Exam 3 Lectures 2-3: Solute Transport |
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Solute Transport
Major concepts
1. Why do most solutes need assistance from a membrane protein in order to cross the membrane?
2. What factors must be considered in order to decide whether or not solute transport is favorable?
3. Why are carriers saturable when channels aren't?
4. How are the proteins required for primary and secondary active transport alike, and how are they different?
Core Knowledge
1. What is the equation for ΔG of transport across the membrane? How does the concentration factor (R T ln [C2/C1]) change when the direction of transport across the membrane changes? How does the charge factor (Z F ΔV) change when the direction of transport across the membrane changes?
2. What are the characteristics of ion channels?
3. How many binding sites does the K+ ion channel have, and how is it selective?
4. What are the characteristics of carriers?
5. What must a primary active transport protein be able to do that secondary active transport proteins don't have to do?
6. What is the cycle of steps used by the Ca2+ ATPase to move Ca2+ across the membrane?
Simple diffusion across the membrane = diffusion through the bilayer
few molecules: O2, CO2, H2O, other small molecules, lipids no hydration shell
Comparing solute movement across a membrane with catalyzing a reaction:
A. A specific protein-solute interaction occurs, that involves
1. removing a hydration shell
2. weak specific (size, shape, chemical) interactions between protein and solute
B. Instead of substrate ↔ product, solute in location 1 ↔ solute in location 2
C. Rate of process can be observed (kinetics), leading to an understanding of process.
D. Terminology: facilitated diffusion, transporters, permeases
For crossing membranes, consider
a. chemical concentration gradient = solute concentration gradient
b. charge gradient = membrane potential, classically measured out → in
c. electrochemical gradient = solute concentration gradient + membrane potential
Thermodynamics of solute movement across a membrane
A. For solutes without a charge
ΔGT = RT ln (C2/C1) where C1 = [S] on the starting side & C2 = ending side [S]
B. For solutes with a charge
ΔGT = RT ln (C2/C1) + Z F ΔV where Z = solute charge, F = Faraday's constant,
and ΔV = transmembrane electrical potential in V
Transport across membranes that requires proteins
Carriers vs Channels
A. Carriers: bind solutes specifically, saturable, have a conformation change,
rate is significantly lower than diffusion
B. Channels: usually less specific than carriers, not saturable, no conformation change,
rate is significantly higher than for carriers
C. Overview of different types and the sequence of discussion
1. Channels (example = K+ channel; read pp 408-410 )
2. Carriers
a. Uniports (example = glucose transporter)
b. Antiports (example = Cl−–HCO3− exchanger) and symports
c. Active transporters (examples = Na+–K+ ATPase and Ca2+ ATPase)
Ion channels
A. Characteristics
1. high flux (107 – 109/ sec) of solute transport
2. not saturable: increasing [ion] does not lead to a decrease in flux
3. specific: Na+, K+, Cl−, etc.
4. usually gated
B. K++ channel as an example
1. K+ chemical gradient is in → out; electrical gradient is out → in
2. Channel is selective (does not allow Na+ movement)
3. Protein structure = 4 identical chains, each with 2 transmembrane α-helices +
short pore helix
4. Pore of channel contains carbonyl oxygens spaced so that K+, but not Na++ , interacts.
Four binding sites: binding K+ in two adjacent sites → repulsion and movement
Carriers
A. Characteristics
1. lower flux than channels
2. increasing [solute] leads to a decrease, a Vmax of flux (measured using v0)
a carrier can be described by its Vmax and Kt of transport Fig. 11-31
3. specific: a carrier has a specific association
B. Glucose transporter of RBC (GLUT1) as an example of a uniport
1. Glucose chemical gradient is out (5 mM) → in (< 1 mM); no electrical gradient
2. Protein structure = one large protein, probably with multiple α-helices spanning
the membrane Fig. 11-30
3. Proposed model of GLUT1 function Fig. 11-32
a. binding glucose (conformation T1 with binding site open to outside)
b. conformation change (conformation T2 with binding site open to cytosol)
c. release of glucose
d. conformation change
4. GLUT1 Kt = 1.5 mM for D-glucose, so the transporter usually functions close to Vmax.
Kt for mannose and galactose = 20 and 30 mM; for L-glucose = 3,000 mM.
5. Other GLUT proteins vary in tissues, Kt values, etc. Table 11–4.
C. Chloride-bicarbonate exchanger of RBC as an example of an antiport
1. Cl− and HCO3− have the same charge and are transported in opposite directions.
2. Transport is charge-neutral and responds only to concentration gradients.
3. In tissues, [HCO3−] is high in cells (produced by carbonic anhydrase).
In lungs, [HCO3−] is low in cells (converted to CO2 by carbonic anhydrase).
D. Active transport = movement, requiring energy, against an electrochemical gradient
1. primary active transport: movement of solute A as a result of an exergonic reaction
ATPases (enzymes) that are carriers
2. secondary active transport: primary active transport → electrochemical gradient for A
electrochemical gradient is then used to move a different solute (B) against its gradient
while solute A moves down its gradient
antiport or symport carriers, but not enzymes
3. Classes of ATPases = P–type, F–type, V–type, and ABC-transporters
4. Na+–K+ ATPase as an example of a P-type ATPase
a. Na+ and K+ concentration gradients Fig. 11–36
b. Model of Na+–K+ ATPase function Fig. 11–37
5. Ca2+ ATPase as an example of a P-type ATPase
a. Cytosol [Ca2+] ∼ 0.100 mM, much lower than sarcoplasmic reticulum, plasma
b. SR ATPase has
I. membrane domain with 2 Ca2+ binding sites;
two conformations: one open to cytosol, one open to SR
II. cytosol section with three domains:
N (nucleotide) domain = ATP-binding site,
P domain = phosphorylation domain with Asp side chain,
A (actuator) domain that causes change in membrane domain
III. Proposed sequence
A. Bind 2 Ca2+
B. Bind ATP
C. Phosphorylate Asp and release ADP →
D. Eversion = conformation change 1
E. Remove phosphate by hydrolysis →
F. Eversion = conformation change 2