Figure 4.

An Na⁺ ion is coordinated in the open conformation. (a) The open conformation coordinates what is assumed to be an Na⁺ ion through three main-chain carbonyls (Pro297, Ala349 and Ile351) and two waters. It is coordinated in a trigonal bipyramidal geometry. The ligation distances are shown. (b) The Na⁺ ion is not found in the closed conformation. Upon substrate-induced conformational change, the loop between helices α15 and α16 shifts, breaking the planar geometry of the main-chain coordinating residues and displacing a water and consequently the Na⁺ ion.