Cold-adapted beta-galactosidase from the Antarctic psychrophile Pseudoalteromonas haloplanktis

The beta -galactosidase from the Antarctic gram-negative bacterium Pseudoal teromonas haloplanktis TAE 79 was purified to homogeneity, The nucleotide s equence and the NH2-terminal amino acid sequence of the purified enzyme ind icate that the beta -galactosidase subunit is composed of 1,038 amino acids with a calculated M, of 118,068. This beta -galactosidase shares structura l properties with Escherichia coli beta -galactosidase (comparable subunit mass, 51% amino sequence identity, conservation of amino acid residues invo lved in catalysis, similar optimal pH value, and requirement for divalent m etal ions) but is characterized by a higher catalytic efficiency on synthet ic and natural substrates and by a shift of apparent optimum activity towar d low temperatures and lower thermal stability. The enzyme also differs by a higher pi (7.8) and by specific thermodynamic activation parameters. P. h aloplanktis beta -galactosidase was expressed in E. coli, and the recombina nt enzyme displays properties identical to those of the wild-type enzyme. H eat-induced unfolding monitored by intrinsic fluorescence spectroscopy show ed lower melting point values for both P. haloplanktis wild-type and recomb inant beta -galactosidase compared to the mesophilic enzyme. Assays of lact ose hydrolysis in milk demonstrate that P. haloplanktis beta -galactosidase can outperform the current commercial beta -galactosidase from Kluyveromyc es marxianus var. lactis, suggesting that the cold-adapted beta -galactosid ase could be used to hydrolyze lactose in dairy products processed in refri gerated plants.