Optimization of the direct conversion of lignocellulose to ethanol by genetically modified Saccharomyces cerevisiae for expression of cellulolytic and oxidative enzymes

Abstract

Bioethanol contributes to reducing energy dependence on fossil fuels. Furthermore, producing it from lignocellulose, obtaining the so-called second-generation ethanol, makes efficient use of agro-industrial waste. Its production consists of the degradation of lignocellulose and the fermentative conversion of the sugars released into ethanol, with Saccharomyces cerevisiae being the main microorganism used. However, this yeast does not produce enzymes that degrade lignocellulose, and this step is commonly performed by another agent. To make it able to carry out this conversion directly, a Brazilian industrial strain was genetically modified to express cellulolytic and oxidative enzymes, producing three variants of the strain. In this context, the objective of this work will be to evaluate the application of these S. cerevisiae variants in the direct conversion of cellulosic residues into ethanol and to optimize the scaling of the process, which is still a big gap among direct conversion studies. To this end, initially, the enzymes expressed in the variants will be characterized concerning the effect of temperature and pH on catalytic activity and thermal stability. Then, small-scale direct conversion tests will be carried out with Avicel®, sugarcane bagasse, and eucalyptus residue using inoculum that will be prepared by combining the different variants. The most efficient inoculums for ethanol production will be evaluated in the scaling of the process. The process of direct conversion of cellulose into ethanol will be evaluated technically-economically and environmentally through industrial scale simulation. Finally, comparison tests will be carried out using the original strain in processes with the addition of a cellulase cocktail. Therefore, this project expects to obtain a process for the production of cellulosic ethanol by direct conversion by modified S. cerevisiae with high yield and economically viable. It is important to highlight that this project is associated with the Thematic Project FAPESP 2015/20630-4 "Biorefinery development integrated to a bioethanol sugar cane plant with zero CO2 emission: routes to convert renewable resources to bio-products and bio-electricity", where the objectives proposed in this doctoral project are directly associated with the objectives of the thematic project mentioned above.