Halophilic enzymes

In fact, halophilic enzymes have been shown to function in low salt environments, where the salt is replaced by a suitable organic solvent typically those that are kosmotropic. For example, an extracellular protease from H. halobium, which requires 4M NaCl for optimal function, is highly active in as little as 0.2 M NaCl in the presence of 40% (v/v) dimethylsulfoxide (DMSO) [10]. The behavior of salts... 

Halophilic enzymes are very unstable in low salt concentrations. Because some of the important fractionation methods in protein chemistry, such as electrophoresis or ion-exchange chromatography, cannot be applied at high salt concentrations, the available fractionation methods are rather limited. This basic difficulty is the main reason why the number of halophilic enzymes studied in pure form is very small. 

Elution of Two Halophilic Enzymes by Decreasing Concentration Gradients of Ammonium Sulfate ... 

A very interesting application of affinity chromatography to the purification of halophilic enzymes was reported by Sundquist and Fahey (1988). These authors have purified the enzymes bis-y-glu-tamylcysteine reductase and dihydrolipoamide dehydrogenase from H. halohium using immobilized metal ion affinity chromatography in high-salt buffers. 

The crystal structures of the E. coli DHFR-methotrexate binary complex (Bolin et al., 1982), of the Lactobacillus casei (DHFR-NADPH-methotrexate ternary complex (Filman et al., 1982), of the human DHFR-folate binary complex (Oefner et al., 1988), and of the mouse (DHFR-NADPH-trimethoprim tertiary complex (Stammers et al., 1987) have been resolved at a resolution of 2 A or better. The crystal structures of the mouse DHFR-NADPH-methotrexate (Stammers et al., 1987) and the avian DHFR—phenyltriazine (Volz et al., 1982) complexes were determined at resolutions of 2.5 and 2.9 A, respectively. Recently, the crystal structure of the E. coli DHFR—NADP + binary and DHFR-NADP+-folate tertiary complexes were resolved at resolutions of 2.4 and 2.5 A, respectively (Bystroff et al., 1990). DHFR is therefore the first dehydrogenase system for which so many structures of different complexes have been resolved. Despite less than 30% homology between the amino acid sequences of the E. coli and the L. casei enzymes, the two backbone structures are similar. When the coordinates of 142 a-carbon atoms (out of 159) of E. coli DHFR are matched to equivalent carbons of the L. casei enzyme, the root-mean-square deviation is only 1.07 A (Bolin et al., 1982). Not only are the three-dimensional structures of DHFRs from different sources similar, but, as we shall see later, the overall kinetic schemes for E. coli (Fierke et al., 1987), L. casei (Andrews et al., 1989), and mouse (Thillet et al., 1990) DHFRs have been determined and are also similar. That the structural properties of DHFRs from different sources are very similar, in spite of the considerable differences in their sequences, suggests that in the absence, so far, of structural information for ADHFR it is possible to assume, at least as a first approximation, that the a-carbon chain of the halophilic enzyme will not deviate considerably from those of the nonhalophilic ones. 

It is tempting to try to explain the halophilic features of ADHFR even in the absence of a detailed kinetic scheme for this enzyme, assuming that the main features of the kinetic scheme of the non-halophilic enzymes hold true also for the halophilic enzyme. The salt concentration might have an effect on the rates of binding or dissociation of the various substrates or on the rate of the hydride transfer reaction. Because, as we saw, the hydride transfer reaction is largely dependent on the protonation of Asp-27, it becomes the rate-limiting step at pH values higher than the pKa of this residue. The effect of salt concentration on the steady-state turnover can be ex-... 

Biotechnologicai Applications of Halophilic Enzymes. Halophilic enzymes are used commercially to produce compatible solutes and valuable extracellular enzymes (87). Halophilic enzymes are also employed in oil recovery and the degradation of industrial pollutants in high salt concentration environments. 

Ryu, K., Kim, J., Dordick, J. S. (1994). Catalytic properties and potential of an extracellular protease from an extreme halophile. Enzyme Microb. TechnoL, 16,266-275. 

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