Analysis of thermal stabilizing interactions in mesophilic and thermophilic adenylate kinases from the genus Methanococcus.

Adenylate kinases (ADKs) from four closely related methanogenic members of the Archaea (the mesophile Methanococcus voltae (MVO), the thermopile Methanococcus thermolithotrophicus (MTH), and the extreme thermopiles Methanococcus igneus (MIG) and Methanococcus jannaschii (MJA)) were characterized for their resistance to thermal denaturation. Despite possessing between 68 and 81% sequence identity, the methanococcal ADKs significantly differed in their stability against thermal denaturation, with melting points ranging from 69 to 103 degrees C. The high sequence identity between these organisms allowed regions of the MVO and MJA ADKs to be exchanged, producing chimeric ADKs with significantly altered thermal stability. Up to a 20 degrees C increase or decrease in stability was achieved for chimeric ADKs, whereas 88% of the original protein sequence was maintained. Based on our previous structural modeling studies, we conclude that cooperative interactions within the hydrophobic protein core play an integral role in determining the differences in structural stability observed between the methanococcal ADKs. From comparisons of the effects of temperature on protein unfolding and optimal enzymatic activity, we also conclude that thermostability and enzymatic temperature optima are influenced differently by molecular modifications and thus that the protein flexibility required for activity and stability, respectively, is not unconditionally linked within the methanococcal ADKs.