nucleotide-binding domain (NBD) of metazoan hexokinase (HK) domain family
Hexokinase (EC 2.7.1.1) possesses the ability to transfer an inorganic phosphate group from ATP to a substrate. It catalyzes the ATP-dependent phosphorylation of aldo- and keto-hexose sugars to the hexose-6-phosphate (H6P). It can catalyze this reaction on glucose, fructose, sorbitol and glucosamine, and as such is the first step in a number of metabolic pathways. Hexokinase contains two structurally similar domains. Some members of the family have two copies of each of these domains. For example, members from 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. In types I and III only the C-terminal half supports catalysis. Their N-terminal half is the regulatory region. In type II, both halves support catalysis. Type IV hexokinase is similar to the yeast enzyme in containing only the two domains and is sometimes incorrectly referred to as glucokinase. 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.
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