FERREDOXIN FROM THE HYPERTHERMOPHILE THERMOTOGA-MARITIMA IS STABLE BEYOND THE BOILING-POINT OF WATER

Heat-stable proteins from hyperthermophilic microorganisms are ideally suited for investigating protein stability and evolution. We measured with differential scanning calorimetry and optical absorption spectro scopy the thermal stability of [4Fe-4S] ferredoxin from Thermotoga mar itima (tfdx), which is a small electron transfer protein. The results are consistent with two-state unfolding at the record denaturation tem perature of 125 degrees C. According to the crystal structure at 1.75 Angstrom resolution, T. maritima ferredoxin contains a significantly i ncreased number of hydrogen bonds that involve charged amino acid side -chains, compared to thermolabile ferredoxins. Thus, our results sugge st that polar interactions substantially contribute to protein stabili ty at very high temperatures. Moreover, because small [4Fe-4S] ferredo xins seem to have occurred early in evolution, the extreme thermostabi lity of tfdx supports the hypothesis that life originated at high temp eratures. (C) 1997 Academic Press Limited.