Where would you find active transport?
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Where would you find active transport? •interface with the environment…. •maintain cell volume •control internal environment •signaling….Ca ++ gradient Lecture 16 Membrane Transport Active transport
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Lecture 16 Membrane Transport Active transport. Where would you find active transport?. interface with the environment…. maintain cell volume control internal environment signaling….Ca ++ gradient. Characteristics of a Transporter. Saturability…characterized by K M and V max - PowerPoint PPT Presentation
Transcript of Where would you find active transport?
Where would you find active transport?
•interface with the environment….
•maintain cell volume
•control internal environment
•signaling….Ca++ gradient
Lecture 16
Membrane TransportActive transport
Characteristics of a Transporter
•Saturability…characterized by KM and Vmax
•Stereospecificity..or specificity unrelared to biophysical characteristics
•Higher rate than expected from oil/water partition coef.
GLUT = sugar transporters
GLUT1-GLUT12
Michaelis-Menten equation for enzyme/transport reactions is very similar to
the Langmuir isotherm
][
][max sK
sVV
m
[s], mM
0 10 20 30 40 500
0.2
0.4
0.6
0.8
1
Km = 1 mM
Km = 10 mM
Vmax
A “simple explanation” says that the rate of reaction should be proportional to the occupancy of the binding site as long as Vmax is constant.
Bacterial Lac permease (lacY): Lactose-proton co-transporter
from Abramson et al. 2003
from Abramson et al. 2003
The Lac permease functional cycle, an example of coupled transport
Note: the proton is always taken up first, but is released at last, which ensures strict coupling of transport without H+ leakage
FzS
SRT Ss )
][
][ln(
2
1energy in gradient:
Example:
Na+-glucose symport: stoichiometry of 2:1
at equilibrium: Δμglu= -2ΔμNa
)][
][ln(2)
][
][ln(2
out
in
in
out
glu
gluRTF
Na
NaRT
)][
][log(
3.2
2)
][
][log(2
out
in
in
out
glu
glu
RT
F
Na
Na
Aspartate Transporter:
Na+ - dependent transport of aspartate
(from Boudker et al., Nature 2007)
apical
basolateral
Tight junction
K +
N a+
N a+
N a+ G LU
G LUK +N a+
G LU
K +
N a+
Na-K ATPase = the primary active transport, generates concentration gradients of Na+ and K+
utilizing ATP Na-Glucose co-transporter, utilizes Na+ gradient as a secondary energy source
Glucose diffusion facilitator (no energy consumed, passive transport)
H2O
G LUGLUT
ATPases that couple splitting of ATP with ion motion across the membrane
pump only protons
ATP synthase(works in reverse)
During contraction of the striated and cardiac muscle, Ca2+ is released into the cytoplasm, but during the relaxation phase it is actively pumped back into SR. Ca2+ ATPae (SERCA) constitutes >80% of total integral protein in SR.
High-affinity stateopen inside
Low-affinity stateopen outside
Muscle Ca2+ pump (SERCA)
The activity of SERCA, especially in the heart is regulated by Phospholamban, a small (single-pass) transmembrane protein. Phosphorylation of phospholamban by PkA removes its inhibitory action and increases the activity of SERCA by an order of magnitude.
The activity of plasma membrane Ca2+ pump (p-class) is regulated by calmodulin, which acts as a sensor of Ca concentration. Elevated Ca2+ binds to calmodulin, which in turn causes allosteric activation of the Ca2+ pump.
Post-Alberts Cycle for the Na+/K+ ATPase
FpHRTFH
HRT
H)
][
][ln(
2
1
HATP n at equilibrium:
Vacuilar or Lysosomal V-type ATPases work in conjunction with Cl- channels
BtuCD ATPase pumps vitamin B12 (ABC transporter)
Many ABC transporters work as flppases or pump lipid-soluble substances (MDR)
MDR1
flippase
