Halophilic Malate Dehydrogenase

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 

Composition Parameters of the hMDH Solution Particles, Calculated from Neutron-Scattering and Mass Density Increments 

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Bonnete, F. Madern, D. Zaccai, G., Stability against denaturation mechanisms in halophilic malate dehydrogenase adapt to solvent connditions, 7. Mol. Biol. 1994, 244, 436-447... 

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 ... 

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. 

Figure 6.4. The effects of different inorganic salts on activity of malate dehydrogenase from an extremely halophilic archaeon (unpublished data of L. Borowitkza and G. Somero).

Halophilic 2-5moll- NaCI Malate dehydrogenase, p-nitrophenylphosphate phosphatase Haloarcula, Halobacteiium, Haloferax, Halorubrum... 

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