transport proteins (1+2) Flashcards
what molecules do not need transporters to enter the cell
CO2, O2 and NO are relatively uncharged small molecules, they are lipophillic (not hydrophobic)
also of course standard lipophillic molecules
water can cross membrane without transporter due to high concentration and it is a small molecule and is uncharged
how are polar molecules in solution
they have hydration shells, energy is required to break hydration shells
this energy barrier prevents charged solutes entering lipid phase of lipid bilayer
how is the charge of cells compared to tissue fluid
generally cells are more negatively charged than tissue fluid
since it is relative by definition tissue fluid is at 0V
what is the free energy associated with the equlibria of a solute in/out of the cell
delta G = RTln (c2/c1) + ZFdeltaV
ZFdeltaV only applies to charged solutes
where Z is charge of solutes and delta V is membrane voltage
where do primary transporters get energy for transport
derive their energy from exergonic chemical reactions e.g active transport
describe concentrations of sodium and calcium in cells
calcium is deliberately removed from cytosol since it is used by most cells to stimulate several pathways
high conc of sodium outside cell is used as a source of free energy to transport other solutes into the cell
what is the pH inside mitochondria, why is this
inside mitochondria there is high pH (low H+), to allow for greater flow of H+ through ATP synthase
what are types of transport protein
channels: either ligand gated or voltage gated (very few freely open)
carriers: uniport, symport, antiport and primary transporters, carriers are never open all the way through
how does speed of channels compare to carriers
channels usually faster than carriers
what type of transport to channels mediate
rapid channels allow hydrated particles through, if dehydration is required they are slower
channels do not require conformational change to allow particles through other than to open/close
channels mediate passive transport, allow solutes to cross membrane, bringing it closer to electrochemical equilibrium
what are gap junctions
gap junctions are channels, they are large channels between cells which allow solutes to pass through without shedding hydration shells
each cell has a hemichannel which is made up of 6 connexin subunits
the channels only open when they contact another hemichannel on a nearby cell and will close on cell damage
the gap junction channels are important for cell to cell communication
diameter of these channels is 2nm, allows for unrestricted diffusion between cells of ions, sugars amino acids, nucleotides
proteins, polysaccharides and nucleic acids cannot get through
what are aquaporins
transport water
rate is 10^9/s
they are tetramers, even though there is a hole in the middle of channel this is not where transport occurs
protons cannot get through aquaporins, neither can hydroxonium ions
channel size of aquaporins is 2.8 angstroms
channel is lined with hydrophillic and hydrophobic residues that help position stream of single file water molecules and prevent proton hopping, protons are also repelled by a central positive region
describe a potassium ion channel
channel of streptomyces lividans:
most plasma membranes have K channels that are permantley open
3 angstrom diameter for channel hole
transport rate is 10^8 ions/s
it is tetrameric
oxygens of carbonyl group of main chain AAs point towards centre of pore as negatively charged residues on inside of pore repels negative particles
selective area of potassium ion channel is 12 angstroms
contains T-V-G-Y-G (threonine, valine, glycine, tyrosine, glycine) sequence, very common in potassium channels
interaction with carboxyl groups allow loss of hydration sphere required by small pore to be energetically favourable
ions with a radius larger than 3 angstroms cannot pass through selectivity filter, however smaller ions such as sodium cannot interact properly
4K+ binding sites means the next potassium can repel the next pushing it through the channel
negative ends at both ends of channel helps attract cations
describe the voltage gated sodium channel
sodium channel can usually transport lithium as well however is too small for potassium
heteromeric tetramer, each subunit has alpha helix on outside containing a row of positive residues
when membrane potential becomes more positive it pushes alpha helices up, as they move they turn, causing conformational change which opens the pore
opening of channel exposes binding site for inactivation plug, causing channel to be closed after 1ms, due to positive residues on plug and negative residues on binding site
selectivity filter in both sodium and calcium channels is formed by extracellular loops between alpha helices 5 and 6, in sodium channels all 4 loops are different
describe the acetylcholine receptor
heteropentameric ligand gated ion channel
containing 2 alpha subunits where Ach binds as well as a beta, gamma and delta subunit
transports 10^ 7 ions/s
pore is 6 angstroms wide
when Ach receptor is not bound to agonist leucine residues of the inner M2 alpha helix point to centre, they are very hydrophobic and so block hydrated ions
there are negative residues on either end of helix attracting cations and repelling anions
when agonist binds it causes M2 alpha helices to turn, hiding leucine residues and exposing small polar residues, which allows hydrated ions to travel through
why are channels faster than carriers
channels are water filled tubes through which solutes can pass, they often posses mechanisms to control what goes through (selectivity) however once open no conformational change is needed
carriers are saturable, channels are not hence why they are faster
compare graphs of rate of transport of carriers and channels, also compare what channels and carriers typically transport
carriers rate is usually a rectangular hyperbola curve till a maximum point
channels have straight line rates to the max
most organic molecules are carried by transporters, transport of solute requires carrier to change shape
solute binding to carriers allows them to be highly specific, they use ATP hydrolysis
channels usually transport ions etc
describe the glucose transporter
GLUT family are uniporters, they all transport D-glucose except GLUT5 which transports fructose
they contain 12 hydrophobic alpha helices
their structure has not yet been determined
9 of the alpha helices have some polar residues, it is possible that when assembled the polar residues face inwards, providing a surface to which glucose can H bond
there is a site which can bind to glucose solute facing either side of the membrane
shape can change either in absence of glucose, direction of transport is controlled by relative glucose concentrations either side of membrane, works with concentration gradient.
different members of GLUT family are regulated differently and expressed in different tissues. Their different Km values contribute to controlling glucose levels in cell
what is physiological range of glucose
5-10mM
compare Km values of GLUT2-4, compare where they exist and how their Km contributes to their function
the Km is the conc of substrate required to acheive half of the Vmax
GLUT 2 has Km of 20mM, in liver and pancreas (roles change depending on glucose conc), allowing a higher variability in rate of glucose transport depending on blood sugar
GLUT4 in muscle and adipose tissue, Km is 5mM, relatively unimportant, however sensitivity to insulin is important
GLUT3 in brain has Km of 1.6mM, good since brain needs good supply of glucose, allowing a relatively constant supply of glucose independent of blood sugar
describe lactose permease
lactose permease:
uses secondary active transport
prokaryotic symporter, allows prokaryotes to take up lactose even if lactose concentration outside cell is very low.
uses secondary active transport, lactose uptake uses H+ gradient created through oxidative phosphorylation to help bring lactose in (H+ is commonly used in bacterial secondary active transport)
H+ binds to COO- site in lactose permease creating COOH, which allows lactose to bind to transporter
when H+ binds to receptor receptor cannot change conformation (face intracellularly) without binding lactose first
H+ is much higher concentration outside cell since most intracellular H+ is pumped outside is ATP synthesis, this means it does not allow lactose to be transported outside cell
what are examples of antiporters, what are antiporters
uses transport of one type of particle inside the cell to facilitate transport of another outside or vice versa
examples: the adenine nucleotide translocator and the sodium calcium exchanger
what is function and mechanism of the adenine nucleotide transporter
adenine nucleotide translocator: supplies the cytosol with ATP produced in the mitochondra, it replaces it with ADP produced from anabolic reactions
the ATP molecule has an extra negative charge over ADP, ATP is -4, ADP is -3
also the inside of the mitochondria is relatively negative compared to the cytosol, thus supporting this mechanism of transport, if a toxin prevents the mitchondria being negative this does not work
what is function and mechanism of sodium calcium exchanger
transports sodium into cell and calcium out, 3 Na+ in for one Ca2+ out
since calcium conc is low in cytosol it uses sodium concentration to pump calcium out and so help maintain low concentration of calcium ions, example of secondary active transport
what type of transport do ion pumps carry out
primary active transport
ion pumps create important concentration gradients, these gradients are used for solute transport or signalling
describe the Na/K ATPase
there are several types of ion pumps, the Na/K+ ATPase which is a P-type pump
P type pumps all have similar amino acid sequences and a particular aspartate residue, which is phosphorylated by ATP during solute transport
energy from ATP hydrolysis is used to drive s olute against its concentration gradient
Na/K ATPase can either bind 2 potassiums or 3 sodiums, it pumps sodium out and potassium into cell
when sodium is bound it also has ability to bind ATP which occurs intracellularly
when ATP is bound it hydrolyses it causing the phosphorylation of the channels aspartate residue (while sodium is still bound)
when channel phosporylates aspartate its ion affinity changes and so releases sodium and binds potassium instead
when potassium ions are bound hydrolysis of phosphate occurs, which causes it to change its affinity to sodium
opening of channel occurs either on inside or outside of cell, depending on what it is transporting, when facing inside it has affinity for sodium, when facing outside it has affinity for potassium
technically the transport direction could occur either way, since conversion of ATP to ADP is reversible however channel cannot pick up a free phosphate group and phosphorylate itself so this does not occur (this process is not reversible), since hydrolysis is so exothermic
E2 conformation is outward facing, E1 conformation is inward facing
how does Km relate to carriers capacity
conc required for their max rate, giving an indication of its capacity
