transport proteins (1+2)

Question 1

what molecules do not need transporters to enter the cell

Answer 1

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

Question 2

how are polar molecules in solution

Answer 2

they have hydration shells, energy is required to break hydration shells

this energy barrier prevents charged solutes entering lipid phase of lipid bilayer

Question 3

how is the charge of cells compared to tissue fluid

Answer 3

generally cells are more negatively charged than tissue fluid

since it is relative by definition tissue fluid is at 0V

Question 4

what is the free energy associated with the equlibria of a solute in/out of the cell

Answer 4

delta G = RTln (c2/c1) + ZFdeltaV

ZFdeltaV only applies to charged solutes

where Z is charge of solutes and delta V is membrane voltage

Question 5

where do primary transporters get energy for transport

Answer 5

derive their energy from exergonic chemical reactions e.g active transport

Question 6

describe concentrations of sodium and calcium in cells

Answer 6

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

Question 7

what is the pH inside mitochondria, why is this

Answer 7

inside mitochondria there is high pH (low H+), to allow for greater flow of H+ through ATP synthase

Question 8

what are types of transport protein

Answer 8

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

Question 9

how does speed of channels compare to carriers

Answer 9

channels usually faster than carriers

Question 10

what type of transport to channels mediate

Answer 10

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

Question 11

what are gap junctions

Answer 11

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

Question 12

what are aquaporins

Answer 12

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

Question 13

describe a potassium ion channel

Answer 13

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

Question 14

describe the voltage gated sodium channel

Answer 14

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

Question 15

describe the acetylcholine receptor

Answer 15

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

Question 16

why are channels faster than carriers

Answer 16

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

Question 17

compare graphs of rate of transport of carriers and channels, also compare what channels and carriers typically transport

Answer 17

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

Question 18

describe the glucose transporter

Answer 18

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

Question 19

what is physiological range of glucose

Answer 19

5-10mM

Question 20

compare Km values of GLUT2-4, compare where they exist and how their Km contributes to their function

Answer 20

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

Question 21

describe lactose permease

Answer 21

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

Question 22

what are examples of antiporters, what are antiporters

Answer 22

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

Question 23

what is function and mechanism of the adenine nucleotide transporter

Answer 23

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

Question 24

what is function and mechanism of sodium calcium exchanger

Answer 24

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

Question 25

what type of transport do ion pumps carry out

Answer 25

primary active transport

ion pumps create important concentration gradients, these gradients are used for solute transport or signalling

Question 26

describe the Na/K ATPase

Answer 26

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

Question 27

how does Km relate to carriers capacity

Answer 27

conc required for their max rate, giving an indication of its capacity