Mutation at a single acidic amino acid enhances the halophilic behaviour of malate dehydrogenase from Haloarcula marismortui in physiological salts

NADP-glutamate dehydrogenase from the halophilic archaeon Haloferax mediterranei: enzyme purification, N-terminal sequence and stability

An NADP(H)-specific glutamate dehydrogenase of Haloferax mediterranei has been purified to apparent homogeneity and characterised. The purified enzyme was stabilized by glycerol in absence of salt. Glutamate dehydrogenase from Hf. mediterranei is a hexameric enzyme with a native molecular mass of 320 kDa composed of monomers each with a molecular mass of 55 kDa. At pH 8.5 the enzyme has Kms of 0.018, 0.34 and 4.2 mM for NADP+, 2-oxoglutarate and ammonium, respectively. Amino acid composition and sequence of the first 16 residues of the N-terminus have been determined.

Halolysin R4, a serine proteinase from the halophilic archaeon Haloferax mediterranei; gene cloning, expression and structural studies

A gene encoding a halophilic serine proteinase, halolysin R4, from a halophilic archaeon Haloferax mediterranei strain R4 was cloned, its nucleotide sequence determined, and expressed in Haloferax volcanii WFD11. The deduced amino-acid sequence (403 aa in length) showed the highest similarity to halolysin 172P1, produced by another halophilic archaeon, strain 172P1 (now designated as Natrialba asiatica). Both halolysins belong to the thermitase branch of class I subtilases, but show long C-terminal extensions of 117 and 123 amino acids, respectively. Removal of this "tail' region from halolysin R4 abolished proteinase activity, indicating it provides an essential (but as yet unknown) function. Substitution of the two cysteine residues in the C-terminal extension with serine decreased enzyme stability in hypotonic solutions, possibly owing to disruption of potential disulfide bonds or perturbation of calcium binding site(s).