Integral components to membrane – synthesized by the cell Oligosaccharides (15 sugar monomers): ▪ Covalently attached to protein core → membrane proteoglycan ▪ Main component of extracellular matrix (ECM)  16 3.3. Carbohydrates - Glycocalyx ▪ Glycocalyx – carbohydrate coat that surrounds the cell Function: ▪ Cell-cell and cell-ECM adhesion ▪ Neutrophils adhesion to endothelial cells during infection ▪ Protection: mechanical, chemical, pathogen damage ▪ Microenvironments Glycocalyx ▪ Signaling ▪ Recognition: ▪ ABO system ▪ Major histocompatibility complex (MHC)  17 3.3. Carbohydrates - Glycocalyx Human ABO blood group antigens Enzyme O (all individuals) Blood group O ▪ Antigen: glycoprotein/glycolipid ▪ Oligosaccharide Enzyme A Enzyme B Blood group A Blood group B Organism will produce antibodies against antigen Blood Group Antigens on Serum Can Receive RBCs Antibodies Blood Types they do not produce A A Anti-B A and O Neutrophils adhesion to endothelial cells B B Anti-A B and O AB A and B None All 1. Recognition (lectin binding) O O Anti-A and O Anti-B 2. Firm adhesion (integrins) 3. Penetration endothelial cell wall 2 1 4. Entry into tissue and migration to site of infection 3 4  18 4.1. Properties – Fluidity Fluid mosaic model – 2D fluid with freely moving phospholipids and proteins Movements: ▪ Lateral diffusion (proteins and lipids) ▪ Lipids: 1-10 μm/second ▪ Proteins: 0.1-1 μm/second Slower for larger proteins ▪ Rotational/spin (protein and lipids) Protein lateral diffusion ▪ Flexion (lipids only) ▪ Flip-flop (lipids only) ▪ Spontaneously – rare ▪ Enzymatic activity – flippases Protein lateral diffusion  19 4.1. Properties – Fluidity A. RESTRICTIONS TO LIPID MOBILITY ▪ Lipids associated with proteins ▪ Lipid rich and protein rich domains B. RESTRICTIONS TO PROTEIN MOBILITY 1. Interacting with surface proteins of other cells (tight junctions) – lateral diffusion limited to membrane domains ▪ Apical domain ▪ Basolateral domain 2. Protein aggregation 3. Interacting with external complexes (e.g. focal adhesions – ECM) 4. Interacting with internal complexes (e.g. cytoskeleton – cortex)  20 4.1. Properties – Fluidity B. RESTRICTIONS TO PROTEIN MOBILITY 3. Interacting with external macromolecules (e.g. focal adhesions – ECM) Cytosol Plasma membrane Integrin (protein) Exterior (fibronectin) 4. Interacting with internal macromolecules (e.g. cytoskeleton – cortex) Exterior Cytosol Cytoskeleton fibers  21 4.2. Properties – Faces ▪ All membranes form closed compartments (i.e. cell, organelles) ▪ Exoplasmic face/leaflet ▪ Cytosolic face/leaflet ▪ Lumen 2x membrane: ▪ Nucleus ▪ Mitochondria ▪ Chloroplast  22 4.3. Properties – Asymmetry ▪ Distinct composition and distribution of lipids, proteins, and carbohydrates between cytosolic and exoplasmic faces – properties of each face Structural Asymmetry Functional Asymmetry A. Lipids ▪ Different phospholipid composition cytosolic vs exoplasmic face ▪ Cholesterol – evenly distributed During apoptosis (programmed cell death) PS translocate to the exoplasmic face – signaling its death Exoplasmic face Cytosolic face  23 4.3. Properties – Asymmetry Extracellular B. Proteins domain ▪ Asymmetric orientation ▪ Anchor proteins ▪ Peripheral membrane proteins Transmembrane domain ▪ Orientation of transmembrane proteins Cytosolic Unique topology: domain ▪ Extracellular domain ▪ Transmembrane domain ▪ Cytosolic domain ▪ Disulfide bonds – only in extracellular d domains and proteins C. Carbohydrate ▪ Exclusive to exoplasmic face: ▪ Glycolipids ▪ Glycoproteins  24 4.4. Properties – Heterogeneity ▪ Variation in composition, organization, and properties within a biological membrane ▪ Compartmentalize membrane into discrete functional domains LIPID RAFTS ▪ Clustered domains enriched in specific lipids (sphingolipids and cholesterol) and proteins (GPI-anchored proteins) ▪ Signaling A. Caveolae ▪ Lipid rafts rich in cholesterol ▪ 60- to 80-nm invaginations of the plasma membrane ▪ Endocytosis ▪ Cell signaling ▪ Regulation of lipid transport