Structures of membrane proteins. Flashcards
What is the speed of ATP powered pumps?
10^0 to 10^3 ions per second.
What is the speed of ion channels?
10^7 to 10^8 ions per second.
What is the main feature of a uniporter?
Moves substance with concentration gradient.
What is the main feature of a symporter?
Moves 1 substance against the concentration gradient and 1 with it. Move in the same direction
What is the main feature of a antiporter?
Moves 1 substance against the concentration gradient and 1 with it. Move in opposite directions.
What proved that aquaporins moved water into the cell?
When a frog lays a oocyte in freshwater the cell does not burst. When mRNA for aquaporins are placed into the cell the cell bursts.
What is in each subunit of an aquaporin?
6 transmembrane helices and 2 short helices per subunit.
What is the order of short and long subunits in the aquaporin?
Long, long, short, long.
How many subunits make up a aquaporin?
8.
How many pores are found in an aquaporin?
- Each subunit has a pore.
How is it possible for the short helices in the aquaporin to span the membrane?
Each one of the short helices goes into one of the membranes and together they can span the membrane.
What parts of the aquaporin are hydrophilic?
The top and the bottom.
How large is the aquaporin pore?
2.8A.
What is the aquaporin too small for?
H3O+.
What two highly conserved residues form the gate of the aquaporin?
Arg195 and His180. Both are these are positive so repel H3O+ residues.
What makes the transport of water through the aquaporin very fast?
The pore is lined with hydrophobic residues.
Explain why no protons are co translated in the aquaporin.
The proton wire is distributed by two Asn residues acting as proton donors.
When a proton wire is in place in the aquaporin what are all water molecules acting as?
Proton donors and acceptors.
What two processes allowed the determination of the Asn residues in the aquaporin?
X ray crystallography and electron microscopy.
What is the electrochemical potential gradient made up of?
Concentration and electron gradient.
What sets up the electron gradient?
K+ channel.
What sets up the chemical gradient?
K+/Na+ pump.
What does the Na+/lysine symporter do?
Pumps lysine up hill using the Na+ gradient.
What three things make up the single subunits in the potassium ion channels?
5s helix outside, 56 helix inside, P loop.
What two things make up the P loop in the single subunits in the potassium ion channel?
P segment and the selectivity filter.
What part of the of the potassium ion channel subunit is partially embedded in the membrane?
The P loop.
How many subunits make up the potassium ion channel?
4.
There are four subunits in the potassium ion channel. How many pores does this result in?
1.
What part of the K channel is homologous in all K channels?
P segment.
What is the role of the p segment in the K channel?
Imparts selectivity.
What does a mutation in the P loop of the K+ channel result in?
Loss of selectivity.
If the P segment in the bacterial P loop is mutated can not be replaced by a mammalian P segment. True or false?
False.
What is the transport rate of the K+ channel?
10^8 ions per second.
What is the empty space in the middle of the K+ channel called?
Vestibule.
What is the diameter of the vestibule in the K+ channel?
10A.
What is the diameter of the P loop/selectivity channel in the K+ channel?
3A.
What part of the K+ channel is hydrophobic?
Selectivity channel.
What part of the K+ channel is hydrophilic?
Vestibule.
Na+ is smaller than K+ but the K+ channel can still distinguish between the two. How?
Both ions are found in the solvated forms. Ka+ coordinates to 8 water molecules and Na+ coordinates to 6. Both of these need to be stripped. This normally costs energy but due to the arrangement of the carbonyl main chains in the selectivity factor this energy can be regained by the formation of 8 bonds, meaning it is only energetically favourable for Ka+ to enter the channel.