Substrate binding properties of potato tuber ADP-glucose pyrophosphorylase as determined by isothermal titration calorimetry

•Substrate binding properties of potato AGPase variants were analyzed by ITC.•The heterotetrameric AGPase, SWTLWT, displays two binding sites for ATP.•The homotetrameric enzymes, SS and LS, contain only a single binding site for ATP.•SWTLWT exhibits higher affinity to ATP due to synergistic interaction of LS and SS.•AGPase does not bind glucose 1-phosphate in the presence or absence of ATPγS. Substrate binding properties of the large (LS) and small (SS) subunits of potato tuber ADP-glucose pyrophosphorylase were investigated by using isothermal titration calorimetry. Our results clearly show that the wild type heterotetramer (SWTLWT) possesses two distinct types of ATP binding sites, whereas the homotetrameric LS and SS variant forms only exhibited properties of one of the two binding sites. The wild type enzyme also exhibited significantly increased affinity to this substrate compared to the homotetrameric enzyme forms. No stable binding was evident for the second substrate, glucose-1-phosphate, in the presence or absence of ATPγS suggesting that interaction of glucose-1-phosphate is dependent on hydrolysis of ATP and supports the Theorell–Chance bi bi reaction mechanism.