Zur Funktion und Struktur synthetisch modifizierter Porine

Ion-channels: goals for function-oriented synthesis

Ion channels provide a conductance pathway for the passive transport of ions across membranes. These functional molecules perform key tasks in biological systems such as neuronal signaling, muscular control, and sensing. Recently, function-oriented synthesis researchers began to focus on ion channels with the goal of modifying the function of existing ion channels (ion selectivity, gating) or creating new channels with novel functions. Both approaches, ion channel engineering and de novo design, have involved synthetic chemists, biochemists, structural biologists, and neurochemists. Researchers characterize the function of ion channels by measuring their conductance in samples of biological membranes (patch clamp) or artificial membranes (planar lipid bilayers). At the single molecule level, these measurements require special attention to the purity of the sample, a challenge that synthetic chemists should be aware of. Ideally, researchers study the function of channels while also acquiring structural data (X-ray, NMR) to understand and predict how synthetic modifications alter channel function. Long-term oriented researchers would like to apply synthetic ion channels to single molecule sensing and to implantat these synthetic systems in living organisms as tools or for the treatment of channelopathies. In this Account, we discuss our own work on synthetic ion channels and explain the shift of our research focus from a de novo design of oligo-THFs and oligo-THF-amino acids to ion channel engineering. We introduce details about two biological lead structures for ion channel engineering: the gramicidin β(6,3) helix as an example of a channel with a narrow ion conductance pathway and the outer membrane porins (OmpF, OmpG) with their open β-barrel structure. The increase and the reversal of ion selectivity of these systems and the hydrophobic match/mismatch of the channel with the phospholipid bilayer are of particular interest. For engineering ion channels, we need to supplement the single-point attachment of a synthetic modulator with the synthesis of a more challenging two-point attachment. The successful function-oriented synthesis of ion channels will require interdisciplinary efforts that include new electrophysiology techniques, efficient synthesis (peptide/protein/organic), and good structural analysis.

Strategies and perspectives in ion-channel engineering

Membranes form natural barriers that need to be permeable to diverse matter like ions and substrates. This permeability is controlled by ion-channel proteins, which have attracted great interest for pharmaceutical applications. Ion-channel engineering (ICE) modifies biological ion channels by chemical/biological synthetis means. The goal is to obtain ion channels with modified or novel functionality. Three functional strategies exist. The first is the manipulation of the wider pores with robust β-barrel structures, such as those of α-hemolysin and porins. The second engineering approach focuses on the modification of narrow (mostly α-helical) pores to understand selectivity and modes of action. A third functional approach addresses channel gating by (photo)triggering the biological receptor that controls the channel. Several synthetis strategies have been developed and successfully utilized for the synthetic modification of biological ion-channels: the S-alkylation of specifically introduced Cys, protein semisynthesis by native chemical ligation, protein semisynthesis by protein trans-splicing, as well as nonsense-suppression methods. Structural studies (X-ray crystallography, NMR spectroscopy) are necessary to support the functional studies and to afford predictable engineering. The reprogramming and re-engineering of channels can be used for sensing applications, treatment of channelopathies, chemical neurobiology, and providing novel lead compounds for targeting ion channels.