The substrate spectrum of TmRK with a set of sugars was studied before and it was shown that the enzyme is very specific for D-ribose . Although TmRK seems to be a rather specific sugar kinase, it phosphorylates the natural nucleosides thymidine and uridine as well as deoxyguanosine, deoxyinosine and deoxycytidine albeit with lower conversion (Figure ). The preference for pyrimidine nucleosides is in good accordance with Pyrobaculum caldifontis PfkB, which preferred cytidine and uridine . Interestingly, while other PfkB members preferred ribonucleosides , TmRK seems to prefer NMP kinases of T. maritima showed a rather narrow substrate scope, except for TmUMPK which accepted all natural riboNMPs (Figure ). The other NMP kinases accepted their preferred substrate next to a few additional substrates. As an example, Tm(d)CMPK accepted CMP, dCMP and UMP.