Evidence for increased local flexibility in psychrophilic alcohol dehydrogenase relative to its thermophilic homologue

The psychrophilic alcohol dehydrogenase (psADH) cloned from Antarctic Moraxella sp. TAE123 exhibits distinctive catalytic parameters in relation to the homologous thermophilic alcohol dehydrogenase (htADH) from Bacillus stearothermophilus LLD-R. Amide hydrogen-deuterium (H/D) exchange studies using Fourier-transformed infrared (FTIR) spectroscopy and mass spectrometry (MS) were conducted to investigate whether the differences are caused by variation in either global or regional protein flexibility. The FTIR H/D exchange study suggested that psADH does not share similar global flexibility with htADH at their physiologically relevant temperatures as has been predicted by the "corresponding state" hypothesis. However, the MS H/D exchange study revealed a more complicated picture concerning the flexibility of the two homologous enzymes. Analysis of the deuteration and exchange rates of protein-derived peptides suggested that only some functionally important regions in psADH that are involved in substrate and cofactor binding exhibit greater flexibility compared to htADH at low temperature (10°C). These observations strongly suggest that variable conformational flexibility between the two protein forms is a local phenomenon, and that global H/D exchange measurement by FTIR can be misleading and should be used with discretion. These results are supportive of the idea that functionally important local flexibility can be uncoupled from global thermal stability. The structural factors underlying the differences in local protein flexibility and catalysis between htADH and psADH are discussed in conjunction with results from crystallographic and fluorescence spectroscopy studies.