FIGURE 11-27 Three-dimensional structure of the two-Na⁺/one-leucine symporter from the bacterium Aquifex aeolicus. (a) The bound L-leucine, two Na⁺ ions, and a Cl⁻ ion are shown in yellow, purple, and green, respectively. The three membrane-spanning α helices that bind the Na⁺ or the leucine are colored brown, blue, and orange. (b, c) Binding of the two Na⁺ ions to carbonyl main-chain or carboxyl side-chain oxygen atoms (red) that are part of helices 1 (brown), 6 (blue), or 8 (orange). It is important that one of the Na⁺ ions is also bound to the carboxyl group of the transported leucine (part b). See H. Krishnamurthy et al., 2009, Nature 459:347–355 for details on the structure and function of this and related Na⁺-linked symporters.

[Data from A. Yamashita et al., 2005, Nature 437:215, PDB ID 2a65.]