Questions: 

Heme serves as a stable prosthetic group and gas sensor of many proteins. However, recent advances in heme research revealed a novel role as a non-genomic modulator of ion channel functions, first exemplified for the Slo1 BK channel (KCNMA1). In the current study, we identified that heme also modulates human ether à go-go (hEAG1, KCNH1) voltage-gated potassium channels. KCNH1 channels are expressed abundantly in neuronal tissues, including the cerebral cortex and the hippocampus, and contribute to modulation of action potential firing patterns.

Methods: 

KCNH1 channels were studied the inside-out patch-clamp configuration upon recombinant expression in Xenopus oocytes.

Results: 

Application of hemin (Fe³⁺-protoporphyrin-IX) to the cytosolic side inhibited KCNH1 channels, while other delayed rectifier K⁺ channels such as KCNA1, KCNA5 and KCNA6 were insensitive to hemin. KCNH1 channel block was complete and exhibited an apparent IC₅₀ of ~4 nM with a Hill coefficient of ~2. At 10 nM the onset of effect followed an exponential time course with a time constant of 61±13 s. Washout of hemin only very slowly recovered the current. Heme (Fe²⁺-protoporphyrin-IX) had the same effect as hemin, while protoporphyrin-IX and Fe²⁺ were inactive. Hemin does not block the ion pore but acts as a gating modifier of KCNH1 channels. At strong depolarizations the effect of hemin is smaller than at voltages of low channel open probability. In addition, hemin (10 nM) shifted the half-activation voltage from -67±3 mV to -52±4 mV, slowed channel activation, while accelerating deactivation kinetics. Physical binding of hemin to KCNH1 proteins was demonstrated in hemin-agarose pull-down assays.

Conclusions: 

Hemin modulates KCNH1 channels about 40-fold more potently than Slo1 BK channels. Free heme/hemin are physiologically relevant modulators of KCNH1 activity and, hence, neuronal excitability.