Volume change associated with the low- to physiological-temperature conformational change in Thermomyces lanuginosus xylanase

Thermomyces lanuginosus is a thermophilic fungus with an optimal thriving temperature of 55°C. We have detected a conformational change in Thermomyces lanuginosus xylanase at 24°C and at pH = 7.0 and 1.0 atm pressure. This conformational change is revealed by an abrupt change in heat capacity as detected by slow-scan-rate differential scanning calorimetry and occurs at a temperature between that where crystals are formed for x-ray structure determination (4°C) and the physiological temperature (55°C). A volume change ΔV ≈-190 L/mol was calculated associated with the conversion from the low-temperature conformation to the higher-temperature conformation. The volume of the enzyme crystal structure, presumed to be the physiological structure, is 26 L/mole 14. We hypothesize this large, negative volume change indicates the low temperature form is either a molten globule or a random coil.