Membrane transport 4
Facilitated diffusion. Ionophores i) - mobile carriers (Valinomycin) and ii) - channel forming carriers (Gramicidin A). Principles of action, molecular structure and selectivity.
Facilitated diffusion is the process of spontaneous passive transport of molecules or ions across a cell's membrane via specific transmembrane integral proteins. Being passive, facilitated transport does not directly require chemical energy from ATP hydrolysis in the transport step itself; rather, molecules and ions move down their concentration gradient.
Ionophores: Substances that selectively catalyze the diffusion of ions through membrane. There are 2 general types:
- 1. Mobile ion carriers; which directly carry a bound ion across the membrane
- 2. Channel-forming carriers; which provide a channel through the membrane for passage of ions down their electrochemical gradient,
Valinomycin (mobile carrier) is highly selective for K+ over 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 water of hydration to form a complex with this ionophore.
Whereas the interior of the valinomycin-K+ complex is polar, the surface of the complex is hydrophobic. Valinomycin enters the lipid core of the bilayer and solubilizes K+ within this hydrophobic environment.
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 translocated either in the complexed or uncomplexed state. The direction of net flux depends on the electrochemical K+ gradient.
Gramicidin A (channel-forming carrier):
Gramicidin A (an antibiotic) consists of 15 D-and L- amino acids, all of which have hydrophobic side chains In an unstable dimer form a transmembrane channel is created between the two associated polypeptide chains, which guides monovalent cations down their electrochemical gradients to the opposite side of the membrane.
The outer surface of the gramicidin dimer, which interacts with the core of the lipid bilayer, is hydrophobic. Ions pass through the more polar lumen of the helix.
Gramicidin channel activity can be observed, if a small number of gramicidin molecules is present in a lipid bilayer separating 2 compartments containing salt solutions. With voltage clamped at some value, current (ion flow through the membrane) fluctuates. Each fluctuation, attributed to opening or closing of one channel, is the same magnitude. The current increment corresponds to current flow through a single channel (drawing - not actual data).