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. 2019 Oct 16;151(12):1413–1429. doi: 10.1085/jgp.201912428

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© 2019 Bozzi et al.

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Figure 1. — Kinetic model of canonical symport. (A) Transport cycle diagram illustrating all possible binding/unbinding/transport events for a canonical symporter. (B) Free energy diagrams for transport events. For a tightly coupled (canonical) symporter, essentially insurmountable kinetic barriers in the protein’s free energy landscape prevent uniport events (dashed lines). Thus, only the empty or fully loaded transporter (solid lines) can efficiently convert from outward-open to inward-open (or vice versa). The combined electrochemical gradients of the primary substrate and the driving ions determine the net direction of cotransport, with a typical physiological situation for symport displayed here (higher concentration of primary substrate inside and higher concentration of driving ion outside).