MOLECULAR BASIS OF MEMBRANE TRANSPORT
Manoj S. Nair, Ph.DPostodoctoral Fellow,
Biochemistry812 Biosciences bldg, 484 w.
12th aveColumbus, OH 43210
Outline of Talk
Types of transport across membranes1. Passive transport2. Carrier mediated 3. Active transport Nernst equilibrium for ion transport Mechanism of ion transport (K-selectivity
filter) Endocytosis of proteins/protein domains
Introduction to Cellular Biophysics
A. Molecular Basis of Membrane Transport.
Essential Cell Biology Alberts, Bray, et al.
Fluid mosaic model of membranes
Dynamic diffusion of lipids in membranes
Active transport
Transport up a concentration gradient
Uses energy (ATP) May also cause
charge gradient across the membrane causing the molecule to move against the membrane potential.
Properties of “Active” membrane pumpsATPases (sometimes GTPases)Example of a Na+/K+ pump
Na+/K+ pump uses 30% resting ATP
Active Pumps are used to transport materials against their electrochemical gradient
Essential Cell Biology Alberts, Bray, et al.
a) Uniport: 1 type of solute is transportedEg: Valinomycin (K+ transport)
Valinomycin is a carrier for K+.
It is a circular molecule, made up of 3 repeats of the sequence shown above.
N C H C OHC
C H
C H 3H 3 C
O
C N
C H
C H 3H 3 C
OHC
C H
C H 3H 3 C
C O C H
C H 3
C
O
H
O
H
3
V a lin o m y c in
L -v a l in e D -h y d ro x y - D -v a l in e L - la c t ic i s o v a le r ic a c id a c id
Valinomycin is highly selective for K+ relative to Na+. The smaller Na+ ion cannot simultaneously interact with all 6 oxygen atoms within valinomycin. Thus it is energetically less favorable for Na+ to shed its waters of hydration to form a complex with valinomycin.
Valinomycin
O O O
O O
Hydrophobic
O
K+
Puckering of the ring, stabilized by H-bonds, allows valinomycin to closely surround a single unhydrated K+ ion.
Six oxygen atoms of the ionophore interact with the bound K+, replacing O atoms of waters of hydration.
Whereas the interior of the valinomycin-K+ complex is polar, the surface of the complex is hydrophobic.
This allows valinomycin to enter the lipid core of the bilayer, to solubilize K+ within this hydrophobic milieu.
Crystal structure
Valinomycin
O O O
O O
Hydrophobic
O
K+
Valinomycin is a passive carrier for K+. It can bind or release K+ when it encounters the membrane surface.Valinomycin can catalyze net K+ transport because it can translocate either in the complexed or uncomplexed state.The direction of net flux depends on the electrochemical K+ gradient.
Val Val
Val-K+ Val-K+
K+
membrane
K+
b) Symport: 2 different solutes transported together in one directionEg: Glucose –Na+ tranporter in epithelial cellsLactose permease: H+ -lactose symport
c)Antiport: 2 different solutes transport in opposite directionsEg: Adenine nucleotide translocase (ATP/ADP exchanger)
Passive transport: Ion channels & Pores
Properties of transmembrane -helices:
Amphiphilic nature
Designer Peptides of Ser & Leu:Formed a hexamer channel in phospholipid membranes.
S.R.Goodman. 1998
What is the mechanism for ion selectivity of channels? This is a frontier of biophysics.
With Passive Channels, ions or other substances move DOWN their electrochemical gradient
+ +
+++
--
-
--
-
Electrochemical Gradient
Basic structure of the potassium channel. Doyle et al. Science, 1998. Nobel Prize in Chemistry in 2003
KcsA Selectivity Filter
KvAP channel voltage sensing paddle
Avidin detection of voltage using biotinlyated KvAP
Receptor-activated gate
• “Gated” channels i.e. channels that open in response to physiological stimuli
Essential Cell Biology Alberts, Bray, et al.