Halophilic proteins

To summarize this set of observations, halophilic proteins adsorb to polysaccharide matrices at high ammonium sulfate concentration. When the matrix is charged the adsorption of proteins having the same charge on the matrix is reduced whereas the adsorption of proteins having opposite charge is greatly facilitated. These observations enabled the development of several powerful purification... 

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. 

Malate dehydrogenase from H. marismortui (AMDH) is the halophilic protein that has been studied most by solution structure methods. A molar mass of 87 kg/mol was determined for the native enzyme. It is stable at high concentrations of NaCl or KC1 and unfolds and dissociates below 2.5 M salt. Pundak and Eisenberg (1981) first measured values for the solvent interactions of AMDH and found that, in contrast to nonhalophilic globular proteins in similar conditions (Bi 0.2—0.3 g/g,B3 0.01 g/g),the halophilic protein bound... 

I. Halophilic EF-Tu and Other Halophilic Proteins under Study... 

Halophilic proteins whose solution structures are currently under study include glyceraldehyde-3-phosphate dehydrogenase from Hal-oarcula vallismortis (Krishnan and Altekar, 1990) and a heme-binding catalase-peroxidase from H.marismortui (F. Cendrin, H. Jouvre, and G. Zaccai, private communication). 

It is hoped that development of expression vectors in halobacteria, as well as development of the methodology for gene expression of halophilic proteins in E. coli and their subsequent reactivation, will enable the use of the site-directed mutagenesis methodology in the elucidation of structural features that are responsible for the halophilic properties of these enzymes. 

In addition to their unusual amino acid content, halophilic proteins need high salt concentrations for maintaining their structure. The volume of halophilic proteins must be measured in these extreme multicomponent solutions. In the case of halophiles, removal of salt would lead to protein denaturation. Some examples for halophilic proteins in concentrated salt solutions are given in Table 11 cf. the values for halophilic malate dehydrogenase and halophilic glutamate dehydrogenase. 

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