Cytosolic glucokinase (GCK) (and three isoforms of hexokinase) catalyse the irreversible reaction of alpha-D-glucose (Glc) and ATP to form alpha-D-glucose-6-phosphate (G6P) and ADP, the first step in glycolysis. In the body, GCK is found only in hepatocytes and pancreatic beta cells. GCK and the hexokinase enzymes differ in that GCK has a higher Km than the hexokinases and is less readily inhibited by the reaction product. As a result, GCK should be inactive in the fasting state when glucose concentrations are low but in the fed state should have an activity proportional to glucose concentration. These features are thought to enable efficient glucose uptake and retention in the liver, and to function as a sensor of glucose concentration coupled to insulin release in pancreatic beta cells. Defects in GCK are can cause maturity-onset diabetes of the young 2 (MODY2; MIM:125851), a heritable early onset form of type II diabetes (Hussain 2010, Osbak et al. 2009).