How do calcium channels transport calcium ions?

Calcium channel activity is crucial for many fundamental physiological processes ranging from the heart beat to synaptic transmission. The channel-forming protein, of about 2000 amino acids, comprises four domains internally homologous to each other. Voltage-dependent Ca2+ channels are the most selective ion channels known. Under physiological conditions, they prefer Ca2+ over Na+ by a ratio of about 1000:1. To explain at the same time the exquisite ion selectivity and the large Ca2+ ion turnover rate of Ca2+ channels (approximately 3 x 10(6) ions/s), two kind models have been proposed. In one, the conduction pathway possesses two high-affinity binding sites. When two Ca2+ ions are bound to each site, the mutual repulsion between them speeds the exit rate for the ions, causing greater ion permeation through the pore. The second model hypothesizes the existence of a single site having a charged structure able to attract multiple, interacting ions, simultaneously. Recent studies that combine mutagenesis and electrophysiology show that the high-affinity binding site is formed by a ring of glutamate residues located in the pore forming region of the Ca2+ channel. As proposed in the second class of models, the results suggest that four glutamate residues, one glutamate donated by each repeat, combine to form a single high-affinity site. In this review the different conduction models for Ca2+ channels are discussed and confronted with structural data.