Block 2 - Transport 1-3 and Channels Flashcards
What are the two categories of active transport?
Primary - ATP
Secondary - electrochemical solute gradients (Na+ gradient, proton gradient, etc.) that is set up using ATP
Both types of active transport require…
An energy source (but that energy source can either be directly used or used to set up and maintain a solute gradient)
What are some examples of primary active transporters?
Na,K-ATPase (NKA) - pump K in and Na out
Ca-ATPase (PMCA) - pump Ca++ out
Ca-ATPase (SERCA) - pump Ca++ into the ER and protons out of the ER
H,K-ATPase (HKA) - pump K+ into the cell and H+ out
V-ATPase - pump protons into vesicles
What are ABC transporters?
ATP Binding Cassette transporters
- ATP dependent transport of large, complicated, organic molecules (bile salts, plant toxins, chemotherapeutic drugs, etc)
- often referred to as “multi drug resistance transporters”
- selective, but broadly
Ion gradients are maintained by…
Energy (ATP used in Na,K-ATPase pumps)
In research studies, when cells were cooled and depleted of ATP what happened to the Na+ and K+ gradients? What happened when they were warmed back up?
They became the same as extracellular
When they warmed, nothing happened until K+ was added to medium, which showed that uptake of K+ was needed for reflux of Na+
What happens when cardiac glycosides like strophanthin and ouabain were added to cells with Na,K-ATPase?
Selective inhibition of Na,K-ATPase, cell can’t maintain the gradient
What is the stoichometry of an Na,K-ATPase pump?
1 ATP
3 Na out
2 K in
True or False: Na ONLY moves out of the cell and K ONLY moves in
False - leak pathways allow some Na in and some K out
Describe the Na,K-ATPase reaction cycle
- E1 picks up 3 Na+and becomes E1P
- drops off 3 Na+outside the cell and becomes E2P
- Picks up 2 K+ and becomes E2
- drops off 2 K+ inside the cell and becomes E1
Besides maintaining an ion gradient, what is the other influence of Na,K-ATPase?
Maintaining normal cell volume
How does Na,K-ATPase maintain cell volume?
When fixed anions are present in the cell, Na+ becomes a relatively fixed cation outside the cell. This results in an influx of water and balance of cell volume
What is the metabolic cost of Na,K-ATPase?
- accounts for about 50% of basal metabolic rate in renal cells, 10-15% of overall BMR
- when Na,K-ATPase is active, O2 consumption is higher
What is the functional unit of Na,K-ATPase?
What happens to them?
A hetero dimer of a-B
- a subunit hydrolyzes ATP and moves Na and K
- B subunit is a glycoprotein that helps localize the mechanism
- y subunit is a FXYD protein that can regulate the pump
What ways can Na,K-ATPase be regulated?
- Long term (hours to days)
- increase or decrease amount of Na,K-ATPase protein - Short term (seconds to minutes)
- change in activity of the already existing proteins by altering their structure or location (in or out of the plasma membrane)
A molecule can simply diffuse across the lipid bilayer if it is relatively….
lipophilic
Net flux of diffusion across the lipid bilayer depends on…
concentration gradient
Hydrophilic molecules can’t diffuse across the lipid bilayer, but they can use two mechanisms to cross the membrane:
carriers and channels
True or False: channels and carriers always work independently. Only one type can be in a given cell.
False - they cooperate and work together to transport things. There are often carriers and channels in the same cell.
What typically is transported through a channel?
low molecular weight hydrophilic solutes like inorganic ions (Na+, Ca++, K+, Cl-)
What typically is transported by a carrier?
polar metabolites, nutrients, large or structurally complex molecules
True or False: a carrier protein creates a pore in the membrane that allows things to cross
False - a carrier is not a pore, but a complex protein that undergoes a series of conformational changes to let a molecule through the membrane
What direction can channels and carriers move things?
channels - always down the gradient
carriers - up or down the gradient depending on the type of carrier
What are the 2 diagnostic characteristics of a carrier?
saturability (due to finite number of transporters and each having a finite turnover)
selectivity (each transporter can accept a limited range of chemical structures as substrates)
What graphs shows information on carrier saturability? Describe the important aspects of the graphs.
Michaelis-Menten Relationship (rate of substrate uptake vs. substrate concentration)
Jmax is the max rate of uptake
Kt is the concentration of substrate to achieve 50% Jmax
Edie-Hofstee plot (linearized form of Michaelis-Menten, plots rate of substrate uptake vs. rate/[S] so that slope is -Kt)
Why is Jmax important?
measure of the transport capacity
Why is Kt important?
intrinsic property of a transporter - change in Kt reflect change in transporter structure
For the rate of a carrier-mediated process to change, what parametersmust have changed?
Either Jmax, Kt, or both
What is competitive inhibition?
an inhibitor molecule competes with the substrate for the active binding site (saturability and selectivity are important factors)
What is noncompetitive inhibition?
function impaired by inhibitor binding to transporters somewhere other than the active site (aka, the substrate can still bind but the transporter is inhibited anyway)
What happens to the Michaelis-Menten plot when a competitive inhibitor is present? What does it mean?
Jmax is unchanged but Kt is increased
at large enough concentrations, it is possible for the substrate to out-compete the inhibitor and fully occupy the binding sites
What concept is the basis for most drug interactions?
competitive inhibition - one drug might compete with another drug for binding sites
What happens to the Michaelis-Menten plot when a noncompetitive inhibitor is present? What does it mean?
Jmax is reduced but Kt is unchanged
even high concentrations of the substrate will not prevent the inhibitor from binding - there will be reduced substrate uptake
What are the categories of carriers?
- facilitated diffusion
- active transporters (primary and secondary)
What is facilitated diffusion?
transporter helps move across the membrane, but cannot catalyze a net flux against an electrochemical gradient
If one side of the membrane is more negative, how will facilitated diffusion change?
If the substrate is positive, more will move to the negative side until the overall charge is balanced
If the substrate is negative, more will move to the “less negative” side of the membrane until the overall charge is balanced
What are main examples of facilitated diffusion?
GLUTs
UT-A1, UT-A2, and UT-B (urea transporters)
Why are there so many glucose transporters if they all do relatively the same thing?
- location
- kinetics
- regulation
aka - some GLUTs have a high Kt so they need a ton of glucose to reach 50% Jmax, while others have a low Kt so they are almost always at 50% Jmax or even 100% Jmax (those with low Kt are good glucose sensors because rate of transport increases as concentration increases)
When intracellular Na+ rises (ex: increase muscle activity) what strategies could a cell employ to export more Na+?
- want to increase affinity for Na+ at the cytoplasmic face
- phosphorylation of the y subunit to influence it’s activity, leading to…
- decrease Kt and increase Jmax
- increase in Na+ efflux
At steady state, which ions are close and far from equilibrium?
Na+ is far, K+ and Cl- are closer
What defines the equilibrium point of an ion?
when the force of an ions concentration gradient is equal and opposite the force of the electrical voltage
Are the equilibrium potentials for Na+, K+ and Cl- positive or negative?
Na+ : positive
K+ and Cl- : negative
Sustaining a store of potential energy requires…
Using ATP to actively build the gradient
Secondary active transport requires two main steps:
- build the gradient using ATP
- allow a solute to cross the membrane using the potential energy of the concentration gradient that was already established
What ratios describe secondary active co-transport?
A in/A out must be < or = B out/B in (one moves down its gradients so the other can moves up its gradient in the same direction)
What ratios describe secondary active counter transport?
A in/ A out must be < or = C in / C out (one moves down its gradients so the other can moves up its gradient but in the opposite direction)
What are examples of cotransport vs. countertransport?
Co - dopamine/Na+
Counter - Ca++/Na+
True or False: secondary transporters can only move one solute
False - they can move multiple
Secondary active transporters on a vesicle typically use the —— gradient while those on the plasma membrane use the —— gradient
Vesicles - H+ gradient
Plasma membrane - Na+ gradient
If you have a 10 fold gradient of Na+ set up, what fold change can you get in a solute?
10 fold (maybe more if you also have electrical potentials working in your favor)
Proteins in the golgi are sorted into vesicles that direct them…
To either the apical or basolateral membrane
True of False: sorting is always the same, and never changes.
False - it is dynamic, can be increased or decreased, can be down regulated, and works in both directions
Regarding protein sorting, the exocrine pancreas is a good example of how…
protein sorting to the apical and basolateral surfaces leads to different function
How do carriers differ from channels?
- carriers never provide a continuous pathway across the membrane, while channels do when they are open
What is the opening and closing of a channel referred to as?
gating
What are the 3 channel characteristics?
- all are integral membrane proteins
- most have a gating mechanism for opening the channel
- all have a selectivity filter for cations vs. anions and often for a particular ion (Na+, K+, etc.)
In the KcsA K+ channel, K+ fits but Na+ doesn’t. What structure makes this possible?
selectivity filter
What is the speed of transport though a channel vs. a carrier?
carrier - 1,000-2,000 particles per second, 1-5 per cycle
channel - 10^6 to 10^8 particles per second when open, 6x10^4 per cycle
Specific channels are characterized by what two things?
conductance -
- electrical conductance is the inverse of electrical resistance (1/R)
- measure of how much electric current occurs with a given driving force
open probability -
- statistical likelihood that the channel will be open
stochastic behavior is somewhat random and uncertain
What is the term for channel opening?
stochastic behavior
How does patch clamp electrophysiology work? Why is it used?
used to study ion channels
measures the electrical current that occurs when ions flow through open channels
When viewing a patch clamp electrophysiology chart, what causes upward and downward deflections?
upward - positive charges leaving or negative charges entering the cell
downward - positive charges entering the cell
The electrical behaviors of an ion channel follows —– Law which is…
Ohm’s Law
I = V/R
I = GV
(I is current, V is voltage, R is resistance, G is conductance)
Channel conductance is the slope of…
the I-V curve
How are channels used by cells?
- excitable cells use channels to execute changes in membrane potential (ex: graded potentials and action potentials)
- epithelial cells use channels (and carriers) to move ions and water across the cellular barrier
The exocrine pancreas uses channels in two main ways. What are they?
- Cl- channels in the pancreatic acinus allow Cl- to enter the lumen for production of NaCl and H20
- Cl- channels in the pancreatic duct allow Cl- to enter the lumen for production of bicarbonate (HCO3-)
