Bioethanol a Microbial Biofuel Metabolite; New Insights of Yeasts Metabolic Engineering

Abstract

Scarcity of the non-renewable energy sources, global warming, environmental pollution, and raising the cost of petroleum are the motive for the development of renewable, eco-friendly fuels production with low costs. Bioethanol production is one of the promising materials that can subrogate the petroleum oil, and it is considered recently as a clean liquid fuel or a neutral carbon. Diverse microorganisms such as yeasts and bacteria are able to produce bioethanol on a large scale, which can satisfy our daily needs with cheap and applicable methods. Saccharomyces cerevisiae and Pichia stipitis are two of the pioneer yeasts in ethanol production due to their abilities to produce a high amount of ethanol. The recent focus is directed towards lignocellulosic biomass that contains 30-50% cellulose and 20-40% hemicellulose, and can be transformed into glucose and fundamentally xylose after enzymatic hydrolysis. For this purpose, a number of various approaches have been used to engineer different pathways for improving the bioethanol production with simultaneous fermentation of pentose and hexoses sugars in the yeasts. These approaches include metabolic and flux analysis, modeling and expression analysis, followed by targeted deletions or the overexpression of key genes. In this review, we highlight and discuss the current status of yeasts genetic engineering for enhancing bioethanol production, and the conditions that influence bioethanol production.

The Xylose Metabolizing Yeast Spathaspora passalidarum is a Promising Genetic Treasure for Improving Bioethanol Production

“…The modern biotechnological applications for generation of alternative and renewable sources of biofuels are receiving more attention due to global worries over the climate change, rapid global warming, and the rising of fossil fuel costs. One of such growing biotechnological trends is the fermentation technology to convert the sugar-rich agriculture waste into bioethanol by conventional or non-conventional yeasts [1][2][3][4]. In general, yeasts have advantages over bacteria for commercial fermentation due to the thickness of their cell walls, less stringent nutritional requirements, large sizes, utmost resistance to contamination, and better growth at acidic pH of bioreactor fermenters.…”

“…Xylose sugar forms up to 15-25% of all angiosperm biomass, Figure 1. Model of yeast fermentation machinery for bioethanol production using the agriculture waste to feed yeasts [4]. The metabolic pathways for xylose and glucose assimilation and fermentation are indicated including the pentose phosphate pathway and glycolysis.…”

“…As a rule of thumb for metabolizing the xylose in most of xylose-fermenting yeasts [4], firstly the xylose is reduced by xylose reductase (XR) to xylitol. In the second step, the xylitol is oxidized by xylitol dehydrogenase (XDH) to xylulose.…”

“…However, xylitol dehydrogenase enzymes use NAD + specifically as a cofactor, which could cause imbalance between the cofactor's source for the XR-XDH pathway and xylitol accumulation under uncontrolled oxygen conditions ( Figure 2) [10]. Model of yeast fermentation machinery for bioethanol production using the agriculture waste to feed yeasts [4]. The metabolic pathways for xylose and glucose assimilation and fermentation are indicated including the pentose phosphate pathway and glycolysis.…”

“…One of pioneer xylose fermenting yeasts is Pichia stipites. P. stipitis is heterothallic ascomycetous yeast, predominantly haploid and related to pentose fermenting yeasts, such as Candida shehatae [1,4,[11][12][13][14][15]. P. stipitis was recently renamed to be Scheffersomyces stipitis [15] and it is natively one of the highest xylose-utilizing and fermenting yeasts.…”