Halophilic Protein Stabilization

Primary, secondary, tertiary, and quaternary structure are familiar concepts for proteins and refer to the amino acid sequence, local folding arrangement, three-dimensional organization, and subunit interactions of polypeptide chains, respectively. Here, tertiary and quaternary structure shall be considered in the most general way, to include also the small molecules or ions that are essential for the conformational stability of the polypeptide chains. This is especially relevant for halophilic proteins, which have extensive interactions with solvent components (water molecules and salt ions). The known structure of a protein (at any level) always results from experiment, and as such is known only within appropriate error limits. 

It is clear that more molecular chronometers need to be analysed. On account of the universality of the core metabolic pathways, a molecular study of the enzymes of central metabolism may prove worthwhile in this context. Moreover, the range of phenotypes within the one domain (e.g. extreme halophilicity and thermophilicity, in addition to mesophilicity) may make a comparative study of these enzymes especially valuable to our understanding of the structural basis for extreme protein stability. For these reasons, a number of laboratories are currently engaged in detailed structure-function investigations of the central metabolic enzymes. For a detailed discussion of the comparative enzymology of these pathways, see ref [1]. 

Fig. 7. Schematic representation of AMDH solution structures. The active structures have two parts a catalytically active core, conceivably similar to that in non-halophilic MDH, and protruding loops, required for stabilization in KCl, NaCl, and MgCl2 solvents. In potassium phosphate the protein dimer is stabilized by the hy-drophobicity of the core and the protruding loops are disordered. In KCl (or NaCl) the protein is stabilized by the interaction of the loops in a specific protein—water-salt hydration network. In MgCl2 a similar structure exists with the same amount of water molecules coordinated by fewer salt ions. In low salt concentration, the protein is unfolded and its hydration is like that of nonhalophilic proteins. From Zaccai el al. (1989), with permission.

There are other proteins from extremely halophilic bacteria that do not require high concentrations of salt for stability or activity [147-149]. This raises the possibility that there are additional proteins in the extreme halophiles that do not require conditions of high ionic strength for stability or activity. The failure to detect such proteins could be due to the conditions used during their isolation and characterization (i.e., high concentrations of salt) that are inimical to such enzymes. 

Further confusion in the order of cations in the Hofmeister series concerning the stability of a biomolecule (the enzyme halophilic malate dehydrogenase) arises from the reversal of the order when the cations are examined at low (< 1M) or at high concentrations (Ebel et al. 1999). The order of efficiency of cations to maintain the folded form of the protein at low concentrations is Ca + Mg + > Li ... 

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