3.1 lonophores: models of carrier proteins
There are two models which explain solute transport mediated by carrier proteins: the mobile carrier model and the pore model. The solute binds the carrier at one side of the membrane and dissociates at the other side according to the mobile carrier model, while the pore model proposes that the carrier protein forms a pore across the membrane through which the solute passes. A certain group of antibiotics can make the membrane permeable to ion. This are called ionophores and are useful compounds to assist the study of membrane transport.
One ionophores, valinomycin, transports ions according to the carrier model, while gramicidin A, another ionophores, makes a pore across the membrane. Valinomycin is a circular molecule consisting of valine, lactate and hydroxyl isovalerate. Hydrophobic methyl and isopropyl group form the surface of the circular molecule while hydrophobic carbonyl group are arranged insite. Cations such as K bind to the hydrophobic interior of the molecule. The complex moves through the hydrophobic membrane to the other side of the membrane where the cation dissociates. The efficiency of a mobile carrier is temperature dependent and becomes less efficiency at low temperature due to decreased membrane fluidity. Uncouplers are mobile carrier of H.
Gramicidin A is a peptide consisting of 15 amino acid residues. This linear peptide has one hydrophilic side and one hydrophobic side. Two molecules of this compound form a hydrophilic pore across the membrane with the interaction between the hydrophobic side of the compound and the membrane lipid. Various cations can move through the pore thus created, the efficiency proteins are mobile carriers while others form pores.