Membrane Structure and Composition

Membrane Structure and Composition

• Membranes are composed of lipids, proteins, and carbohydrates
• These components dictate several biologic functions such as cell signaling and transport
• Selective permeability supports physical and chemical compartmentalization of intracellular enzyme systems
• Membranes contain enzymes and receptors that allow cells to respond to external signals and generate chemical and electrical signals
• Permeability is controlled by molecular channels and pumps
• External surface composition determines cell-to-cell recognition processes involved in cell adhesion and immune responses

Membrane Components

• Lipids include phospholipids, sphingolipids, and cholesterol
• Phospholipids contain two fatty acids attached to glycerol in addition to a phosphate group
• Fatty acids can be saturated or unsaturated
• Most phospholipids have ethanolamine, choline, inositol, or serine esterified to the phosphate
• Sphingolipids include sphingomyelin, cerebrosides, and gangliosides
• Cerebrosides and gangliosides are sugar-containing lipids primarily in the plasma membrane
• Sphingomyelin is prominent in myelin sheaths
• Cholesterol is in the plasma membrane, with its hydroxyl group on the surface at the water interface
• Membranes are generally 40-50% protein, but can range from 20% in the myelin membrane to 80% in the inner mitochondrial membrane
• Protein and lipid composition is unique for each membrane, and distribution is asymmetric

Integral Membrane Proteins

• Integral membrane proteins may penetrate the membrane or exist as transmembrane proteins interfacing with both the cytosol and external environment
• They interact strongly with membrane lipids through hydrophobic side chains, removable only by destroying membrane structure
• They are usually composed of multiple α-helices with hydrophobic side chains
• Cylindrical arrays form pores for transport of polar molecules

Peripheral Membrane Proteins

• Peripheral membrane proteins are loosely associated with the surface of either side of the membrane
• They interact with the membrane through hydrogen bonding or salt-bridging
• They can be removed without disrupting the structure of the membrane
• Membrane carbohydrates exist only as extracellular covalent attachments to lipids and proteins
• Carbohydrate structures are highly variable and may be highly antigenic, contributing to the immune recognition of cells

Membrane Structure

• Membranes selectively separate internal and external aqueous compartments using a phospholipid bilayer
• The bilayer consists of two monolayers (leaflets) of phospholipids
• Hydrophilic phosphate head groups face the aqueous solution
• Hydrophobic fatty acid tails face the center of the bilayer
• Bilayers form sheetlike structures measuring 60-100 Å in thickness, held together by noncovalent forces
• Although the bilayer is symmetric regarding amphipathic lipid orientation, the composition is asymmetric
  • Exterior monolayer: contains mostly sphingomyelin and phosphatidylcholine
  • Interior monolayer: contains mostly phosphatidylserine and phosphatidylethanolamine
• Cholesterol is more evenly distributed, with composition determined by membrane function
• Membrane proteins are distributed asymmetrically to localize enzyme activity, energy transduction, facilitated transport, and receptors
• Peripheral proteins may contain a lipid anchor extending into the membrane
• Membrane composition and asymmetry are maintained by adding new membrane structure to existing structure
• Self-assembly permits self-sealing of damage to the phospholipid bilayer

Fluid Properties of Membranes

• The assembly of proteins and lipids into a membrane creates a fluid mosaic
• Both proteins and lipids undergo two-dimensional lateral diffusion in the membrane
• Transverse diffusion is energetically unfavorable, proteins and lipids do not "flip-flop" without flippase enzymes
• Fluidity is affected by factors:
  • Long-chain saturated fatty acids: Interact strongly and reduce fluidity
  • Double bonds: Increase fluidity, greater with cis-configuration than with trans-configuration
  • Cholesterol: Prevents movement of fatty acid chains and reduces fluidity
  • Temperature: Fluidity increases with temperature

Membrane Transport

• Membranes block hydrophilic molecules and restrict diffusion through the lipid bilayer
• Movement across the membrane is governed by simple diffusion, facilitated diffusion, and active transport

Simple Diffusion

• Water, gases (O2, CO2, NO), and lipophilic molecules cross membranes
• It occurs down a concentration gradient in either direction
• A steep concentration gradient produces faster diffusion compared to a shallow gradient
• Smaller molecules diffuse faster than larger molecules
• Simple diffusion is not saturable; the rate increases linearly with the increase in substrate concentration gradient

Facilitated Diffusion

• When molecules are excluded from simple diffusion due to size or charge, facilitated diffusion mechanisms exist
• Specialized carrier proteins in the membrane either diffuse across the membrane with their substrate or extend across the membrane, forming a channel
• Similarities to simple diffusion:
  • Diffusion occurs down a concentration gradient
  • Energy is supplied by the gradient, not cellular energy
• Differences from simple diffusion:
  • Facilitated diffusion is faster than simple diffusion
  • The carrier has specificity for the transported substance
  • Facilitated diffusion displays saturation kinetics
• Carrier proteins such as translocases, porters, and permeases show:
  • Structural specificity for transported molecules
  • Dissociation constant (Tm) for the transported molecule, analogous to Km for enzymes
  • Inhibition by agents that block the transport of specific molecules
  • Exhibited saturation kinetics (Vmax)
• Facilitated diffusion has three primary modes:
  • Ion channels: protein-lined channels that selectively allow ions to flow at a high rate when they are open
    • Formed by multiple transmembrane domains of the specific ion channel protein
    • Some channels are gated and open transiently in response to specific signals
  • Uniporters: facilitate diffusion of single substances, such as glucose or a specific amino acid
    • GLUT family of sodium-independent glucose transporters passively transport glucose, galactose, and fructose into most cells
  • Cotransporters: transport more than one molecule simultaneously
  • Symporters: carry two different molecules in the same direction at the same time
  • Antiporters: carry two different molecules in opposite directions at the same time
• Example of symporter: Glucose transporter in the kidney and intestine
• Example of antiporter: Chloride-bicarbonate transporter in erythrocyte membranes

Active Transport

• Carrier proteins transport molecules against a gradient
• Can be directly coupled to hydrolysis of ATP
• Specific for the molecules being transported
• Demonstrates saturation kinetics
• Can be specifically inhibited
• No ATP hydrolysis without transport
• Example of active transport is the sodium/potassium ATPase antiporter
• Maintains low intracellular Na+ and high intracellular K+
• Pumps 3 Na+ out and 2 K+ in for every ATP hydrolyzed

Intracellular Signal Transduction

• Hormones are physiologic signals that influence cellular metabolism by triggering coordinated intracellular responses
• The signal travels from the hormone molecule to the change in activity of the target enzymes
• It is transmitted through a signal transduction cascade
• Since each reaction step in the cascade produces a catalyst as its product, each step serves to amplify the signal

Plasma Membrane Receptors

• Plasma membrane receptors are transmembrane proteins that generate an intracellular response following binding of hormones, cytokines, or other signals on the exterior
• They share several characteristics with enzymes:
  • Hormone binding induces a conformational change in the receptor protein
  • Hormone binding demonstrates reversibility
  • Hormone binding demonstrates inhibition
• The response to a given hormone can be positive or negative depending on which receptors are present

Cyclic Adenosine Monophosphate System

• When epinephrine and glucagon bind to their receptors, they send a wave of phosphorylation through the cell that leads to coordinated changes in metabolism
• The initial signal that is generated in this pathway is the second messenger molecule, cyclic adenosine monophosphate (cAMP)
• cAMP is synthesized by a membrane-bound adenylate cyclase when the hormone binds to the receptor
• Cyclic AMP concentration is determined by the balance between adenylate cyclase and cAMP phosphodiesterase activity
• Increased concentration of cAMP converts more protein kinase A to its active form
• Protein kinase A regulates a variety of target proteins and enzymes by phosphorylation with ATP

G-Protein-Mediated Signal Transduction

• Epinephrine and glucagon receptors activate a G-protein complex that interacts with adenylate cyclase
• G-protein-coupled receptors contain seven a-helical domains extending across the membrane
• Hormone binding causes a change in intracellular domain, allowing interaction with the heterotrimeric Gs protein
• The α-subunit of the Gs protein releases bound GDP and binds GTP
• The α-subunit-GTP complex dissociates from the β-γ dimer and interacts with adenylate cyclase
• Binding one hormone molecule causes the formation of several subunits; this amplifies he hormonal signal
• The α-subunit deactivates itself within minutes by hydrolyzing GTP to GDP GTPase activity; the GDP remains bound
• The α-subunit-GDP complex reassociates with the β-γ dimer to form an inactive complex
• Spontaneous GTP hydrolysis gives G-proteins an automatic deactivating mechanism

Desensitization to Epinephrine

• The epinephrine receptor undergoes accommodation to unchanging concentrations of epinephrine
• As the Gs subunits dissociate from the receptor, the β-adrenergic receptor kinase phosphorylates the cytoplasmic domain of the receptor
• The phosphorylated domain will not interact with G protein even with epinephrine bound to the receptor
• The kinase phosphorylates only the hormone-receptor complex
• Epinephrine levels must increase to generate a new active hormone-receptor complex

Phosphoinositide Cascade

• Some hormones such as angiotensin II, epinephrine (α1-receptors), vasopressin, and oxytocin stimulate the action of phospholipase C in the plasma membrane
• Phospholipase C hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG)
• IP3 causes a rapid release of Ca++ from the endoplasmic reticulum by opening Ca++ channels
• Cytosolic Ca++ then binds to calmodulin
• The Ca++-calmodulin complex then activates Ca++-calmodulin-dependent protein kinases
• The Ca++-calmodulin complex also activates a Ca++-ATPase pump, quickly restoring low intracellular [Ca++]
• Smooth muscle contraction is activated by Ca++ through this signaling mechanism
• Uncomplexed Ca++ also activates protein kinase C, which plays a role in platelet activation and prostaglandin action
• DAG increases the activity of protein kinase C by increasing its affinity for Ca++
• Protein kinase C regulates target proteins by serine and threonine phosphorylation
• The phosphoinositide-related hormones activate the Gq protein by allowing it to bind GTP
• Phosphatases degrade IP3 to inositol, and DAG is degraded to phosphatidic acid

Tyrosine Kinase Receptors

• Insulin and several other growth factors communicate their signal through tyrosine kinase receptors
• They span the plasma membrane with only one α-helix
• The intracellular domain has two types of tyrosine kinase catalytic activity:
  • The receptors phosphorylate themselves (autophosphorylation)
  • They phosphorylate tyrosine residues on target proteins
• Insulin receptor is a tetramer that is stabilized by internal disulfide bonds
• Upon binding insulin to the external domain, the internal tyrosine kinase domain phosphorylates tyrosine residues on insulin receptor substrate 1 (IRS-1) to transduce the insulin signal by two pathways:
  • IRS-1 converts phosphatidylinositol in the plasma membrane to PIP2, leading to activation of protein kinase B
  • IRS-1 converts inactive ras (another type of G-protein) into its active GTP-bound form which activates MAP kinase
• The insulin signal is terminated by endocytosis of the insulin-receptor complex in endosomes

Intracellular Receptors of Lipophilic Hormones

• Cytosolic receptors bind lipophilic hormones, such as the steroid hormones or retinoic acid
• Cytosolic receptor-hormone complexes are transported to the nucleus, where they regulate gene expression
• Their action is much slower than membrane receptor pathways, taking hours to days to reach full effect

Clinical Aspects of Intracellular Signaling

• Several bacterial toxins catalyze the covalent attachment of adenosine diphosphate (ADP)-ribose to G-proteins
  • Cholera toxin ADP-ribosylates the Gs α-subunit, leading to excessive water and electrolyte secretion
  • Pertussis toxin ADP-ribosylates the Gi α-subunit which produces whooping cough
  • Diphtheria toxin ADP-ribosylates eEF-2, blocking polypeptide synthesis.
• Erectile Dysfunction:
  • Erection of the penis involves release of NO in the corpus cavernosum as a result of sexual stimulation
  • NO activates guanylate cyclase, increasing cGMP levels, producing smooth muscle relaxation in the corpus cavernosum and allowing inflow of blood
  • Drugs enhance the effect of NO by inhibiting phosphodiesterase type 5