Francisco Pérez-Pomares

Occurrence of two different glutamate dehydrogenase activities in the halophilic bacterium Salinibacter ruber.

Salinibacter ruber, an extremely halophilic member of the domain Bacteria, has two different cytoplasmic glutamate dehydrogenase activities, marked as GDHI and GDHII. GDHI showed a strong dependence on high salt concentrations for stability, but not for activity, displaying maximal activity in the absence of salts. GDHII depended on high salt concentrations for both activity and stability. It catalyzed amination of 2-oxoglutarate with optimal activity in 3 M KCl at pH 8. No activating effect was found when NaCl was replaced by KCl. Only GDHII displayed activity in the deamination reaction of glutamate with an optimal pH of 9.5. Both enzymes were activated by certain amino acids (L-leucine, L-histidine, L-phenylalanine) and by nucleotides such as ADP or ATP. A low-molecular-mass cytoplasmic fraction was found to be a highly effective activator of GDHII in the presence of high NaCl concentrations.

NAD-glutamate dehydrogenase from Halobacterium halobium: inhibition and activation by TCA intermediates and amino acids.

A variety of metabolites have been found to elicit a form of inhibition or activation on an NAD-specific glutamate dehydrogenase (NAD-GDH, EC 1.4.1.2) from Halobacterium halobium. The purified halophilic enzyme was tested with several compounds known to be allosteric modifiers of mammalian glutamate dehydrogenases to determine their effects on enzyme activity. GTP, ATP, ADP and AMP did not affect the enzyme, so these effectors of bovine glutamate dehydrogenase do not play a role in the regulation of the halophilic enzyme. However, the halophilic enzyme was subject to strong inhibition by TCA intermediates. When measuring the initial rate of the reaction, the oxidative deamination of L-glutamate was inhibited by TCA metabolites such as: fumarate, oxalacetate, succinate and malate; by substrate analogues such as: NADP+, D-glutamate and glutarate; and by dicarboxylic compounds such as adipate. On the other hand, all the amino acids tested were activators of this enzyme, except the D-isomer of the substrate L-glutamate that acted as an inhibitor. The relative effectiveness of each inhibitor or activator (Ki or Ka values) was correlated with the dipole moment (mu), HOMO and LUMO molecular orbital energies, optimal distance between two carboxyl groups, and hydrophobicity. Compounds with high dipole moment acted as good activators while compounds with low dipole moment were inhibitors. We have also found that the best activators were amino acids with no polar lateral chain.

Amino acid residues involved in the catalytic mechanism of NAD-dependent glutamate dehydrogenase from Halobacterium salinarum.

The pH dependence of kinetic parameters for a competitive inhibitor (glutarate) was determined in order to obtain information on the chemical mechanism for NAD-dependent glutamate dehydrogenase from Halobacterium salinarum. The maximum velocity is pH dependent, decreasing at low pHs giving a pK value of 7.19+/-0.13, while the V/K for l-glutamate at 30 degrees C decreases at low and high pHs, yielding pK values of 7.9+/-0.2 and 9.8+/-0.2, respectively. The glutarate pKis profile decreases at high pHs, yielding a pK of 9. 59+/-0.09 at 30 degrees C. The values of ionization heat calculated from the change in pK with temperature are: 1.19 x 10(4), 5.7 x 10(3), 7 x 10(3), 6.6 x 10(3) cal mol-1, for the residues involved. All these data suggest that the groups required for catalysis and/or binding are lysine, histidine and tyrosine. The enzyme shows a time-dependent loss in glutamate oxidation activity when incubated with diethyl pyrocarbonate (DEPC). Inactivation follows pseudo-first-order kinetics with a second-order rate constant of 53 M-1min-1. The pKa of the titratable group was pK1=6.6+/-0.6. Inactivation with ethyl acetimidate also shows pseudo-first-order kinetics as well as inactivation with TNM yielding second-order constants of 1.2 M-1min-1 and 2.8 M-1min-1, and pKas of 8.36 and 9.0, respectively. The proposed mechanism involves hydrogen binding of each of the two carboxylic groups to tyrosyl residues; histidine interacts with one of the N-hydrogens of the l-glutamate amino group. We also corroborate the presence of a conservative lysine that has a remarkable ability to coordinate a water molecule that would act as general base.

An optimized method to produce halophilic proteins in Escherichia coli

Background The homologous and heterologous expression of genes is a prerequisite for most biochemical studies of protein function. Many systems have been carried out for protein production in members of the Bacteria and Eukarya, however members of the Archaea are less amenable to genetic manipulation. Only a few systems for high-level gene expression have been developed for halophilic microorganisms. Because of this, mesophilic hosts, in particular Escherichia coli, have been used to produce halophilic proteins for biochemical characterization and crystallographic studies. Expression in E. coli has the advantage to be faster and it will easily allow production on a commercial scale. In contrast, difficulties are encountered since enzymes from extreme halophiles require the presence of high salt concentration for activity and stability, and the overexpressed product will need either reactivation or refolding in a salt solution, and so the purification techniques should be compatible with the high salt concentration required.

Heterologous overexpression of a halophilic α-amylase

Background Extracellular hydrolytic enzymes such as α-amylases are widely used in diverse applications in different industrial areas. α-amylase (EC 3.2.1.1) is an important endo-type carbohydrase that hydrolyzes α-1,4 glycosidic linkages of D-glucose oligomers and polymers. This enzyme has been found in organisms of the three Domains, being a key enzyme of carbohydrate metabolism. Haloferax mediterranei is an extremely halophilic Archaea that requires high salt concentrations to grow. This microorganism is able to grow in a minimal medium with ammonium acetate as the only source of carbon and nitrogen.H. mediterranei shows α-amylase extracellular activity when grows in this minimal medium in the presence of starch. The main role of this enzyme is the starch metabolism in the extracellular medium, so a lot of microorganisms depend on amylases for survival [1].

Recent Advances in the Nitrogen Metabolism in Haloarchaea and Its Biotechnological Applications

Halophilic archaea belong to the third domain of life, which live and survive in a highly salty environment. Nitrate assimilation is one of the main processes of the N-cycle, allowing the use of NO3−, NO2− and/or NH4+ as N source for growth. This pathway in general termed “Assimilatory nitrate pathway or assimilatory nitrate reduction” includes not only assimilatory nitrate reduction but also the assimilation of nitrite and ammonium. In the assimilatory nitrate reduction, NO3− is finally reduced to NH4+ by two sequential reactions catalysed by a ferredoxin-dependent nitrate reductase (Nas; EC 1.6.6.2) and a ferredoxin-dependent nitrite reductase (Nir; EC 1.7.7.1). The glutamine synthetase/glutamate synthase pathway (GS-GOGAT; EC 6.3.1.2, EC 1.4.7.1, respectively) or l-glutamate dehydrogenase (GDH; EC 1.4.1.2) are responsible for incorporating NH4+ into carbon skeletons. This chapter reviews current knowledge on nitrogen metabolism in haloarchaea with emphasis on assimilatory nitrate reduction, proteins involved and its regulation.