Although ATP is the most common phosphoryl group donor for kinases, certain hyperthermophilic archaea, such as Thermococcus litoralis and Pyrococcus furiosus, utilise unusual ADP-dependent glucokinases (ADPGKs) and phosphofructokinases (ADPPKKs) in their glycolytic pathways [, , ]. ADPGKs and ADPPFKs exhibit significant similarity, and form an ADP-dependent kinase (ADPK) family, which was tentatively named the PFKC family []. A ~460-residue ADPK domain is also found in a bifunctional ADP-dependent gluco/phosphofructo-kinase (ADP-GK/PFK) from Methanocaldococcus jannaschii (Methanococcus jannaschii) as well as in homologous proteins present in several eukaryotes [, ]. Structure determination for eukaryotic ADPGK revealed an overall similar fold to archaeal orthologues with some differences in secondary structural elements. In the nucleotide-binding loop of eukaryotic ADPGK there is a disulfide bond between conserved cysteines; one of the cysteines coordinating the AMP defines an apparently nucleotide-binding motif unique to eukaryotic ADPGKs. Mammalian enzymes are specific for glucose [].The whole structure of the ADPK domain can be divided into large and small α/β subdomains. The larger subdomain, which carries the ADP binding site, consists of a twisted 12-stranded β-sheet flanked on both faces by 13 α-helices and three 3(10) helices, forming an α/β 3-layer sandwich. The smaller subdomain, which covers the active site, forms an α/β two-layer structure containing 5 bβ-strands and four α-helices. The ADP molecule is buried in a shallow pocket in the large subdomain. The binding of substrate sugar induces a structural change, the small domain closing to form a complete substrate sugar binding site [, , ].
