Isolement, sélection de souches levuriennes de sols arides sahariens (El-M’gheir) productrices de polygalacturonase

Abstract

The general purpose of this study is to search for performing yeasts in the production of thermostable polygalacturonase for possible industrial applications. A total of 20 strains, belonging to the yeast biodiversity of palm and steppe soils in the region of ElM’gheir (Province of El-Oued in the south-east of Algeria), were isolated. Conventional methods coupled with molecular biology (sequencing of the D1/D2 region of the gene encoding 26S rRNA and ITS) identified five different species: Clavispora lusitaniae, Cryptococcus magnus, Meyerozyma guilliermondii, Aureobasidium pullulan sand Yarrowia lipolytica. The agar plate testing method allowed the isolation of a single pectinolytic strain: Aureobasidium pullulans (S6). In order to reduce the cost of the polygalacturonase production, tomato pomace was used. This substrate was targeted by its richness in soluble carbohydrates (20.25%), in mineral matter (5.83%) and in proteins (18.31%). The study showed that the selected yeast Aureobasidium pullulans gives a better production of enzyme with a concentration of 4% in tomato pomace. The inducing substrate is lactose 1% causing an enzyme increase of 642%.Optimization of the medium components was achieved with the aid of response surface methodology. The composition of the optimized medium was as follows: pH 5.16, lactose 1.84 g/L and CaCl2 0.089 g/L. Practical validation of the optimum medium provided polygalacturonase activity of 22.05 U/ml, which was 5-fold higher than in unoptimized conditions. Batch cultivation in a 20 l bioreactor, the maximum production of the PGase was obtained within 32 of culture corresponding to a maximum of the yeast growth. The mechanism of the enzyme production is therefore of associated type. The chromatographic profile on a Sephacryl S-200 revealed two pectinolytic activities confirmed by the elution of two peaks on DEAE-Sepharose, it is therfore two isoenzymes: the PG1 and the PG2. After these steps, the two forms of isoenzymes were purified respectively with purification rates of 161.1 and 153.4 and yields of 27.39% and 26.45%. The PG1 and PG2 were glycoproteins. The molecular mass determined with SDS-PAGE was estimated as 113.79 KDa for the PG1 and 71.44 KDa for the PG2. The thin layer chromatography revealed enzymes that hydrolyse polygalacturonic acid to monogalacturonic acid, which shows that the enzymes are exopolygalacturonases (E.C 3.2.1.67). These two isoenzymes have different electrical charges and molecular weights with optimum pH of 10 and 5 for the PG1 and PG2 respectively. The PG1 has a good stability in the pH zone of 7-11 whereas the PG2 is stable in another pH zone varying from 4 to 9 with thermostabilities of 5 h at 60°C. At 80°C and 90°C, the PG1 retains 76.6% and 70% of its activity respectively. As for the PG2, it maintains 88.58% of its activity at 80°C, 70% at 90°C after a 5 h heat treatment. From the point of view of application, the two purified exo-PGases were able to improve the clarification of lemon juice. In view of these physicochemical properties and the thermostability, the two purified exo-PGases PG1 and PG2 could be applied industrially for the clarification of lemon juice or other industries: food industries, textile, paper and waste processing.