where would you find active transport? interface with the environment. maintain cell volume control...
DESCRIPTION
GLUT = sugar transporters GLUT1-GLUT12TRANSCRIPT
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 sKsVV
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
FzSSRT Ss )
][][ln(2
1energy in gradient:
Example:
Na+-glucose symport: stoichiometry of 2:1
at equilibrium: Δμglu= -2ΔμNa
)][][
ln(2)][][
ln(2out
in
in
out
gluglu
RTFNaNa
RT
)][][
log(3.22)
][][
log(2out
in
in
out
gluglu
RTF
NaNa
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)
H2OG 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
FpHRTFHH
RTH
)][][
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
