Glucose modulation of glucokinase activation by small molecules

Small molecule activators of glucokinase (GK) were used in kinetic and equilibrium binding studies to probe the biochemical basis for their allosteric effects. These small molecules decreased the glucose K-0.5 (similar to 1 mM vs similar to 8 mM) and the glucose cooperativity (Hill coefficient of 1.2 vs 1.7) and lowered the k(cat) to various degrees (62-95% of the control activity). These activators relieved GK's inhibition from glucokinase regulatory protein (GKRP) in a glucose-dependent manner and activated GK to the same extent as control reactions in the absence of GKRP. In equilibrium binding studies, the intrinsic glucose affinity to the activator-bound enzyme was determined and demonstrated a 700-fold increase relative to the apoenzyme. This is consistent with a reduction in apparent glucose K-D and the steady-state parameter K-0.5 as a result of enzyme equilibrium shifting to the activator-bound form. The binding of small molecules to GK was dependent on glucose, consistent with the structural evidence for an allosteric binding site which is present in the glucose-induced, active enzyme form of GK and absent in the inactive apoenzyme [Kamata et al. (2004) Structure 12, 429-438]. A mechanistic model that brings together the kinetic and structural data is proposed which allows qualitative and quantitative analysis of the glucose-dependent GK regulation by small molecules. The regulation of GK activation by glucose may have an important implication for the discovery and design of GK activators as potential antidiabetic agents.