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Lecture 4 - Membrane - Structure and Composition_notes
Learning Objectives
Explain the diverse functions and critical roles of the plasma membrane in cellular processes.
Describe the detailed structure of the plasma membrane, including its layers and components.
Identify various molecules crucial for membrane function and their interactions.
Describe the chemical properties of key membrane molecules: lipids, proteins, and carbohydrates.
Classify three distinct types of membrane proteins, elaborating on their key characteristics and functions.
Describe the Fluid Mosaic Model, including its historical significance and implications for membrane structure and function.
Explain the asymmetry of membrane bilayers and interpret relevant experimental evidence supporting asymmetric distribution of lipids.
Observing a Cell and Membrane Composition
Cells are enclosed by a defined boundary known as the plasma membrane, critical for maintaining homeostasis.
Historical Context:
Robert Hooke: First observed cells in cork (1665), introducing the term "cell".
Charles Overton: Conducted studies in 1889 on molecular permeability, discovering that nonpolar chemicals easily penetrate membranes, and proposed a lipid-based boundary layer.
Lipid Properties:
Lipids are dynamic molecules, allowing other lipophilic substances to freely enter and transition through the membrane, contributing to its function as a selective barrier.
Discovery of Lipid Bilayers
Gorter and Grendel’s Experiment (1925): Extracted lipids from Red Blood Cells and demonstrated that when spread out on a water surface, the lipid layer covers a surface area approximately twice that of the cells themselves, indicating the presence of a bilayer.
Functions of Cell Membrane
Key Functions:
Compartmentalization: Organizes cellular contents and separates biochemical processes.
Scaffold for Biochemical Activities: Serves as a foundation for proteins that conduct enzymatic reactions.
Selective Permeability Barrier: Regulates the entry and exit of substances, vital for ion balance and nutrient uptake.
Signal Transmission: Facilitates communication between cells and their environment by receiving and transmitting signals.
Cell-environment Interactions: Mediates interactions between the cell and its external environment, influencing processes like adhesion and signaling.
Energy Generation and Transfer: Plays a role in energy transduction paths, particularly in bioenergetic functions of mitochondria.
Composition of Cell Membrane
Components: Major constituents include lipids, carbohydrates, and proteins, each contributing to the membrane's properties and functions.
Fluid Mosaic Model
Proposed by S.J. Singer and Garth L. Nicolson (1972), suggesting that the plasma membrane is a dynamic mosaic of different types of lipids, proteins, cholesterol, and carbohydrates. This model emphasizes that components can move laterally within the layer without compromising membrane integrity, allowing fluidity essential for cellular function.
Freeze-fracture and Etching Technique
Procedure:
Cells are rapidly frozen in liquid nitrogen to preserve structural integrity.
The frozen specimen is fractured along lines of weakness inherent within the lipid bilayer. The fracture occurs on lines of weakness like between the lipid bilayer of the plasma membrane.
Freeze etching uses a vacuum to remove surface ice.
A carbon-platinum shadowing technique is employed to create a replica for detailed examination via electron microscopy.
Structural Components of Biomembranes
Lipids
Lipids, especially phospholipids, are the main building blocks of biological membranes.
Characteristics of Phospholipids:
Hydrophobic Tail: Composed of fatty acid chains that are repelled by water.
Hydrophilic Head: Contains a phosphate group that is attracted to water, enabling the formation of bilayers.
Phospholipids are described as amphipathic because they possess both hydrophilic and hydrophobic properties, facilitating bilayer formation.
Phospholipid Arrangement in Water
In aqueous environments, phospholipids self-assemble into distinct structures: liposomes, bilayers, and micelles, each serving unique functions in cellular processes.
Characteristics of Cellular Membranes
Membranes form sealed structures with defined outer and inner surfaces, preventing the exposure of hydrocarbon chains to the aqueous environment, thus maintaining cellular integrity. Phospholipids configure themselves into bilayers that yield distinct cytoplasmic and exoplasmic faces.
Types of Lipids in Biomembranes
Principal Classes:
Include sterols, phosphoglycerides, and sphingolipids, which all contribute to the structural diversity and functionality of membranes.
Phosphoglycerides Structure
Characterized by two fatty acyl chains linked to a glycerol phosphate backbone, with a polar head group attached to the phosphate.
Characteristics:
They are the most abundant phospholipid class in biomembranes, contributing significantly to membrane fluidity and integrity.
Classification of Phosphoglycerides
Diverse types based on their head group nature, including:
Phosphatidylcholine: The most abundant in mammalian cells.
Phosphatidylserine and Phosphatidylethanolamine: Involved in signaling and membrane curvature.
Phosphatidylinositides: Key players in intracellular signaling pathways.
Sphingolipids Overview
Derived from sphingosine, which includes long-chain fatty acids connected via amide bonds. Sphingolipids play crucial roles in signaling and structural functions within the membrane.
Polar Head Groups in Sphingolipids
Some sphingolipids, like sphingomyelin, feature phosphate-containing polar heads indicating their function as phospholipids; others, containing sugar head groups, function as glycolipids, often found in nerve tissues.
Sterols
Cholesterol and Its Analogs: Serve as important sterols in membranes, impacting fluidity and structural integrity.
Cholesterol is the major sterol in mammalian cells, often making up 50%-90% of membrane structures, while ergosterol is prevalent in fungi; unique sterols are also found in plant cells, such as stigmasterol and sitosterol.
Effects of Cholesterol on Membrane Fluidity
Bidirectional Effects:
At high temperatures, cholesterol decreases membrane fluidity, whereas at low temperatures, it increases fluidity.
Cholesterol acts to prevent lipid packing and the formation of crystalline structures, thus maintaining membrane flexibility and functionality in various environmental conditions.
Cholesterol acts as a buffering molecule in membranes and prevents abrupt changes in membrane fluidity over a range of temperatures.
Dynamic Nature of Lipids in Membranes
Lipids within membranes exhibit lateral diffusion and rotate freely, allowing for dynamic adaptation to the cellular environment.
They do not spontaneously migrate from one leaflet to the other. Migration requires moving the polar head group through the hydrophobic core. Achieved with help of enzyme called Flippases.
FRAP Experiment: Demonstrates how lipid migration is facilitated by enzymes known as "flippases", highlighting the energy-dependent aspects of membrane dynamics.
Lipid Dynamics Continued
Lipids can freely diffuse within their specific regions but face restrictions when attempting to cross into different regions, illustrating a more organized structure compared to pure lipid bilayers.
protein rich regions separate lipid rich regions. Lipids can diffuse within but not between.
lateral diffusion is slowed in the plasma membrane compared to pure bilayer
Factors Impacting Membrane Properties
1. Temperature
Reducing temperature shifts the membrane phase from a fluid to a gel-like state, which significantly alters lipid diffusion rates and membrane behavior.
undergoes a phase transition.
rate of diffusion of the lipid drops at phase-transition temperature
at physiological temperatures, the hydrophobic interior of natural membranes has low viscosity and fluid like consistency.
2. Saturation State of Phospholipids
Fatty acid chains in lipids can be:
fully saturated (No double bonds) and are solid at room temp. Eg. animal fat like butter
Cis-Unsaturated fatty acids (contains double bond) are liquid at room temp. Eg. plant fats such as olive oil.
presence of double bonds creates a kink in the chain and increases membrane fluidity.
3. Lipid Composition
The presence of different lipid types, such as cholesterol and sphingomyelin, can alter the thickness and curvature of the membrane, directly affecting its functional properties.
Lipid composition:
Location: Golgi membranes have more sphingolipids. ER membrane has more phospholipids.
Need: Cells lining intestinal track have higher sphingolipid concentration to counter harsh conditions encountered by these cells.
Cholesterol impacts fluidity of the membrane.
cholesterol acts as a buffering molecule in membrane and prevents abrupt changes in membrane fluidity over a range of temperature.
The lipid composition impacts the thickness and curvature of membranes.
cholesterol and sphingomyelin causes increase in membrane thickness. size and shape of head groups and tails influences curvature.
Regulation of Lipid Composition
Enzymatic processes mediate the remodeling and synthesis of membrane lipids, allowing cells to adapt their membrane compositions in response to changing environmental conditions.
Membrane remodeling is mediated by enzymes
Desaturase: converts single bonds in fatty acyl chains to form double bonds
Phsopholipases and Acyl transferases: reshuffles hydrocarbon chains between different lipid molecules
Flippases: catalyzes movement of lipid molecules from one leaflet to another
Most membrane lipids are synthesized in the ER and are inserted in the membrane via trafficking of vesicles.
Some organelles have resident enzymes to alter lipids or can make unique ones
Asymmetry in Membranes and Its Functional Importance
Asymmetrical arrangements of phospholipids across leaflets reflect their diverse functional roles, such as in cell signaling and stability. Specific lipids correlate with functional necessities crucial for cellular activities.
Composition of the cytoplasmic and extracellular leaflets differ:
Cytoplasmic leaflet: Rich in phosphatidylserine and phosphatidylethanolamine, which play roles in cell signaling and apoptosis.
Extracellular leaflet: Contains higher levels of sphingolipids and cholesterol, contributing to membrane fluidity and protection against environmental stress. Plays a role that can be bound by other molecules for signaling.
Lipid Rafts
Lipid rafts are specialized microdomains within membranes approximately 50 nm in diameter. Cholesterol contributes significantly to maintaining the structural integrity of these rafts, which facilitate important protein interactions and signaling pathways, contributing to the overall functionality of the cell membrane.
Membrane Complexity Beyond Lipids
In addition to lipids, carbohydrates and proteins are integral components of membranes, enhancing functionality, stability, and communication.
Membrane Proteins Overview
A significant number of genes encode for membrane proteins, whose abundance and function vary depending on cell type. They are pivotal in processes such as transport, signaling, cellular adhesion, and energy generation.
Functions include:
Transport
Cellular Signaling
Cell to Cell adhesion
Generating energy
antigen presentation and immune regulation
Types of Membrane Proteins
Integral Membrane Proteins: Span the entire lipid bilayer and are embedded within the membrane.
Peripheral Membrane Proteins: Associated through non-covalent interactions and can detach with changes in ionic conditions.
Lipid-anchored Proteins: Tethered to membrane lipids via covalent bonds, creating an asymmetrical orientation crucial for function.
Integral Membrane Proteins Characteristics
Spans the entire lipid bilayer.
These proteins exhibit amphipathic properties, facilitating interaction with both the hydrophobic lipid tails and the hydrophilic environment.
Integral proteins may form hydrophilic channels, assisting in transport across the bilayer.
To form hydrophobic membrane spanning regions Integral proteins may possess alpha-helical or beta-barrel structures that enable them to span the membrane, thus participating in selective transport and communication essential for cell survival.
Membrane spanning ⍺-helix has a continuous segment of 20-25 hydrophobic amino acids
The ⍺-helix places the hydrophobic side chains on the
outside ---> forms Van der Waal’s interaction with the fatty
acyl side chains
The hydrophilic peptide bonds connecting the amino acids are tucked inside (away from the fatty acid tails) ----> stabilized by H-bonds with each other
single a-helical domain is sufficient to incorporate an integral membrane protein onto a membrane
Glycophorin A (single pass), GPCRs & Aquaporins (multi-pass)
Predicting Transmembrane Domains
Assess “hydrophobicity” of each amino acid in the protein sequence.
Hydrophobicity is determined by: lipid solubility, energy needed to transfer form a non polar to an aqueous medium, hydrophobicity plot
Evaluating the hydrophobicity of amino acid sequences can predict potential transmembrane domains within integral proteins, aiding in their characterization.
Beta-sheet Barrel Transmembrane Proteins
These proteins include porins, which form channels in membrane structures of organelles, facilitating the passage of small molecules and ions.
No continuous segment of hydrophobic amino acids
In each subunit, 16-B stands form a sheet that twists into a barrel shaped structure with a pore at the center
outward facing side chains on each b strand is hydrophobic
B strands form a ribbon like band that encircles the outside of the barrel
inside of the barrel is hydrophilic
Detergents and Integral Proteins
The role of detergents in solubilizing integral proteins is highlighted, differentiating between ionic and non-ionic varieties, with implications for biochemical studies.
Detergents have a polar head group and a non-polar hydrocarbon chain
they solubilize integral proteins by binding to the hydrophobic regions (in the membrane fatty acid tails)
ionic will denature
non ionic do not denature
Transmembrane Protein Orientation
The importance of understanding the orientation (cytosolic vs. exoplasmic) of transmembrane proteins is emphasized due to its relevance in their functional roles within the cell.
Peripheral Membrane Proteins Characteristics
Peripheral proteins attach to membranes via non-covalent interactions with polar head groups or integral membrane proteins
network of peripheral proteins on the cytoplasmic side of the plasma membrane
Lipid-anchored Membrane Proteins
These proteins comprise covalent linkages to specific lipids, leading to functional asymmetry between the cytoplasmic and exoplasmic leaflets of the membrane.
covalently attached lipids anchor water soluble proteins to the membrane
Cytoplasmic Lipid-anchored Proteins
Detailed insights into processes such as fatty acylation and prenylation explain how these proteins are anchored to the cytoplasmic side of the membrane.
Acylation: covalent bond is formed between fatty acids and N-terminal glycine in target proteins
Prenylation: covalent bond is established between a lipid containing 3-4 isoprene units and cysteine residue in a protein at or near its c-TERMINUS
Exoplasmic Lipid anchored membrane protein
Discusses GPI (glycosylphosphatidylinositol) complexes and their components, showcasing how these structures influence membrane dynamics and cell interactions.
GPI anchored proteins: target proteins are attached to a GPI complex via an amide bond at its C-terminus
The exact structure of GPI anchors vary in different cell types
GPI complex consists of several components:
phosphatidylinsositol fatty acyl side chain extend into lipid bilayer
several sugar residues
phophoethanolamine: covalently connects the anchor to the C-terminus of the protein
GPI anchors are glycolipids.
Glycoproteins and Glycolipids
The presence of carbohydrate chains in glycoproteins and glycolipids is crucial for extracellular interactions, playing vital roles in processes like immune response and cell recognition.
Carbohydrate chains can be covalently linked to serine, threonine, or asparagine residues of transmembrane proteins (glycoproteins) or to lipids (glycolipids)
glycoproteins and glycolipids are always on the exoplasmic side and protrude outwards
they mediate interaction with extracellular proteins, growth factors, antibodies
