Lecture 16
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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

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Where would you find active transport?

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Where would you find active transport

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


Where would you find active 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.


Where would you find active transport

GLUT = sugar transporters

GLUT1-GLUT12


Where would you find active transport

Vmax

1

Km = 1 mM

0.8

Km = 10 mM

0.6

0.4

0.2

0

0

10

20

30

40

50

[s], mM

Michaelis-Menten equation for enzyme/transport reactions is very similar to the Langmuir isotherm

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.


Where would you find active transport

Bacterial Lac permease (lacY): Lactose-proton co-transporter

from Abramson et al. 2003


Where would you find active transport

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

from Abramson et al. 2003


Where would you find active transport

energy in gradient:

Example:

Na+-glucose symport: stoichiometry of 2:1

at equilibrium: Δμglu= -2ΔμNa


Where would you find active transport

Aspartate Transporter:

Na+ - dependent transport of aspartate

(from Boudker et al., Nature 2007)


Where would you find active transport

apical

Na-K ATPase = the primary active transport, generates concentration gradients of Na+ and K+

utilizing ATP

Tight junction

Na-Glucose co-transporter, utilizes Na+ gradient as a secondary energy source

GLUT

Glucose diffusion facilitator (no energy consumed, passive transport)

H2O

basolateral


Where would you find active transport

ATPases that couple splitting of ATP with ion motion across the membrane

ATP synthase

(works in reverse)

pump only protons


Where would you find active transport

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.


Where would you find active transport

Muscle Ca2+ pump (SERCA)

High-affinity state

open inside

Low-affinity state

open outside


Where would you find active transport

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.


Where would you find active transport

Post-Alberts Cycle for the Na+/K+ ATPase


Where would you find active transport

Vacuilar or Lysosomal V-type ATPases work in conjunction with Cl- channels

at equilibrium:


Where would you find active transport

BtuCD ATPase pumps vitamin B12 (ABC transporter)


Where would you find active transport

Many ABC transporters work as flppases or pump lipid-soluble substances (MDR)

MDR1

flippase


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