Abstract
Extracellular pullulanases were purified from cell-free culture supernatants of the marine thermophilic archaea Thermococcus litoralis (optimal growth temperature, 90 degrees C) and Pyrococcus furiosus (optimal growth temperature, 98 degrees C). The molecular mass of the T. litoralis enzyme was estimated at 119,000 Da by electrophoresis, while the P. furiosus enzyme exhibited a molecular mass of 110,000 Da under the same conditions. Both enzymes tested positive for bound sugar by the periodic acid-Schiff technique and are therefore glycoproteins. The thermoactivity and thermostability of both enzymes were enhanced in the presence of 5 mM Ca, and under these conditions, enzyme activity could be measured at temperatures of up to 130 to 140 degrees C. The addition of Ca also affected substrate binding, as evidenced by a decrease in K(m) for both enzymes when assayed in the presence of this metal. Each of these enzymes was able to hydrolyze, in addition to the alpha-1,6 linkages in pullulan, alpha-1,4 linkages in amylose and soluble starch. Neither enzyme possessed activity against maltohexaose or other smaller alpha-1,4-linked oligosaccharides. The enzymes from T. litoralis and P. furiosus appear to represent highly thermostable amylopullulanases, versions of which have been isolated from less-thermophilic organisms. The identification of these enzymes further defines the saccharide-metabolizing systems possessed by these two organisms.