Thesis (MSc)--University of Stellenbosch, 2006. / ENGLISH ABSTRACT: The euascomycetous fungus Aureobasidium pullulans produces xylanolytic accessory enzymes, including an α-L-arabinofuranosidase. The deduced amino acid sequence of the abfA gene encoding α-L-arabinofuranosidase was 69-76% identical to family 54 glycoside hydrolases. The abfA gene encoded a 498 amino acid polypeptide including a signal peptide consisting of 20 amino acids. The mature protein had a calculated molecular weight of 49.9 kDa. One putative N-glycosylation site was found and an iso-electric point of 4.97 was calculated. A. pullulans AbfA was also found to consist of an N-terminal catalytic domain (residues 1-317) and a C-terminal arabinose-binding domain (residues 318-442). The abfA gene from the colour-variant strain of A. pullulans, NRRL Y-2311-1, was recently transferred in Saccharomyces cerevisiae Y294. The yeast culture was grown on synthetic defined medium and α-L-arabinofuranosidase was expressed successfully, secreted from the cells and was purified from the supernatant in a single step using gel filtration. It had an apparent mobility of 52.7 kDa on SDS-PAGE and 36 kDa estimated by gel filtration. The heterologous enzyme was characterized according to pH and temperature dependence and stability, apparent mobility and kinetic properties. The temperature optimum of the recombinant α–L-arabinofuranosidase was 55 °C and it was stable over 3 h at 40 °C. The enzyme displayed optimum activity between pH 4 and 4.5 and was stable at pH 4 over 3 h. Kinetic analysis on p-nitrophenyl-α-arabinofuranoside yielded a Km of 1.43 mM and a Vmax of 23.7 U/mg. Product inhibition was observed and a Ki of 28 ± 3 mM was determined during assaying in the presence of arabinose. A specific activity of 3.85 ± 0.008 U/mg was determined on p-nitrophenyl-α-L-arabinofuranoside and no activity was found on chromogenic substrates which contained a β-linked arabinofuranosyl. The enzyme showed low activity against the 1,5-α-L-arabino-oligosaccharides and cleaved arabinose from corn fibre, oat spelt arabinoxylan and to a lesser degree wheat arabinoxylan. No release of arabinose was observed from larch wood arabinogalactan, α-1,5-debranched arabinan and lignin-arabinose substrates. Linkage preference showed less activity against α-1,5-linked than α-1,2 or α-1,3-linked arabinofuranosyl subunits. Synergism between α–L-arabinofuranosidase and endo-β-1,4-xylanase occurred when measuring the increase in arabinose during wheat arabinoxylan degradation. A. pullulans NRRL Y-2311-1 was grown on synthetic defined medium and native α-L-arabinofuranosidase was expressed and secreted into the culture medium. The native enzyme was partially purified from the supernatant in two steps using gel filtration. The native α–L-arabinofuranosidase had an apparent mobility of 51.5 kDa on SDS-PAGE, displayed optimum activity at 50°C and pH 3. Kinetic analysis on p-nitrophenyl-α-arabinofuranoside gave a Km of 8.33 mM and a Vmax of 1.54 U/mg, and the enzyme showed slight activity against 1,5-α-L-arabinotriose. The properties of the native enzyme were similar to that of the heterologous α–L-arabinofuranosidase. Hydrolysis of sugar cane bagasse by heterologous α–L-arabinofuranosidase and xylanase revealed that pre-treatment with liquid ammonium was more effective in releasing component sugars than a pre-treatment with water at 140º C. A three-dimensional homology model of the heterologous α–L-arabinofuranosidase was constructed using the solved crystal structure of arabinofuranosidase (AkabfB) from Aspergillus kawachii, which was 71 % identical. / AFRIKAANSE OPSOMMING: Die euaskomisetiese swam Aureobasidium pullulans produseer xilanolitiese ensieme, insluitend ‘n α-L-arabinofuranosidase. Die afgeleide aminosuurvolgorde van die abfA geen wat α-L-arabinofuranosidase enkodeer, was 69-76% identies aan familie 54 glikosied hidrolase. Die abfA geen enkodeer ‘n 498 aminosure polipeptied, insluitend ‘n sein peptied bestaande uit 20 aminosure. Die volwasse proteïen het ‘n berekende molekulêre gewig van 49.9 kDa. Een moontlike N-glikosilasie plek is gevind en ‘n iso-elektriese punt van 4.97 is bereken. A. pullulans AbfA bestaan uit ‘n N-terminaal katalitiese gebied (residu 1-317) en ‘n C-terminaal arabinose-bindingsgebied (residu 318-442). Die abfA geen van die kleur-variante ras van A. pullulans, NRRL Y-2311-1, is onlangs na Saccharomyces cerevisiae Y294 oorgedra. Die giskultuur is op ‘n sinteties gedefinieërde medium gegroei en α-L-arabinofuranosidase was suksesvol uitgedruk, uitgedra van af die selle en van uit die supernatant gesuiwer in ‘n enkele stap deur gel filtrasie te gebruik. Dit het ‘n berekende mobiliteit van 52.7 kDa op SDS-PAGE en 36 kDa geskat deur middel van gel filtrasie. Die heteroloë ensiem is gekarakteriseer volgens pH en temperatuurafhanklikheid en stabiliteit, klaarblyklike mobiliteit en kinetiese eienskappe. Die temperatuur optimale van α–L-arabinofuranosidase was 55 °C en dit was stabiel oor 3 ure teen 40 °C. Die ensiem het optimale aktiwiteit tussen pH 4 en 4.5 getoon en was stabiel teen pH4 en oor 3 ure. Kinetiese analiese op p-nitrofeniel-α-L-arabinofuranosied het ‘n Km van 1.43 gelewer en ‘n Vmax van 23.7 U/mg. Produkinhibisie is opgelet en ‘n Ki van 28 ± 3 mM is vasgestel gedurende toetsing in die teenwoordigheid van arabinose. ‘n Spesifieke aktiwiteit van 3.85 ± 0.008 U/mg is vasgestel op p-nitrofeniel-α-L-arabinofuranosied en geen aktiwiteit was gevind op chromogeniese substrate wat ‘n β-verbinding arabinofuranosiel bevat het nie. Die ensiem het ‘n lae aktiwiteit getoon teen die 1,5-α-L-arabino-oligosakkaried en het die arabinose van mielievesel, hawerspelt arabinoxilaan en tot ‘n mindere mate koring arabinoxilaan geskei. Geen vrystelling van arabinose is van af lorkehout arabinogalactan, α-1,5-onvertakte arabinan en lignin-arabinose substrate nie opgemerk. Verbindingsvoorkeure het minder aktiwiteit teen α-1,5-verbindings as α-1,2 of α-1,3- verbindings arabinofuranosiel subeenhede getoon. Sinergisme tussen α–L-arabinofuranosidase en endo-β-1,4-xilanase het plaasgevind met die bepaling van meer arabinose gedurende koring arabinoxilaan degradasie. A. pullulans NRRL Y-2311-1 is op sinteties gedefinieerde medium gegroei en α-L-arabinofuranosidase was uitgedruk en uitgeskei in die kultuur medium. Die inheemse ensiem was gedeeltelik gesuiwer van die supernatant in twee stappe met die gebruik van gel filtrasie. Die inheemse α–L-arabinofuranosidase het ‘n klaarblyklike mobiliteit van 51.5 kDa op SDS-PAGE, het optimum aktiwiteit vertoon by 50°C en pH 3. Kinetiese analiese op p-nitrofeniel-α-arabinofuranosiede het ‘n Km van 8.33 mM en ‘n Vmax van 1.54 U/mg, en die ensiem het effense aktiwiteit teen 1,5-α-L-arabinotrios getoon. Die eienskappe van die inheemse ensiem was soortgelyk aan die van die heteroloë α–L-arabinofuranosidase. Hidroliese van suikerrietbagasse met heteroloë α–L-arabinofuranosidase en xilanase het aan die lig gebring dat vooraf behandeling met vloeistof ammonium meer effektief is in die vrystelling van komponent suikers as ‘n vooraf behandeling met water teen 140º C. ‘n Driedimensionele homologiese model van die heteroloë α–L-arabinofuranosidase is gekonstrueer deur die gebruik van die verklaarde kristal struktuur van arabinofuranosidase (AkabfB) van Aspergillus kawachii, wat 71% identies was.