Coupling of catalytic and channel function in the Ca2+ transport ATPase

A Kinetic Model with Ordered Cytoplasmic Dissociation for SUC1, an Arabidopsis H + /Sucrose Cotransporter Expressed in Xenopus Oocytes

“…This is interesting because it suggests that SUC1 may function as a pore and it belongs to the superfamily of transporters identified by Marger and Saier (1993), which includes facilitated diffusion carriers. This type of mechanism was proposed for the Na + / glucose cotransproter (Hopfer & Groseclose, 1980) and there are also some similarities with the first-on first-off type mechanism for Ca + translocation through the SR Ca 2+ ATPase (Inesi & Kirtley, 1990) and the channeltype structure proposed for exchange of Na + through the Na + /K + pump (Gadsby et al, 1993). Recent cotransporter kinetic models have moved closer to the channel mechanism with the development of a multi-substrate single-file model (Su et al, 1996).…”

Short and Long Range Functions of Amino Acids in the Transmembrane Region of the Sarcoplasmic Reticulum ATPase

“…Accordingly, a model was constructed by clustering transmembrane helices M4, M5, M6, and M8 and thereby forming a channel that can admit two Ca 2ϩ in single file (19). Rotation of the helices for optimal positioning of acidic side chains within the lumen of the channel is favored by the amphiphilic character of the helices.…”

The Cytoplasmic Loop between Putative Transmembrane Segments 6 and 7 in Sarcoplasmic Reticulum Ca2+-ATPase Binds Ca2+ and Is Functionally Important

“…These residues are Glu-309, Glu-771, Asn-796, Thr-799, Asp-800, and Glu-908, located in the putative transmembrane segments M4, M5/M6, and M8. A model was built in which these transmembrane segments are clustered and form a channel that can admit two Ca 2ϩ in a single row (since these bound ions appear to be stacked inside the ATPase structure (21,22)). All of these residues, with the exception of Glu-908 (18,23), have been found to be essential for occlusion of Ca 2ϩ .…”