ABSTRACT
The large amounts of lactose generated from the dairy industry and the microbial production of bioethanol from whey offer an alternative to costly production methods. Saccharomyces cerevisiae is the most intensively used microbial cell in bioethanol production, but S. cerevisiae cells lack lactose-hydrolyzing enzymes and genes. The addition of substrate range to S. cerevisiae provides a vital opportunity to develop its potential for biofuel production. Several approaches for engineering the metabolic pathway have been used to construct lactose-utilizing S. cerevisiae strains, particularly strains expressing the lactose-hydrolyzing genes of the yeast Kluyveromyces. Introduction of novel genes and pathways and the optimization of its cellular machinery by metabolic engineering of yeast are intensifying its applications as microbial fuel cell factories. When glucose is present, the enzymes essential for the utilization of galactose are synthesized at low rates or not at all. An effective way to improve ethanol productivity is by modifying the GAL system. The GAL gene regulatory system of S. cerevisiae is a tightly regulated system. Gal6, Gal80, and Mig1 are the negative regulators of the GAL system, and the knockout of the Gal6, Gal80, and Mig1 by homologous recombination results in an increase in ethanol productivity. Non-Saccharomyces yeasts such as Kluyveromyces are also possible solutions for inexpensive ethanol production from whey. Understanding the various strategies for the improvement of the yeast cell may provide an opportunity to develop efficient strains that will produce valuable bioethanol from renewable resources.
