Structure and function of archaeal ADPGK

There are certain primary structure elements that are conserved throughout the ADP- dependent kinase family and ribokinase superfamily. The sequence identity between the archaeal and the eukaryotic homologs of ADPGKs is generally very low with less than around 20% identity. An alignment of members of the ribokinase family, including ADPGK, ADPPFK, adenosine kinase and ribokinase as examples, is shown in Figure 4. The sequence motif for the active site is GXGD, positioned at the end of a helix, where the aspartate residue (Figure 4, alignment position 567) acts as a catalytic base. This sequence motif is highly conserved in other members of the ribokinase family. It has been shown that mutation of this aspartate to a different amino acid results in the loss of activity (Ito et al. 2001). However, the aspartate residue appears to be not strictly mandatory, as a cysteine residue as been observed in this position in other ribokinase-like enzymes (Ito et al. 2003). In the 4-methyl-5-β- hydroxyethylthiazole kinase from B. subtilis, mutation of this cysteine residue to aspartate results in a nine-fold gain of activity (Campobasso et al. 2000). Upon closing of the two domains the terminal phosphate group is stabilized by a conserved arginine residue. This stabilises the phosphate at the catalytic centre and the glucose molecule. The interaction of the catalytic aspartate and the hydroxyl group in 6′- position of the bound glucose triggers the transfer reaction (Ito et al. 2003).

10 Figure 4: Sequence alignment of members of the ribokinase family.

Sequence alignment of members of the ribokinase family of proteins. Alignment was made with Clustal Omega (Sievers et al. 2011). Pink shading indicates conserved sequence features (see text).

hsADPGK = H. sapiens ADPGK, mmADPGK = M. musculus ADPGK, dr = Danio rerio ADPGK,

tlADPGK = T. litoralis ADPGK, pfADPGK = P. furiosus ADPGK, pfADPPFK = P. furiosus

ADPPFK, tgAK = Toxoplasma gondii adenosine kinase, hsAK = H. sapiens adenosine kinase, hsRK =

H. sapiens ribokinase, ec = E. coli ribokinase

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