Short-term control of glucokinase activity: role of a regulatory protein

Glucokinase is one of the four hexokinases present in mammalian tissues. It is expressed in two cell types that have to respond to changes in the blood glucose concentration, the liver parenchymal cell and the beta-cells of pancreatic islets. The former are responsible for the metabolism and storage of an important part of the ingested glucose, whereas the latter secrete insulin in response to an increase in the blood glucose level. One major characteristic of glucokinase is that it has a relatively low affinity for glucose and displays positive cooperativity for this substrate, despite the fact that it is a monomeric enzyme. Furthermore, unlike other hexokinases, it is not inhibited by micromolar (physiological) concentrations of glucose 6-phosphate but by a regulatory protein that transduces the effect of fructose 6-phosphate and of fructose 1-phosphate. The purpose of this review is to describe these aspects of the regulation of glucokinase.