<p>Hexokinase is an important enzyme that catalyses the ATP-dependent conversion of aldo- and keto-hexose sugars to the hexose-6-phosphate (H6P). The enzyme can catalyse this reaction on glucose, fructose, sorbitol and glucosamine, and as such is the first step in a number of metabolic pathways [<cite idref="PUB00001951"/>]. The addition of a phosphate group to the sugar acts to trap it in a cell, since the negatively charged phosphate cannot easily traverse the plasma membrane.</p><p>The enzyme is widely distributed in eukaryotes. There are three isozymes of hexokinase in yeast (PI, PII and glucokinase): isozymes PI and PII phosphorylate both aldo- and keto-sugars; glucokinase is specific for aldo-hexoses. All three isozymes contain two domains [<cite idref="PUB00001951"/>]. Structural studies of yeast hexokinase reveal a well-defined catalytic pocket that binds ATP and hexose, allowing easy transfer of the phosphate from ATP to the sugar [<cite idref="PUB00014215"/>]. Vertebrates contain four hexokinase isozymes, designated I to IV, where types I to III contain a duplication of the two-domain yeast-type hexokinases. Both the N- and C-terminal halves bind hexose and H6P, though in types I an III only the C-terminal half supports catalysis, while both halves support catalysis in type II. The N-terminal half is the regulatory region. Type IV hexokinase is similar to the yeast enzyme in containing only the two domains, and is sometimes incorrectly referred to as glucokinase.</p><p>The different vertebrate isozymes differ in their catalysis, localisation and regulation, thereby contributing to the different patterns of glucose metabolism in different tissues [<cite idref="PUB00014214"/>]. Whereas types I to III can phosphorylate a variety of hexose sugars and are inhibited by glucose-6-phosphate (G6P), type IV is specific for glucose and shows no G6P inhibition. Type I enzyme may have a catabolic function, producing H6P for energy production in glycolysis; it is bound to the mitochondrial membrane, which enables the coordination of glycolysis with the TCA cycle. Types II and III enzyme may have anabolic functions, providing H6P for glycogen or lipid synthesis. Type IV enzyme is found in the liver and pancreatic beta-cells, where it is controlled by insulin (activation) and glucagon (inhibition). In pancreatic beta-cells, type IV enzyme acts as a glucose sensor to modify insulin secretion. Mutations in type IV hexokinase have been associated with diabetes mellitus.</p><p>All hexokinases contain one or two strongly conserved regions, which has been shown to be involved in substrate binding [<cite idref="PUB00000147"/>]. The signature patterns of this entry are derived from such conserved regions.</p>