Pumps, Transporters, Channels Flashcards
disulfide bond on glycosylation site
makes it more compact
Plasma membrane permiability
- hydrophobic and small polar molecules (H2O) can pass lipid bilayer ions and larger polar molecules can not
organelle membranes
allow compartmentalization of cellular functions
challenge for the cell
maintaining concentration gradient against leaky pores and water efflux, this takes energy
along solute gradient
spontaneous
against solute gradient
requires energy driving it
osmosis
hypertonic, isotonic, hypotonic
hypertonic
more ions outside cell so water goes out
isotonic
equilibrium so = water going in and out
hypotonic
more ions inside than outside so water goes into cell
electrochemical gradient
chemical gradient creates membrane potential b/c ions trying to flow in; more efficient to move pos molecules across gradient than negative
membrane potential
outside of cell diff concentration than inside of cell
simple difusion
molecule just crosses plasma membrane
passive transport
go through channel mediated or transport mediated passage
active transport
use energy to move molecule against concentration gradient
passive transport
- Simple diffusion (along a gradient)
- Osmosis (hydrostatic pressure)
- Facilitated diffusion (protein mediated) Diffusion facilitated by channels and solute carriers, allows molecules to pass otherwise impermeable membrane along their concentration gradient
Carrier
transmembrane proteins that expose solute binding site; transport involves conformational changes that exposes solute binding site to other side of membrane where solute is released
channels
pore forming transmembrane protein that allow flux of solute molecules to cross membrane; allows rapid transport bc weak pore interaction with molecule ; open and close via conformational changes in inner helices that line the pore
ionophores
channel forming protein that allows ion to cross membrane by sheathing its charge; found in microorganisms; work for passive transport
Rumensin
used in beef and dairy to improve growth rates and prevent coccidiosis (ionophore)
ion channels used for
passive fast transport
have selective hydrophobic pore
provide pathway for charged ions to penetrate hydrophobic cell membrane; ions can pass passively through water filled pore formed by channel
Different modes of ion channel regulation
- voltage gated channels
- ligand gated channels
- mechanically gated channels (affected by things like stress)
Why don’t small ions like Na+ pass through K+ channel
b/c channel is highly selective for K+; selectivity filters read radius and charge of ion and only if its right ion does it go through
- when K+ goes through ion dehydrated when entering filter and electrostatic interactions w/ water are replaced by carbonyl oxygen atoms lining pore, oxygen atoms spaced too far apart to strip Na+ ions of water molecules in K+ channel so they can’t enter
Mechanosenstive channels
controled by mechanics, react to hight of membrane, stress on membrane ect.
Voltage- gated channels
wide transmembrane helices that read membrane potential via voltage sensor
voltage gated cat ion channels
composed of four highly similar segments clusters around central pore; loop btwn 2 membrane spanning domains makes pore
- cell depolarizes and positively charged AAs move within charged voltage fields -> conformational change and gating of channel
voltage gated anion channels
cl-; mediate hyperpolarizatoin of muscle and nerve
3 states of ion channel
- Closed
- Inactivated
- Open
change in membrane voltage opens channel allowing ions to permeate, channel inactivates while cell depolarized bc separate inactivation particle binds to vestibule; membrane depolarizes and voltage gate closes and inactivation peptide binding is relieved
Ligand gated ion channels
Neurotrasmitter-relsease triggers channel opening (chemical signal -> electrical signal)
- opened by binding of one or more ligands to external channel surface
- mediate excitatory and inhibitory postsynaptic potentials
- channels also gated by intracellular ligands and mechanical stimuli
- transport can be triggered by second messengersr biding
Acetylcholine receptor
at neuromuscular junction; ligand-gated channel; binding of two acetylcholine molecules opens the channel that has relatively broad selectivity for cations
interplay of multiple channel systems
multiple channels work together from chem signals -> electrical signals
Transporters
- require large conformational changes
- can work with or against gradient where channels can only go in passive direction
- stronger interaction with substrate so works slower than channels
Basic principles of transporter fx
- Strong interaction with substrate on one side
- Reversible conformational change (with and without substrate)
- Exposure of substrate binding site on opposite side of bilayer and release of substrate
- “Alternate Access” model (open on 1 side close open on other)
Transporters (carriers) vs channels
Transporters Channels
- strong interaction w - weak interaction with
cargo cargo
- slow transport - rapid transport
-can work bidirectional - unidirectional (only
(w/ or against gradient) along a gradient)
Active transport
requires energy to transport molecules against electrochemical; gradient (energy in form of ATP or light); these transporters aka pumps
3 ways of driving active transport
- Coupled Transporter
- ATP driven pump
- Light- Driven Pump
Coupled transport
aka co-transport; use energy stored in electrochemical gradient
ATP driven pump
consumes 1 ATP molecule per cycle; this is facilitated by energy ATP consumption
Light-Driven pump
facilitated by light
Uniporter
passive or active transport; regulation by voltage, stress, or ligand
symport
Active transport; 2 molecules transported in same direction (ie from 1 side of membrane to other)
antiport
Active transport; molecules transported in opposite direct w/ respect to membrane
Na+ coupled glucose transporter
Uses Na+ gradient to transport glucose (Na+ binds increasing affinity for glucose and allowing both to be transported)
- cooperative binding sites
- undergoes conformational change upon transport
- toggles between several states
** Still active transport (secondary) bc still uses ATP bc will eventually have to pump Na+ out of cytosol w/ ATP driven pump
Primary active transport and secondary active transport both
Use energy to relocate substrates across membranes; transport against concentration gradient
primary active transport
ATPases (consuming energy in form of ATP)
Secondary active transport
coupled to Na+ (plasma membrane) or H+ (bacterial and organelles) co-transport using energy stored in electrochemical gradient
- Na+ and H+ will subsequently be pumped out of cytosol by ATP-driven Na+ or H+ pumps that maintain Na+ or H+ gradient across cell membrane
Trans-celluar transport of glucose
Gut -> Blood
Gut has low glucose gradient so use Na+ powered symporter to cross into interstitum from gut lumen
Blood has low glucose concentration so passive transport into blood + Na+/K+ pump to maintain cellular [Na+] which uses ATP
Sodium pump
restores Na+ and K+ gradient across plasma membrane by moving Na+ and K+ against concentration gradients via antiporter; driven by ATP hydrolysis (~1/3-2/3 cellular energy consumption dedicated to maintaining this gradient)
Pumping directions:
Na+ inside cell -> outside
K+ outside cell -> inside
* also called Na+ pump, Na+/K+ pump and ATPase
* without this pump Na+ and Cl- would leak into cell -> swelling
P-type transport ATPases mechanism
Overall mechanism applies to
Na+/ K+ pump
Ca2+ pump
H+ pump
In Na+/ K+ example
sodium binds ATP hydrolyzes -> conformational change -> sodium release on other side -> K+ binds -> dephosphorylaion -> K+ released on other side
P-type transport ATPases
- Na+/ K+ antiporter for maintenance of cellular [Na+]/[K+]
- Ca2+ ATPase; removal of Ca2+ from cytosol after signaling; Steep [Ca2+] gradient across plasma membrane (or SR/ ER)
- H+/K+ pumps; avid secretions in stomach
Ca2+ gradient
- maintained by two systems in plasma membrane
- antiporter system (Na+ driven Ca2+ exchange) and Ca2+ pump
- Structure Ca2+ pump:
- unphosphorylated- Ca2+ binding cavity exposed to cytosol
- phosphorylated- Ca2+ released on luminal side
Ca2+ storage
stored in ER/ SR so intracellular Ca2+ pumped by Ca2+-ATPases in the plasma membrane and ER (SR)membranes (gets pumped into SR)
Types ATPases
- P-type ATPases (phosphorylatoin dependent)
- F-type ATPases
- V-type ATPases
F-type ATPases
- structurally unrelated to P-type ATPase
- located in bacteria, mitochondria, chloroplasts
- usually run in reverse (ATP production); ATP synthase
- transmembrane units cross protein across gradient passively powering ATP synthase (turns basically a cytosolic turbine which rotates as protons move through channels)
V-type ATPases
- structurally related to F-type ATPases
- located in lysosomes, synaptic vesicles, plant vacuoles
- acidification of organelle interior
How does ATP synthase work
proton driven conformational change in membrane -> rotate stalk -> protons released on other side membrane by exit channel -> rotation -> enzymatic units binding phosphate and ADP -> ATP
ABC transporters
- characterized by two highly conserved ATP-binding cassettes
- constitute largest family of membrane transport proteins
- in bacteria use energy stored in ATP or proton gradient to import nutrients or small molecules
- in eukaryotes medically relevant ABC transporters = multi drug resistant proteins found over expressed in tumors (can pump hydrophobic drugs out cytosol making tumor cells resistant to cytotoxic drugs ie chemo)
- chloroquine transporter in Plasmodium falciparum responsible for resistance against antimalarial drugs
- cystic fibrosis transmembrane conductance regulator found mutated in CF (looks like a transporter but functions like a channel)
