2.B.20 The Prodigiosin (Prodigiosin) Chloride/Bicarbonate Exchanger Family
Gale 2011 discusses three classes of synthetic compounds that were modified to bind and transport anions across lipid bilayer membranes. All of these compounds were originally designed as anion receptors that form stable complexes with anions but were then further developed as transporters. By studying structurally simple systems and varying their properties to change the degree of preorganization, the affinity for anions, or the lipophilicity, they have rationalized why particular anion transport mechanisms (cotransport or antiport) occur in particular cases. For example, the chloride transport properties of receptors based on the closely related structures of isophthalamide and pyridine-2,6-dicarboxamide: the central ring in each case was augmented with pendant methylimidazole groups designed to cotransport H+ and Cl-. The more preorganized pyridine-based receptor was the more efficient transporter, a finding replicated with a series of isophthalamides in which one contained hydroxyl groups designed to preorganize the receptor. This latter class of compound, together with the natural product prodigiosin, can transport bicarbonate (as part of a chloride/bicarbonate antiport process) across lipid bilayer membranes.
Gale 2011 also studied the membrane transport properties of calixpyrroles. Although the parent meso-octamethylcalixpyrrole functions solely as a Cs+/Cl- cotransporter, other compounds with increased anion affinities can function through an antiport process. One example is octafluoro-meso-octamethylcalixpyrrole; with its electron-withdrawing substituents, it can operate by chloride/bicarbonate antiport. Moreover, calixpyrroles with additional hydrogen bond donors can catalyze chloride/nitrate antiport. Thus, increasing the affinity of the receptor allows the compound to transport an anion in the absence of a cation. Simple thioureas are highly potent chloride/bicarbonate antiport agents that function at low concentrations, although the urea analogues are inactive. The higher hydrophobicities and lower polar surface areas of the thiourea compounds compared to their urea analogues provide clues to the high potency of these compounds.
Anionophores, small molecules capable of facilitating the transmembrane transport of anions have been made, based on the structure of prodigiosin (Hernando et al. 2018). These include four anionophores, 1a-d, all including a 1,2,3-triazole group. These compounds are efficient anion exchangers in model phospholipid liposomes. The changes in the hydrogen bond cleft modified the anion transport selectivity exhibited by these compounds compared to prodigiosin and suppressed the characteristic high toxicity of the natural product. Chloride efflux and iodide influx in living cells was demonstrated. They permeabilize cellular membranes to halides with efficiencies close to the natural anion channel CFTR at doses that do not compromise viability. pH gradients stimulated activities as for the airway epithelia of Cystic Fibrosis patients (Hernando et al. 2018).
Synthetic anion transporters that facilitate transmembrane H+/Cl- symport have anti-cancer potential due to their ability to neutralize pH gradients and inhibit autophagy. Compared to prodigiosin, synthetic anion transporters usually have low-to-modest H+/Cl- symport activities. A chloride-selective tetraurea macrocycle has a record-high H+/Cl- symport activity similar to that of prodigiosin and demonstrates voltage-switchable transport properties that are linked to the lack of uniport activity (Wu et al. 2019). By studying anion binding affinities and transport mechanisms of four other anion transporters, Wu et al. 2019 also showed that the lack of uniport and the voltage-dependent H+/Cl- symport originate from strong binding to lipid phosphate headgroup that hampers the diffusion of the free transporters through the membranes, leading to an unusual symport mechanism that involves only charged species.