Enzymes from thermophilic and hyperthermophilic organisms are invaluable catalysts for organic synthesis and biotechnology due to their stability and function at elevated temperatures. While the temperature dependency of catalysis with normal substrates has been studied for several of these enzymes, little is known about the effects of temperature on substrate specificity. Here we report the characterization of a novel thymidine kinase from the hyperthermophilic Thermotoga maritima which shows high substrate specificity at the organism's native growth temperature (82 degrees C) but turns promiscuous at 37 degrees C. Our experiments link this temperature-dependent substrate promiscuity to conformational changes in the protein below 70 degrees C, shifting the enzyme's rate-determining step to a substrate-independent transition in the homotetramer structure.