The gene responsible for ribokinase (RK) in human/eukaryotic cells has not yet been identified/characterized. Blast searches with E. coli RK have identified a human protein showing significant similarity to the bacterial RK. The cDNA for this protein was expressed in E. coli and the recombinant protein efficiently phosphorylated ribose to ribose-5-phosphate using ATP, confirming its identity as RK. In contrast to ribose, the enzyme exhibited very little to no phosphorylation of D-arabinose, D-xylose, D-fructose and D-galactose. The catalytic activity of human RK was dependent upon the presence of inorganic phosphate, as observed previously for E. coli RK and mammalian adenosine kinases (AK). A number of activators and inhibitors of human AK, produced very similar effects on the human and E. coli RKs, indicating that the catalytic mechanism of RK is very similar to that of the AKs.