Figure 2. Binding of CIT induces conformational changes at the allosteric site, the active site, and the heterodimer interface.

(a) Comparison of the α^Mgγ and α^Mgγ^Mg+CIT structures. The α and γ subunits in the α^Mgγ structure are colored in lemon and cyan, respectively, and these in the α^Mgγ^Mg+CIT structure in magenta and orange, respectively. Major conformational changes are observed at the allosteric site, the active site, and the β5–β6 loop, the α7 helix and the β7 strand at the heterodimer interface. The orientations of the α6 and α7 helices in both subunits are indicated with dashed arrows. Some key residues involved in the conformational changes are shown with side chains. The zoom-in panel on the right top shows the conformational changes of the β7^A and β7^G strands. For clarity, only the hydrogen-bonding interactions between the main chains are shown and the side chains are omitted. The zoom-in panel in the right bottom shows the loop-to-helix transition of the N-terminal region of the α7^A and α7^G helices. (b) Structure of the allosteric site in the α^Mgγ (left panel) and α^Mgγ^Mg+CIT (right panel) structures. In the α^Mgγ structure, the hydrogen-bonding interactions between the residues of the CIT-binding subsite and the surrounding residues (colored in grey) are indicated with dashed lines. In the α^Mgγ^Mg+CIT structure, the interactions between CIT and the surrounding residues and the Mg²⁺ are indicated with dashed lines. (c) Comparison of the allosteric site in the α^Mgγ and α^Mgγ^Mg+CIT structures. (d) Comparison of the active site in the α^Mgγ and α^Mgγ^Mg+CIT structures. (e) A schematic diagram showing the hydrogen-bonding interactions among the β5–β6 loop, the α7 helix, the β7 strand, and the α5 helix in the α^Mgγ and α^Mgγ^Mg+CIT structures. The interactions in the α^Mgγ structure are indicated with green lines; these disrupted upon the CIT binding are indicated with dashed green lines and the newly formed interactions with red lines. (f) Structure of the heterodimer interface in the α^Mgγ (left panel) and α^Mgγ^Mg+CIT (right panel) structures. For clarity, only the hydrogen-bonding interactions altered upon the CIT binding are indicated with dashed lines.