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A schematic representation of the transport cycle of a carrier (dark green). The alternating-access mechanism requires interconversion between functional conformational states: outward-facing open (OF-o), outward-facing occluded (OF-occ), inward-facing open (IF-o), and inward-facing occluded (IF-occ). Uncoupled water or ion transport can occur due to channel-like transient intermediates (light green). Reprinted with permission from ref (787). Copyright 2013 Li et al.
