Ion transport across membranes can occur actively through ion pumps or passively through channels or carriers. Passive transport of metal ions can be carrier-mediated by ionophores, which are ligands that encapsulate metal ions and have organic groups to transport them across membranes. Examples of ionophores include valinomycin and nonactin, which selectively transport potassium ions and sodium ions respectively by binding them. Ionophores disrupt ion balance in cells and can act as antibiotics, though they cannot distinguish between microbial and host cells.

1. METAL ION TRANSPORT
-Jaiswal Priyanka Balister
MSc - I

2. Types of Ion transport
• Ion transport can be either active transport or passive transport.
• Active transport is the pumping of molecules or ions through a
membrane against their concentration gradient
• It requires :
- A transmembrane protein (Ion Pump).
- Energy in the form of ATP.
• Passive transport can be either channel mediated or carrier
mediated. It does not require energy for the transport of ions.

4. Passive Transport
• Passive transport of metal ions may occur in two
ways:
1. the cation may be encapsulated within a
macrocyclic ion carrying ligand (ionophores)
having organic groups outside – Carrier
mediated.
2. The metal ions may pass through an ion
permeable channel extending through the
membrane.

5. Ionophores
• Ionophores are naturally occuring ligands having the ability
to encapsulate metal ion and at the same time provide a
group of organic groups outside the complex.
• These compounds resemble crown ethers and cryptates by
having several oxygen or nitrogen atoms spaced along a
chain or ring that can wrap around a metal ion.
• Examples: valinomycin, nonactin, gramicidin, etc.
• The transport kinetics of lipid-soluble ionophores has been
studied by the planar lipid bilayer technique.
• They are obtained mostly from marine organisms and fungi.

6. How Ionophores work?
• Some ionophores increase the passive diffusion across a lipid
bilayer by providing a hydrophilic pocket that binds a solute and
sequesters it away from the hydrophobic lipid interior.
• These ionophores generally show specificity of transport
because the pocket binds some ions better than others.
• For these types of ionophores, the ionophore-ligand complex is
believed to diffuse across the lipid bilayer, carrying the ligand
with the ionophore.
• Other ionophores form an aqueous channel across the lipid
layers.

7. • These channel-forming ionophores are less specific but still show
specificity due to the size and shape of the channel.

8. Valinomycin
• It resembles a cyclic peptide.
• The ionophore is a 12 unit depsipeptide, has
three repeated sequences of L-valine-D-
hydroxyvaleric acid-D-valine-L-lactic acid.
• The resulting 36 membered flexible macrocycle
can adopt several geometries depending upon
the polarity of the surrounding medium and the
presence or absence of the metal ion within its
cavity.
• The 6 carbonyl oxygens facing the inside of the structure chelate a
single K+ in an octahedral co-ordination. It forms an 1:1 complex
with K+, which fits precisely into the cavity.
• The K+ complex is nearly 1000 times stronger than that made by
Na+ ion, which is only loosely held within the cavity.

9. Nonactin
• On coordination to an alkali metal ion, the conformation changes to
form a nearly cubic arrangement of four carbonyl O and four ether O
around the cation.
• It is a naturally occuring ionophore and resembles
a crown ether.
• Nonactin is a 32 membered cyclic tetralactone that
forms crystalline complexes with K+, Na+, NH4
+
of various ion sizes.
• In the metal-free form, the carbonyl oxygens point
outward and the 4 oxygens from the 4
tetrahydrofuran rings form nearly a square at the
centre.

10. Ionophores as Antibiotics
• Ionophores can carry both K+ and Na+ ions through the
nonpolar cell membranes in either directions.
• This may disrupt the balance in the cells- a situation that
may be fatal for single celled species, viz. bacteria.
• The ionophores thus possess the potentiality of antibiotics
(transport antibiotics). But they are therapeutically useless
as they cannot distinguish microbial cells from the host
animal cells.
• Some limited veterinary applications are reported.

11. References
1. General & Inorganic Chemistry, Vol 2, R.Sarkar.
2. Inorganic Chemistry, 5th Ed., Atkins, Overton, Rourke,
Weller, Armstrong.
3. Quantitative Human Physiology: An Introduction, Joseph
Feher.
4. Cell Physiology Sourcebook: Essentials of Membrane
Biophysics, Nicholas Sperelakis.