Chapter 9 Membrane Transport

Concentration of ions in and out of neuron

Na+ higher extracellulary

K+ lower extracellulary

K+ ion & other ion movement results in…

relative positive charge outside of cell, relative negative inside cell. This resulting charge impablance generates a voltage across the membrane (membrane potential delta psi)

Δψ units

volts

Δψ =

Rt/Zf * ln(ion,in / ion, out)

Rt/Zf * ln(ion,in / ion, out) what is R, F, Z, T?

R = gas constant 8.3145 J/kmol

F = Faraday consantan, 96485 J/vmol

Z = net charge per ion

T = temp in Kelvin

Δψ =, for monovlanet ion (Z=1) at 20 degrees celsius

Δψ = 0.058 vlog10 ([ion in ]/[ion out/)

typical membrane potential of animal cells, what it indicates

-70 mV; indicates that inside/cytosel is more negative than outside/extracellular fluid

how are action potentials fired

Nerve stimulated → PM Na+ channels open → Na+ enters cell

increased voltage from -70 to threshold (up to +50 mV), reveral of membrane potential causes cell depolarization, which causes an action potential

impacts of APs

1) triggers opening of nearby voltage-gated K+ channels

2) stimulates opening of additional Na+ channels further along the axon → chain reaction causes action potential to travel

can AP signal go backwards? why/why not?

can’t go backwards

once ion channels have shut, they remain closed for a few milliseconds

purpose of myelin sheath

ensures rapid propagation of action potentials by insulating the axon

structure/components of myelin sheath

several layers of membrance (derived from another cell) coiled around axon

rich in sphingolipids, contains little proteins

how does myelin sheath increase AP propagation

by insulation

prevents ion movement except at nodes between myelin axon segments, so AP ‘jumps’ from node to node, which makes the AP travel ~20X faster than in unmyelinated axon

2 large groups of transport proteins

Na+ and K+ channels

important distinction in types of transport proteins

whether they require a source of free energy to operate

Neuronal Na+ and K+ channels considered ___ transporters because ___

passive transporters; they provide means for ions to move down a concentration gradient, so thermodynamically favored

equation for free energy change while transporting an ion

ΔG= RT In ([X]in/[X]out) + ZFΔψ

what is true about transport for anionic substances with charge Z

transport may not be thermodynamically favored, depending of mV Δψ, even if correct concentration gradient alone favors transport

the protein initially establishing and maintaining cells’ Na+ and K+ gradients is what kind of transporter? What does this mean?

is an active transporter; needs free energy of ATP hydrolysis reaction to move ions against their concentration gradient

porins

simplest form of passive transporter

structure of porins (subunits, secondary structures, side chains)

trimers with each subunit forming 16- or 18-stranded membrane beta barrels

there are loops connections these beta barrels

some may have several carboxylate side chains

selectivity and open-ness of porins

exhibit varying degrees of solute selectivity depending on geometry of barrel interior and nature of side chains projecting into it.

always considered open

some may have several carboxylate side chains, making porins weakly selective for cationic substances

structure of general ion channels

multimers of identical or similar subunits with alpha-helical membrane-spanning segments

example of well known ion channel and its structure

K+ channel in Streptomyces lividans

tetrameric, each subunit includes 2 long and 1 short alpha helices

K+ selectivity filter

structure in ion channels that selects for K+ to go through while preventing other, smaller ions like Na+ form just diffusing through

how does K+ selectivity filter specifically select for K+/prevent Na+

narrows to about 3 angstroms at one point

carbonyl oxygens coordinate desolvated K+ ions (which have diameter of 2.67 angstroms) to go through

desolvated Na+ ions (diameter is 1.90 angstroms) are too small to coordinate with these carbonyl groups so they are excluded from pore/can’t cross

gated channels

channels that undergo conformation changes in response to some kind of environmental factor to open/close channel

what would happen if channels, such as K+ and Na+, were always open in neurons/nerves?

action potentials couldn’t fire/propagate and membrane potential would reach equilibrium. Cells would die

how does antifungal agent amphotericin B work

it kills a variety of pathogenic fungi by forcing ion channels open, causing lack of APs and eventual cell death

3 main mechanisms of gated channels

1) voltage-gated, opens in response to depolarization

2) reposition of N-terminal segment of protein to block cytoplasmic pore opening

3) response to membrane tension

aquaporins

channel allowing water to pass through lipid bilayer (water travels both via diffusion and aquaporins)

AQP1 and structure

best-defined aquaporin

is homotetramer with subunits consisting mostly of 6 membrane-spanning alpha helices, plus 2 shorter helices that lie within dimensions of bilary

can H3O+ or protons freely pass through aquaporins? why/why not?

no! aquaporins interrupt H-bonded chain of water (H3O+) molecules in pores → occurs when asparagine side chains transiently form H bond to passing water molecules

GLUT and steps to work

glucose-transporting family of gated channels

1) extracellular glucose binds to outward-facing conformations of protein

2) ligand binding triggers conformational change (via rocking motion) that exposes bound glutamate to intracellular side of membrane

3) bound glucose dissociates

4) inward-facing conformation switches back to outward-facing, ready to bind another glucose

can move glu in either direction depending on [glu]in and [glu]out

uniport transport system

moves single substance at a time, ex gluT

symporter transport system

transports 2 different substances across the membrane in the same direction

antiporter transport system

moves 2 different substances in opposite directions across membrane

what is Na, K-ATPase

active antiport transporter protein that largely maintains Na+/K+ concentrations inside/outside of eukaryotic cells

what provides Na, K-ATPase energy

ATP hydrolysis provides free energy; allows Na+ to be pumped in and K+ to be pumped out against their concentration gradients. Energetically favorable ATP → ADP + Pi drives energetically unfavorable transport of Na+ and K+

what happens in each Na, K-ATPase cycle

each cycle hydrolyzes 1 ATP, then pumps 3 Na+ out and 2 K+ in. this is a multistep process involving phosphorylated protein intermediates, which ensures that the transporter opens in only one direction

Na, K-ATPase structure

ATP binding site and aspartate residue that becomes phosphorylated during reaction cycle are located in cytoplasmic domains

what type of ATPase is Na, K-ATPase? other types?

p-type ATPase (P = phosphorylation)

other types: v-type ATPases in plant vacuoles, F-type ATPases (work in reverse) that synthesize ATP in mitochondria and chloroplasts

what do ABC transporters do

protect cells from toxic substances that insert themselves into the lipid bilayer and alter membrane structure and functio

ABC transporter in humans

P-glycoprotein

how do ABC transporters generally work

ATP-dependent conformation changes in cytoplasmic portion of protein are coupled to conformation changes in membrane-embedded portion of protein. this is how GLUT works, but in a reverse orientation

lipid flippases

ABC transporter that is specific to lipids

secondary active transport

when transporters take advantage of a concentration (or other) gradient already established by another pump

final step of neurotransmission?

release of neurotransmitters to postsynaptic cell

nt between motor neurons and muscles

acetylcholine, which is stored in synaptic vesicles

steps of nt release from axon terminals

1) AP reaches axon terminal, v-gated Ca2+ channels open and allow influx of extracellular Ca2+ ions

2) increase in local intracellular Ca2+ concentration triggers vesicle exocytosis

3) Ach diffuses across synaptic cleft and binds to integral membrane protein receptors on the muscle cell surface

then Ach degraded/recycled

how is Ach degraded at neural synapse

by acetylcholinesterase

how is Ach/other nts recycled

they’re transported back to presynaptic cell by secondary active transporters

ex: serotonin is recycled

SSRIs, how they work

selective serotonin reuptake inhibitors; block transporter recycling serotonin back to its presynaptic cell, so it continues to bind and serotonin signaling is enhanced

what do SNARE proteins do

physically pair 2 membranes together and induce them to fuse

structure of SNARE proteins

SNARE = soluble N-ethylmaleimide-sensitive-factor attachment protein receptor

integral membrane proteins

how is SNARE complex formed, is this favorable?

2 PM SNARE proteins and 1 synaptic vesicle SNARE form a complex with 4 alpha helices. formation of this complex is thermodynamically favored therefore membrane fusion is spontaneous

factors that may contribute to bilayer membrane shape changes in living cells

both SNAREs’ exerted tension

enzyme removal of acyl or head groups of lipids

endocytosis

opposite of exocytosis; new vesicle is formed by budding inwards from plasma membrane

2 main kinds of endocytosis

pinocytosis, receptor-mediated endocytosis

pinocytosis

formation of small intracellular vesicles that contain extracellular fluid plus whatever solutes are present

receptor-mediated endocytosis

materials brought into cell must first bind specifically to a protein receptor on the cell surface

coat proteins, example

what vesicles are usually coated with, coating drives spherical budding of vesicle

ex: clathrin

exosomes AKA microvesicles

smal extracellular membrane-encolsed particles that are involved in cell-cell communication and are present in bodily fluids (so can help with diagnostics for diseases, for example)