Pseudomonas aeruginosa PAO1 phosphoglucose isomerase was purified as an active soluble form by a single-step purification using Ni-NTA chromatography that showed homogeneity on SDS-PAGE with molecular mass similar to 62 kDa. The optimum temperature and pH for the maximum isomerization activity with d-galactose were 60 A degrees C and 7.0, respectively. Generally, sugar phosphate isomerases show metal-independent activity but PA-PGI exhibited metal-dependent isomerization activity with aldosugars and optimally catalyzed the d-galactose isomerization in the presence of 1.0 mM MnCl2. The apparent Km and Vmax for d-galactose under standardized conditions were calculated to be 1029 mM (+/- 31.30 with S.E.) and 5.95 U/mg (+/- 0.9 with S.E.), respectively. Equilibrium reached after 180 min with production of 567.51 mu M d-tagatose from 1000 mM of d-galactose. Though, the bioconversion ratio is low but it can be increased by immobilization and enzyme engineering. Although various l-arabinose isomerases have been characterized for bioproduction of d-tagatose, P. aeruginosa glucose phosphate isomerase is distinguished from the other l-arabinose isomerases by its optimal temperature (60 A degrees C) for d-tagatose production being mesophilic bacteria, making it an alternate choice for bulk production.