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Two new studies show that porphyrin molecules--nitrogen-containing macrocycles that often coordinate iron or other metals--are capable of binding potassium channel proteins in at least two very different ways. The findings suggest that heme, which is a natural iron-coordinating porphyrin that functions as a catalytic center in many enzymes, might also play a solo role as a cell-signaling molecule. And they point to artificial symmetric porphyrins as a promising start toward designing clinically important potassium channel inhibitors. Heme (otherwise known as iron protoporphyrin IX) is a key biological cofactor. Hemoglobin, the protein that carries oxygen through the blood, binds oxygen via its four heme cofactors. And heme acts as the catalytic center in a variety of enzymes, including the various cytochrome P450 enzymes that metabolize drugs in the body. But free-floating heme may also play an important biological role, according to a new study led by associate professor of physiology Toshinori Hoshi of the University of Pennsylvania. Hoshi, staff scientist Xiang Dong Tang, and coworkers recently showed that free heme can bind to and inhibit the action of a calcium-dependent potassium channel [Nature, 425, 531 (2003)]. Hoshi's team began to investigate the interaction of heme with this channel after an analysis of its amino acid sequence turned up a known heme-binding motif. This motif, which contains two cysteines and a histidine, is located in the portion of the potassium channel that protrudes into the cytoplasm between the domains that are thought to regulate channel opening. Sure enough, Hoshi's team found that heme does bind to this motif. Electron paramagnetic resonance and UV-visible spectra of heme bound to a 23-residue peptide fragment encompassing the motif suggest that the histidine side chain coordinates to the heme's iron center. This mode of heme coordination is similar to that seen in cytochrome proteins. Using electrophysiological techniques to monitor the ability of single channels to transport potassium ions across membranes, Hoshi's team has shown that heme binding inhibits the channel's function by decreasing the frequency of channel openings. In vivo, free heme--high concentrations of which are released after strokes, heart attacks, and other severe injuries--may block this potassium channel, preventing it from carrying out its normal cell-protective duties, Hoshi suggests. Nobody expected that free heme might play such a role, Hoshi says. But "now that we have provided the first example of a fast heme-signaling system, other examples are likely to follow," he tells C&EN. His team is currently trying to test whether these channels are tagged with heme after cellular injury. SYNTHETIC Trauner expects his symmetric tetraphenylporphyrin ligands (purple) to bind to the extracellular side of potassium channels (gray). COURTESY OF D. TRAUNER Dirk Trauner, an assistant professor of chemistry at the University of California, Berkeley, is also interested in the interaction of hemelike molecules with potassium channels. Trauner believes man-made, symmetric porphyrins are a promising start toward designing drugs for autoimmune diseases, epilepsy, cardiac arrythmia, and diabetes. "We were prompted to develop our ligands by analyzing X-ray structures of potassium channels and their interactions with naturally occurring peptide toxins," Trauner says. In these channels, the pore that potassium ions go through is located at the center of symmetry of four identical protein subunits. So Trauner settled on tetraphenylporphyrins in the hopes that the fourfold symmetrical molecules might interact with all four subunits simultaneously. Such polyvalent ligands should bind to the extracellular side of the channel with high affinity, he tells C&EN. In fact, the water-soluble tetraphenylporphyrins that Trauner, graduate student Stefan N. Gradl, and their coworkers synthesized bind to voltage-gated potassium ion channels with nanomolar affinities [J. Am. Chem. Soc., 125, 12668 (2003)]. Porphyrins that bear phenyl groups with positively charged substituents are particularly good inhibitors of channel function--although none of the derivatives Trauner's team has tested completely blocks potassium ion flow across the membrane. "The modular composition of our ligands allows easy modifications and should provide a large set of synthetic probes that discriminate among different potassium channels," the authors write. Trauner is particularly interested in making metalloporphyrins and fluorescent porphyrins with larger substituents. According to Maria L. Garcia, a distinguished senior investigator in Merck Research Laboratory's department of ion channels and a coauthor on both papers, these ligands could be used to "dial in" channel specificity to selectively modulate the action of specific channels in vivo. |
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