Reduction of Folate by Dihydrofolate Reductase from Thermotoga maritima

Mammalian dihydrofolate reductases (DHFR) catalyse the reduction of folate more efficiently than the equivalent bacterial enzymes, despite typically having similar efficiencies for the reduction of their natural substrate dihydrofolate. In contrast, we show here that DHFR from the hyperthermophilic bacterium Thermotoga maritima is able to catalyse reduction of folate to tetrahydrofolate with a similar efficiency to reduction of dihydrofolate under saturating conditions. NMR and mass spectrometry experiments showed no evidence for production of free dihydrofolate during either the EcDHFR- or TmDHFR-catalysed reductions of folate, suggesting that both enzymes perform the two reduction steps without release of the partially reduced substrate. Our results imply that the reaction proceeds more efficiently in TmDHFR than in EcDHFR because the more open active site of TmDHFR facilitates protonation of folate. Because T. maritima lives under extreme conditions where tetrahydrofolate is particularly prone to oxidation, this ability to salvage folate may impart an advantage to the bacterium by minimising wastage of a valuable cofactor.