Figure 5. A schematic diagram showing the molecular mechanism of the allosteric regulation of the αγ heterodimer.

In the absence of any activators, the active site adopts an inactive conformation unfavorable for the ICT binding, and the enzyme is in the basal state which has a high S_0.5,ICT with a low catalytic efficiency. The binding of CIT induces conformational changes at the allosteric site, which are transmitted to the active site through conformational changes of the structure elements at the heterodimer interface, including the β5–β6 loop, the α7 helix, and the β7-strand in both the α and γ subunits, leading to the conversion of the active site from the inactive conformation to the active conformation favorable for the ICT binding. Hence, the enzyme assumes the partially activated state which has a moderately decreased S_0.5,ICT with a moderately increased catalytic efficiency. The binding of ADP in the presence of CIT does not induce further conformational changes at the allosteric site and the active site, but establishes a more extensive hydrogen-bonding network among CIT, ADP and the surrounding residues through the metal ion, which conversely enhances or stabilizes the CIT binding. Hence, the binding of CIT and ADP together has a synergistic activation effect, and the enzyme assumes the fully activated state which has a substantially decreased S_0.5,ICT with a significantly increased catalytic efficiency.