Purification and characterization of three extracellular (1→3)-β-d-glucan glucohydrolases from the filamentous fungus Acremonium persicinum

Abstract

Three (1-->3)-beta-D-glucanases (GNs) were isolated from the culture filtrates of the filamentous fungus Acremonium persicinum and purified by (NH4)2SO4 precipitation followed by anion-exchange and gel-filtration chromatography. Homogeneity of the purified proteins was confirmed by SDS/PAGE, isoelectric focusing and N-terminal amino acid sequencing. All three GNs (GN I, II and III) are non-glycosylated, monomeric proteins with apparent molecular masses, estimated by SDS/PAGE, of 81, 85 and 89 kDa respectively. pI values for the three enzymes are 5.3, 5.1, and 4.4 respectively. The pH optimum for GN I is 6.5, and 5.0 for GN II and III. All three purified enzymes displayed stability over the pH range 4.5-10.0. Optimum activities for GN I, II and III were recorded at 65, 55 and 60 degrees C respectively, with both GN II and III having short-term stability up to 50 degrees C and GN I up to 55 degrees C. The purified GNs have high specificity for (1-->3)-beta-linkages and hydrolysed a range of (1-->3)-beta- and (1-->3)(1-->6)-beta-D-glucans, with laminarin from Laminaria digitata being the most rapidly hydrolysed substrate of those tested. K(m) values for GN I, II, and III against L. digitata laminarin were 0.1, 0.23 and 0.22 mg/ml respectively. D-Glucono-1,5-lactone does not inhibit any of the three GNs, some metals ions are mild inhibitors, and N-bromosuccinimide and KMnO4 are strong inhibitors. All three GNs acted in an exo-hydrolytic manner, determined by the release of alpha-glucose as the initial and major product of hydrolysis of (1-->3)-beta-D-glucans, and confirmed by viscometric analysis and the inability to cleave periodate-oxidized laminarin, and may be classified as (1-->3)-beta-D-glucan glucohydrolases (EC 3.2.1.58).