2.B.1 The Valinomycin Carrier (Valinomycin) Family
Valinomycin is an ionophoric antibiotic that functions as a carrier to shuttle K+ across a biological membrane, down its electrochemical gradient, thereby eliminating the membrane potential and the K+ concentration gradient. It is synthesized by Streptomyces fulrissimus and contains in sequence: D-valine, L-lactate, L-valine, and D-hydroxy isovalerate repeated three times. It is extremely selective for K+ and renders both artificial and biological membranes permeable to this cation. The valinomycin-K+ complex shields the hydrophilic groups of valinomycin on the interior of the complex when K+ is bound and leaves the hydrophobic rich chains on the external surface exposed to the membrane phospholipid side chains.
The generalized reaction normally catalyzed by valinomycin is:
K+ (out) K+ (in).
That for anionophoric valinomycin derivatives is:
anion (out) ⇌ anion (in).
Berezin 2015 used theory and modeling to investigate K+ valinomycin (Vln) carrier as a classical anionophore. This author discussed formation of the ion pair Vln:K+ or Vln:Cl- versus encapsulation of the anion into a Vln:Cl- complex as two alternative mechanisms that could account for high anionophoretic activity of the cyclic peptide (Riddell and Zhou 1994). Constructing kinetic models helped distinguish between the two (Berezin 2015).