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

Characteristics of a Transporter Saturability…characterized by K M and V max 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 [s], mM K m = 1 mM K m = 10 mM V max A “simple explanation” says that the rate of reaction should be proportional to the occupancy of the binding site as long as V max is constant.

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

energy in gradient: Example: Na + -glucose symport: stoichiometry of 2:1 at equilibrium: Δμ glu = -2Δμ Na

Aspartate Transporter: Na + - dependent transport of aspartate (from Boudker et al., Nature 2007)

apical basolateral Tight junction 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) H2OH2O GLUT

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, Ca 2+ is released into the cytoplasm, but during the relaxation phase it is actively pumped back into SR. Ca 2+ ATPae (SERCA) constitutes >80% of total integral protein in SR.

High-affinity state open inside Low-affinity state open outside Muscle Ca 2+ 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 Ca 2+ pump (p-class) is regulated by calmodulin, which acts as a sensor of Ca concentration. Elevated Ca 2+ binds to calmodulin, which in turn causes allosteric activation of the Ca 2+ pump.

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

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