Halophilic Adaptation: Novel Solvent Protein Interactions Observed in the 2.9 and 2.6 Å Resolution Structures of the Wild Type and a Mutant of Malate Dehydrogenase from Haloarcula marismortui

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Abstract
Previous biophysical studies of tetrameric malate dehydrogenase from the halophilic archaeon Haloarcula marismortui (Hm MalDH) have revealed the importance of protein−solvent interactions for its adaptation to molar salt conditions that strongly affect protein solubility, stability, and activity, in general. The structures of the E267R stability mutant of apo (−NADH) Hm MalDH determined to 2.6 Å resolution and of apo (−NADH) wild type Hm MalDH determined to 2.9 Å resolution, presented here, highlight a variety of novel protein−solvent features involved in halophilic adaptation. The tetramer appears to be stabilized by ordered water molecule networks and intersubunit complex salt bridges “locked” in by bound solvent chloride and sodium ions. The E267R mutation points into a central ordered water cavity, disrupting protein−solvent interactions. The analysis of the crystal structures showed that halophilic adaptation is not aimed uniquely at “protecting” the enzyme from the extreme salt conditions, as may have been expected, but, on the contrary, consists of mechanisms that harness the high ionic concentration in the environment.