Ion channels as enzymes: analogy or homology?

Ion channels render nerve and muscle excitable. A typical channel protein can mediate the passive transfer of millions of ions per second across the membrane. Thus, channels catalyse the transmembrane flux of ions, fulfilling criteria traditionally associated with enzymes. Is this a semantic coincidence, or do channels and enzymes in fact rely upon similar structural principles? A general answer remains elusive given the paucity of crystallographic data on channels. Nevertheless, emerging evidence points to fundamental similarities between the pores of channels and the active sites of enzymes of resolved structure. Shared features include narrow clefts lined by protein loops, and specific binding of transition intermediates during catalysis. The often cited analogies between channels and enzymes might therefore reflect basic design homologies.