ION CHANNELS AS ENZYMES - ANALOGY OR HOMOLOGY

Ion channels render nerve and muscle excitable. A typical channel prot ein can mediate the passive transfer of millions of ions per second ac ross the membrane. Thus, channels catalyse the transmembrane flux of i ons, fulfilling criteria traditionally associated with enzymes. Is thi s 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, emerg ing evidence points to fundamental similarities between the pores of c hannels and the active sites of enzymes of resolved structure. Shared features include narrow clefts lined by protein loops, and specific bi nding of transition intermediates during catalysis. The often cited an alogies between channels and enzymes might therefore reflect basic des ign homologies.