Mannose metabolizing enzymes from the red alga Galdieria sulphuraria

The unicellular thermo-acidophilic rhodophyte Galdieria sulphuraria is capable of growing at high rates on D-mannose, a sugar toxic to many higher plants. Mannose is introduced into metabolism through an ATP-dependent fructokinase which phosphorylates fructose (K-m = 0.21 mM) as well as mannose (K-m = 0.48 mM) but not glucose; The product, mannose-6-phosphate, is converted to fructose-6-phosphate by a mannose-6-phosphate isomerase (K-m = 1.2 mM). The fructokinase was purified by chromatography on DEAE-Fractogel and hydroxylapatite. The enzyme is a homodimer with an M-r of 75 000. The mannose-6-phosphate isomerase was purified by PEG precipitation and affinity chromatography on Affinity-Gel Blue A. The enzyme is a monomer with an M-r of 48 000. The pathway of mannose utilization in G. sulphuraria strongly resembles the reactions of mannose non-sensitive higher plants, while plants with mannose sensitivity lack activity of mannose-6-phosphate isomerase. (C) 1997 Elsevier Science Ltd. All rights reserved.