Chapter 11 Biological membranes and transport

Membranes (lipid bilayers)

define external boundaries of cells and organelles; control molecular traffic across the boundary (selectively permeable), organize complex reaction sequences and cell to cell communication

Membranes are..

flexible, self sealing, can fuse, and undergo fission

Phosphatidylethanolamine

phospholipid that makes up a large abundance of the inner monolayer of the phospholipid bilayer

Phosphatidylcholine

phospholipid that makes up a large abundance of the outer monolayer of the phospholipid bilayer

Relative proportions of protein and lipids

vary according to the type of cell the membrane is a part of as well as the organelle the membrane belongs to reflecting the diversity of biological functions for different cells

Micelles

spherical structures that contain anywhere from a few dozen to a few thousand amphipathic molecules; hydrophobic regions aggregated in the interior, where water is excluded, and their hydrophilic head groups at the surface,

Micelle formation

occurs when the cross sectional area of the head group is greater than the greater than the acyl side chain(s) aka the tail

Bilayer

lipid aggregate that forms two lipid monolayers (leaflets) into a two dimensional sheet

Bilayer formation

occurs when the cross section areas of the head group and acyl side chain(s) (tails) are similar (like in glycerophospholipids and sphingolipids); unstable due to hydrophilic interactions with water, creating a liposome

Liposome

lipid aggregate that forms a hollow sphere/vesicle; state in which bilayers achieve maximal stability in aqueous environments and contain a separate aqueous compartment that encloses water

Fluid mosaic model

structure of biological membranes

Phosphatidylserine

phospholipid that is typically in the inner monolayer; if shifted to the RIGHT, the cell plays its role in clot formation OR signals for apoptosis

Integral proteins

membrane bound proteins that have stretches of amino acids that traverse the lipid bilayer; classified based on their spatial relationship with the plasma membrane

Peripheral proteins

membrane associated proteins that associate with the membrane or with membrane bound proteins through charge charge, hydrogen bonding or van der Waals interactions, or those bound to the bilayer with a prosthetic group

Peripheral protein extraction

extraction that occurs via changes in pH or ionic strength, removal of Ca2 by a chelating agent, or addition of urea or carbonate

Integral protein extraction

extraction that occurs via detergents, which disrupt the hydrophobic interactions with the lipid bilayer and form micelle

Integral proteins attached to a membrane lipid covalently

can be released by treatment with phospholipase C

Integral/transverse protein structure

composed of an alpha helix with hydrophobic amino acids in the hydrophobic tail region of the membrane with multiple charge groups present to stabilize the helix

Bacteriorhodopsin

The most studied transmembrane protein. It is a light driven proton pump from Halobacterium salinarum; folds into seven hydrophobic alpha helices that traverse the lipid bilayer that each have hydrophobic amino acids with nontransmembrane parts being the only hydrophilic residues

Hydropathy plots

show the hydrophobic nature of the amino acid side chains for two integral membrane proteins; peaks above zero indicate hydrophobic amino acids and x axis indicates amount of amino acids (residues)

Tyrosine and Tryptophan

residues of membrane proteins that tend to cluster at the water lipid interface

Porins

proteins that allow certain membrane proteins; made up of beta strands which transverse the bilayer in antiparallel (barrel) orientation; residues on outside of barrel are hydrophobic; under strict control

Porins example

glucose (GLUT) transporters, FepA (iron uptake), OmpLA (phospholipase dimer), Maltoproin (maltose transporter (trimer)

Lipid linked membrane proteins

Covalently attached lipids that anchor membrane proteins to the lipid bilayer

Membrane dynamics

pliable, able to change shape while maintaining integrity; individual lipid molecules, membrane bound proteins, also move relative to each other

Gel phase/paracrystalline state

degree of disorder observed at cold temperatures, almost no movement of the lipid molecule

Liquid ordered state

degree of disorder observed at warm temperatures, molecular motion is evident as well as lateral movement

Liquid disordered state

degree of disorder at high enough temperatures where there is near total disorder within the acyl region of the bilayer

Fatty acid composition

changes at different temperatures with some the ratio of unsaturated fatty acids to fatty acids decreasing indicating an increase in saturated fatty acids

Flippase

move aminophospholipids from outer (exoplasmic) plasma membrane to inner (cytosolic) membrane

Floppase

move aminophospholipids from inner (cytosolic) plasma membrane to outer (exoplasmic) membrane

Scramblase

move lipid in either direction toward equilibrium

Spectrin and ankyrin

cytoskeletal proteins that anchor membrane spanning proteins in place

Spectrin

long filamentous cytoskeletal protein crosslinked at complexes containing actin that helps stabilize the membrane against deformation

Ankryin

cytoskeletal protein anchored in the membrane by a covalently bound palmitoyl side chain

Membrane fusion requirements

membranes must recognize each other, surfaces must be close to each other (apposed), be locally disrupted to result in hemifusion, and fusion

Hemifusion

the fusion of the outer leaflet (single layer of bilayer) of each membrane.

Fusion proteins

mediate the appropriate time to fuse membranes for regulated endocytosis

Kinds of membrane fusion events

budding of vesicles from Golgi complex, exocytosis, endocytosis, fusion of endosome and lysosome, viral infection, fusion of sperm and egg, fusion of small vacuoles, separation of two plasma membranes at cell division

Neurotransmitter fusion at a synapse

approach, binding and zipping of vSNARE(on vesicle) and tSNARE (on terminal cell) causing the outer leaflets of vesicle and cell to fuse then the inner leaflets until complete fusion opens a pore to release the vesicle’s contents

SNAREs

family of proteins that are involved in the fusion of neurotransmitters

Simple diffusion

movement of solutes particles across a permeable divider from a region of high concentration to lower concentration

Facilitated diffusion/passive transport

integral membrane proteins which lower the energy barrier for molecular movement across a membrane by providing a path for the molecules; movement with electrochemical gradients

Classes of facilitated diffusion

carriers (classic transporters) and channels (ion channels)

GLUT1 glucose transporter

structure that has 12 membrane spanning helices that, when side by side assemble helices that produce a channel lined with hydrophilic amino acids that can hydrogen bond glucose molecules to pass it through the membrane

ClHCO3 exchanger

cotransport system found in erythrocytes that’s primary role is to increase CO2 carrying capacity of the blood

Uniport

transporters that carry only one substrate ex, erythrocyte glucose transporter

Symport cotransporter

transporters that move two substrates simultaneously in the same direction

Antiport cotransporter

transporters that move two substrates simultaneously in opposite directions

Active transport

movement of solute molecules against the electrochemical gradient; energy required for this transport

Primary active transport

transport coupled with an exergonic chemical reaction such as ATP hydrolysis, ATP to ADP +Pi+energy

Secondary active transport

transport coupled with an exergonic flow of a different solute with electrochemical gradient

Na+ K+ATPase

example of a primary active transporter where binding affinity of the sodium and potassium are altered when the transporter opens into/out of the cell

ATP binding cassette transporters

a large family of ATP dependent transporters involved in transporting many different kinds of compounds out of cells against concentration gradients; some have low specificity, others have very high transport specificity

ABC transporters

found extensively in most all plants, animals and simpler organisms, large family of ATP dependent transporters that pump amino acids, peptides, proteins, metal ions, various lipids, bile salts, and many hydrophobic compounds, including drugs, out of cells against a concentration gradient; good for drug design bc associated with antibiotic resistance

Secondary active transport example

primary transport of H+ powers the secondary transport of lactose through a symport lactose transporters where lactose and H+ enter the cell

Symport lactose transporter mechanism

lactose binds to the middle of the transporter helices that will then undergo a conformation to open inwards and change conformation via changes to charged side chains releasing lactose into the periplasm then the cytoplasm

Aquaporins

provide channels for rapid movement of water molecules across all plasma membranes; humans have 10 different and Arabidopsis thaliana (plant) has 38 different genes for these; sometimes appear in high numbers like 2x10^5 per blood cell

AQP1

aquaporin made up of four monomers, each forming a channel through which water molecules can move single file

10^9 s^neg1

rate at which AQP1 facilitates water movement

Come back to slide 29

Ion selective channels/Gated ion channels

type of channel protein that is found in all cells and used to regulate the cytosolic concentrations of specific ions ex. K+ channels and Na+ channels

Difference between ion transporters and gated ion channels

Rate of transfer can be several orders of magnitude faster than transporters, Channels are not saturable (have no maximum w/ High substrate concentration), and transporters are Channels are gated: ligand gated, voltage gated

Ligand gated channels

channel that’s generally oligomeric, binding of an extracellular or intracellular small molecule forces an allosteric transition in the protein which opens or closes the channel

Voltage gated ion channels

protein channel where a change in transmembrane electrical potential (Vm) causes a charged protein domain to move relative to the membrane, opening or closing the ion channel.

What is the difference between a gated ion channel and a transporter (pump)?

an ion channel has a single gate while a transporter has 2 alternating gates

Gated Receptor desensitization

occurs when there is long term exposure to a ligand for gated receptors

Toxin targets and man made nerve agents

gated ion channels are targets as they’re underscored due to the potency of this