The physiological role of glucokinase binding and translocation in hepatocytes

Abstract

The compartmentation of glucokinase in the hepatocyte is regulated by the extracellular glucose concentration and by substrates that alter the concentration of fructose 1-phosphate in the hepatocyte. At low glucose concentrations, that mimic the fasted state, glucokinase is sequestered in an inactive state bound to the 68 kDa regulatory protein in the nucleus. In these conditions the rate of glucose phosphorylation is less than 15% of the total glucokinase activity. An increase in extracellular glucose concentration, within the range occurring in the portal vein in the absorptive state, or low concentrations of fructose or sorbitol (precursors of fructose 1-phosphate), cause the translocation of glucokinase from the nucleus to the cytoplasm and this is associated with a corresponding increase in glucose phosphorylation. The effect of glucose on translocation is mimicked by mannose which is also phosphorylated by glucokinase as well as by competitive inhibitors of glucokinase (mannoheptulose and 5-thioglucose) which are not phosphorylated. Various lines of evidence suggest that the action of these analogues is most likely due to binding to an allosteric or non-catalytic site. The saturation curve of glucose phosphorylation in intact hepatocytes is sigmoidal with an S0.5 of approximately 20 mM and a Hill coefficient approximately 2. This saturation curve can be explained by the activity of glucokinase in the cytoplasmic compartment. Translocation of glucokinase from the nucleus to the cytoplasm in response to precursors of fructose 1-phosphate (which cause dissociation of glucokinase from the regulatory protein) is associated with stimulation of glucose phosphorylation, glycolysis and glycogen synthesis. Using Metabolic Control Analysis to determine the Control Coefficient (Control Strength) of cytoplasmic (free) glucokinase on glucose metabolism it can be shown that the free glucokinase activity has a very high control strength on glycogen synthesis (CFGKJ > 1), indicating a major role of translocation of glucokinase in the control of hepatic glycogen synthesis. Overexpression of glucokinase in hepatocytes by adenovirus-mediated glucokinase overexpression is associated with a marked increase in glycogen synthesis. The relation between glycogen synthesis and enzyme overexpression is sigmoidal with an enzyme concentration causing half-saturation (S0.5) in the physiological range. The high Control Coefficient of glucokinase on hepatic glycogen synthesis explains the abnormalities of hepatic glycogen synthesis in patients with a single mutant allele of the glucokinase gene (Maturity Onset Diabetes of the Young, type 2).