Halophilic enzymes: proteins with a grain of salt

Halophilic enzymes, while performing identical enzymatic functions as their non-halophilic counterparts, have been shown to exhibit substantially diff erent properties, among them the requirement for high salt concentrations, in the 1-4 M range, for activity and stability, and a high excess of acidic over basic amino residues. The following communication reviews the functio nal and structural properties of two proteins isolated from the extremely h alophilic archaeon Haloarcula marismortui: the enzyme malate-dehydrogenase (hMDH) and the 2Fe-2S protein ferredoxin. It is argued that the high negati ve surface charge of halophilic proteins makes them more soluble and render s them more flexible at high salt concentrations, conditions under which no n-halophilic proteins tend to aggregate and become rigid. This high surface charge is neutralized mainly by tightly bound water dipoles. The requireme nt of high salt concentration for the stabilization of halophilic enzymes, on the other hand, is due to a low affinity binding of the salt to specific sites on the surface of the folded polypeptide, thus stabilizing the activ e conformation of the protein. (C) 2000 Elsevier Science B.V. All rights re served.