Primary structure and characterization of an exopolygalacturonase from Aspergillus tubingensis

• Published in: European journal of biochemistry Vol. 240; no. 3; pp. 738 - 746
• Main Authors: Kester, H.C.M, Kusters-van Someren, M.A, Muller, Y, Visser, J
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.09.1996
• Subjects: acid hydrolysis; Amino Acid Sequence; amino acid sequences; amino acids; Aspergillus; Aspergillus - enzymology; Aspergillus - genetics; Aspergillus niger; Aspergillus tubingensis; Base Sequence; biosynthesis; catalytic activity; chemistry; Cloning, Molecular; DNA, Fungal; DNA, Fungal - genetics; Enzyme Inhibitors; Enzyme Inhibitors - pharmacology; enzymology; exopolygalacturonase; fungi; galacturonic acid; genbank/x99795; gene expression; Gene Expression Regulation, Fungal; genes; Genes, Fungal; genetics; glycolysis; Glycoside Hydrolases; Glycoside Hydrolases - chemistry; Glycoside Hydrolases - genetics; Glycoside Hydrolases - metabolism; Hexuronic Acids; Hexuronic Acids - pharmacology; Hydrogen-Ion Concentration; hyphae; Isoelectric Point; kinetic analysis; Kinetics; metabolism; Moleculaire genetica van industriële micro-organismen; Molecular Genetics of Industrial Micro-organisms; Molecular Sequence Data; Molecular Structure; Molecular Weight; pharmacology; plant biochemistry; plant breeding; plant genetics; plant physiology; polygalacturonase; polymerization; pyruvate kinase; Recombinant Fusion Proteins; Recombinant Fusion Proteins - biosynthesis; Recombinant Fusion Proteins - chemistry; Recombinant Fusion Proteins - genetics; sequence analysis; Sequence Homology, Amino Acid; subsite structure; Substrate Specificity; VLAG
• ISSN: 0014-2956; 1432-1033
• DOI: 10.1111/j.1432-1033.1996.0738h.x

From the culture fluid of the hyphal fungus Aspergillus tubingensis, an exopolygalacturonase with a molecular mass of 78 kDa, an isoelectric point in the pH-range 3.7-4.4 and a pH optimum of 4.2 was purified. The enzyme has been characterized as an exopolygalacturonase [poly(1,4-alpha-D-galacturonide) galacturonohydrolase] that cleaves monomer units from the non-reducing end of the substrate molecule. Km and Vmax for polygalacturonic acid hydrolysis were 3.2 mg ml-1 and 3.1 mg ml-1 and 255 U mg-1 and 262 U mg-1 for the wild-type and recombinant enzymes, respectively. The kinetic data of exopolygalacturonase on oligogalacturonates of different degree of polymerization (2-7) were interpreted in terms of a subsite model to obtain more insight into catalysis and substrate binding. On oligogalacturonates of different degrees of polymerization (2-7), the Michaelis constant (Km) decreased with increasing chain length (n). The Vmax value increased with chain length up to n = 4, then reached a plateau value. The enzyme was competitively inhibited by galacturonic acid (Ki = 0.3 mM) as well as by reduced digalacturonate (Ki = 0.4 mM). The exopolygalacturonase gene (pgaX) was cloned by reverse genetics and shows only 13% overall amino acid sequence identity with A. niger endopolygalacturonases. The exopolygalacturonase is most related to plant polygalacturonases. Only four small stretches of amino acids are conserved between all known endogalacturonases and exopolygalacturonases. Expression of the pgaX gene is inducible with galacturonic acid and is subject to catabolite repression. A fusion between the promoter of the A. niger glycolytic gene encoding pyruvate kinase and the pgaX-coding region was used to achieve high level production of exopolygalacturonase under conditions where no endopolygalacturonases were produced.