Fructokinase (FRK; EC 2.7.1.4) catalyzes the phosphorylation of D-fructose to D-fructose 6-phosphate (F6P). This irreversible and near rate-limiting step is a central and regulatory process in plants and bacteria, which channels fructose into a metabolically active state for glycolysis. Towards understanding the mechanism of FRY, here we report the crystal structure of a FRK homolog from a thermohalophilic bacterium Halothermothrix orenii (Hore_18220 in sequence databases). The structure of the Hore_18220 protein reveals a catalytic domain with a Rossmann-like fold and a beta-sheet "lid" for dimerization. Based on comparison of Hore_18220 to structures of related proteins, we propose its mechanism of action, in which the lid serves to regulate access to the substrate binding sites. Close relationship of Hore_18220 and plant FRK enzymes allows us to propose a model for the structure and function of FRKs. (C) 2010 Elsevier Inc. All rights reserved.