A recombinant mannose-6-phosphate isomerase from Geobacillus thermodenitrificans (GTMpi) isomerizes aldose substrates possessing hydroxyl groups oriented in the same direction at the C2 and C3 positions such as the d- and l-forms of ribose, lyxose, talose, mannose, and allose. The activity of GTMpi for d-lyxose isomerization was optimal at pH 7.0, 70A degrees C and 1 mM Co2+. Under these conditions, the k (cat) and K (m) values were 74,300 s(-1) and 390 mM for d-lyxose and 28,800 s(-1) and 470 mM for l-ribose, respectively. The half-lives of the enzyme at 60, 65, and 70A degrees C were 388, 73, and 27 h, respectively. GTMpi catalyzed the conversion of d-lyxose to d-xylulose with a 38% conversion yield after 3 h, and converted l-ribose to l-ribulose with a 29% conversion yield.