The role of the regulatory protein of glucokinase in the glucose sensory mechanism of the hepatocyte

Glucokinase has a very high flux control coefficient (greater than unity) o n glycogen synthesis from glucose in hepatocytes (Agius et al., J. BioL Che m, 271, 30479-30486, 1996). Hepatic glucokinase is inhibited by a 68 kDa gl ucokinase regulatory protein (GKRP) that is expressed in molar excess. To e stablish the relative control exerted by glucokinase and GKRP, we applied m etabolic control analysis to determine the flux control coefficient of GKRP on glucose metabolism in hepatocytes. Adenovirus-mediated overexpression o f GKRP (by up to 2-fold above endogenous levels) increased glucokinase bind ing and inhibited glucose phosphorylation, glycolysis, and glycogen synthes is over a wide range of concentrations of glucose and sorbitol, It decrease d the affinity of glucokinase translocation for glucose and increased the c ontrol coefficient of glucokinase on glycogen synthesis. GKRP had a negativ e control coefficient of glycogen synthesis that is slightly greater than u nity (-1.2) and a control coefficient on glycolysis of -0.5. The control co efficient of GKRP on glycogen synthesis decreased with increasing glucokina se overexpression (4-fold) at elevated glucose concentration (35 mM), which favors dissociation of glucokinase from GKRP, but not at 7.5 mM glucose. U nder the latter conditions, glucokinase and GKRP have large and inverse con trol coefficients on glycogen synthesis, suggesting that a large component of the positive control coefficient of glucokinase is counterbalanced by th e negative coefficient of GKRP. It is concluded that glucokinase and GKRP e xert reciprocal control; therefore, mutations in GKRP affecting the express ion or function of the protein may impact the phenotype even in the heteroz ygote state, similar to glucokinase mutations in maturity onset diabetes of the young type 2. Our results show that the mechanism comprising glucokina se and GKRP confers a markedly extended responsiveness and sensitivity to c hanges in glucose concentration on the hepatocyte.