2CB3: Chapter 4 - Part 1

Membrane Proteins and Lipids: Key Facts and Functions

• Membrane proteins can be integral, peripheral, or lipid-anchored, each with different interactions with the lipid bilayer
• Integral proteins, which make up 25-30% of all encoded proteins, act as receptors, channels, and agents for electron transport
• Isolating integral membrane proteins requires detergents like ionic SDS or nonionic Triton X-100
• Some challenges in studying integral proteins include low numbers per cell, instability in detergents, and glycosylation
• Transmembrane domains in integral proteins can be identified by a string of nonpolar amino acids spanning the lipid bilayer
• Hydropathy plots can help identify transmembrane segments based on amino acid sequence
• Peripheral proteins are easily solubilized and have a dynamic relationship with the membrane
• Lipid-anchored proteins are covalently linked to lipid molecules and can act as adhesion molecules or enzymes
• Membrane fluidity is crucial for the movement of molecules and is affected by the physical state of membrane lipids
• At higher temperatures, membrane lipids exist in a relatively fluid state, while at lower temperatures, they form a frozen crystalline gel
• Saturated fatty acids resemble a straight, flexible rod, while unsaturated fats can be cis or trans, with crooks in the chain at the sites of a double bond
• The structure of the lipid bilayer depends on temperature, with individual lipids able to rotate around their axis or move laterally within the plane

Overview of Plasma Membrane Functions and Composition

• Plasma membrane functions include compartmentalization, scaffold for biochemical activities, selective permeability barrier, solute transportation, response to external signals, intercellular interaction, and energy transduction
• Membranes enclose cells and intracellular compartments, sequestering hydrolytic enzymes and acid hydrolases in membrane-bound vacuoles
• Membranes act as scaffolds and sites for enzyme localization, such as CO2 fixation in the outer surface of chloroplasts
• Gated channels in membranes control the movement of selected molecules, allowing rapid penetration of water molecules through the plasma membrane
• Membrane proteins facilitate the movement of substances against concentration gradients, such as H+ ions
• Membrane receptors transduce signals from outside the cell in response to specific ligands, like the plant hormone Abscisic acid controlling Calcium efflux and stomatal closure
• Membranes mediate recognition and interaction between adjacent cells, allowing the flow of material through plasmodesmata
• Membranes are involved in processes converting one type of energy into another, such as the conversion of ADP to ATP in the inner mitochondrial membrane
• Studies on plasma membrane structure led to the discovery that the outer layer of cells is composed of lipids, based on the principle "like dissolves in like"
• Early 19th-century studies by E. Gorter and F. Grendel proposed that cell membranes might contain a lipid bilayer, supported by RBC experiments
• Cell physiologists determined that membranes contain proteins, which are single proteins or part of complexes, explaining the permeability and surface tensions of membranes
• Membranes are lipid-protein assemblies held together by noncovalent bonds, with the lipid bilayer as a structural backbone and barrier, and proteins determining specialized activities. Lipid composition includes phosphoglycerides, sphingolipids, and cholesterol, with phosphoglycerides having hydrophilic head groups and hydrophobic fatty acyl chains.

Membrane Lipids and Fluidity

• Cis fats are beneficial, promoting good cholesterol, while trans fats are harmful to cardiovascular health
• Factors affecting membrane fluidity include the degree of saturation of fatty acids, the length of fatty acyl chains, and the presence of cholesterol
• Cholesterol abolishes sharp transition temperatures and creates a condition of intermediate fluidity in membranes
• Membrane lipid composition changes in response to temperature fluctuations, with cells remodeling phospholipids for cold resistance
• Desaturation of single bonds in fatty acyl chains and reshuffling of chains between phospholipid molecules contribute to membrane remodeling
• Phospholipids can move laterally within the same leaflet of the plasma membrane with ease, while flipping between leaflets is less favorable
• Enzymes called flippases facilitate the movement of certain phospholipids between membrane leaflets
• Membrane proteins are not stagnant and can move within the plasma membrane
• Cell fusion techniques have revealed that membrane proteins can move between fused cells
• Fluorescence recovery after photobleaching (FRAP) and single particle tracking (SPT) are used to measure the diffusion rates of membrane proteins
• The extent of fluorescence recovery in FRAP provides a measure of the percentage of mobile molecules
• Protein mobility in the membrane can be limited by interactions with other integral proteins, the cytoskeleton, and directed interactions with other proteins