Evaluation of culture conditions for assimilation of xylose and secretion of enzymes and peptides by yeasts isolated from the environment

Yeasts are chemorganotrophic organisms that mainly use glucose as a source of energy and carbon. In addition to glucose, other fermentable sugars are found in abundance in nature and have been underutilized in the industry, of which xylose stands out. For some yeasts, such as Saccharomyces cerevisiae, the use of pentoses is limited by the lack of specific membrane transporters and intracellular enzymes for the metabolization of this sugar. However, some yeasts are able to use xylose as a carbon source and bioconvert it into products such as etanol, organic acids or peptides. This implies the existence of a transport system and intracellular enzymes to metabolize it. The fermentation of pentoses is an essential step to improve the yield in the production of ethanol and organic acids. In this context, the prospection of depolymerizing auxiliary enzymes of lignocellulosic material, such as β-glycosidases and α-Larabinofuranosidases, also plays an important role in obtaining fermentable sugars. In addition, few studies are available regarding the production of bioactive peptides by yeasts, which can be promising sources of their production. Thus, the present work sought to investigate the consumption of xylose, the production of peptides with biological activity and the production of β-glycosidases by the species Pichia ofunaensis and Trichosporon multisporon, as well as the production of α-L-arabinofuranosidases by Aureobasidium pullulans and A. leucospermi. The enzymes were prospected using wheat bran as a substrate in solid state fermentation and then the functional biochemical characterization of the enzymes was performed. Regarding xylose assimilation, were evaluated the cell growth and consumption of this sugar by the yeasts P. ofunaensis and T. multisporon cultivated in YEPX medium at different initial pH: 4.5; 6.5 and 8.5; and temperatures: 28, 32 and 36 ºC. Finally, the presence of bioactive peptides with antimicrobial activity in the media in which the yeasts were grown was evaluated. In our results, we observed greater activity for β-glycosidases at pH 5.5-6 and 50-60 ºC (P. ofunaensis, with 0.40 U mL-1), and pH 5.5-6 and 55 ºC (T. multisporon, with 0.21 U mL-1). Both β-glycosidases were tolerant to ethanol, methanol, isopropanol and acetone at the concentrations evaluated. αL-arabinofuranosidases exhibited maximum activity at pH 5.5-6.5 and 60-70 ºC (A. pullulans, with 0.11 U mL-1), and pH 5.0-7.5 and 60 ºC (A. leucospermi, with 0.12 U mL-1). The α-Larabinofuranosidase produced by A. pullulans exhibited greater tolerance to ethanol than the enzyme produced by A. leucospermi. As for the consumption of xylose, yeasts showed better performance at initial pH 4,5, indicating that the transport of xylose has been carried out in association with H +. In addition, P. ofunanesis showed faster consumption of xylose at 32/36 ºC, while T. multisporon at 28 ºC. The analysis of the antibiogram for StapHylococcus aureus and Escherichia coli did not result in the formation of an inhibition halo, thus indicating that molecules that had bactericidal / bacteriostatic action were not secreted by the yeasts, concluding that the production of bioactive peptides of antimicrobial character by these yeasts did not occur. The information obtained here demonstrated the capacity of xylose consumption by the yeasts P. ofunaensis and T. multisporon and the biochemical properties of the enzymes secreted by the yeasts. These results will contribute to future studies of cloning and heterologous expression of these enzymes, as well as to further investigate the transporters involved in the transport of xylose. (AU)