non-halophilic proteins, which bind similar
amounts of water but do not bind salt. Halophilic
proteins maintain functionally active conformations
in the presence of high concentrations of antichaotropic
salts, whereas in the presence of chaotropic
salts, their conformations become inactive.
A number of studies have suggested that the
halophilic adaptation correlates with an increase
in acidic amino acids in the protein composition.5–8
A statistical analysis of a dozen halophilic proteins
confirmed the acidic nature of the halophiles.9
The relationship between acidic residues and salt
binding was suggested by a stabilization model
proposed for the tetrameric malate dehydrogenase
(MalDH) from Haloarcula marismortui.4,10,11 The
amino acid residues located at the protein surface
have been proposed to bind in a network of
hydrated salt ions that cooperatively contribute
to the stabilization. The structure of the [2Fe-2S]
ferredoxin from H. marismortui at 1.9A ° resolution
revealed an acidic surface as well as extensive
hydration networks.12 The significance of acidic
amino acids in halophilic adaptation has been
challenged, however, by Bohm & Jaenicke13,14 and
Elcock,15 following their statistical and theoretical
model studies of the halophilic dihydrofolate
reductase (H2 folate reductase).