Pentose Metabolism in Saccharomyces cerevisiae: The Need to Engineer Global Regulatory Systems

Abstract

Extending the host substrate range of industrially relevant microbes, such as Saccharomyces cerevisiae, has been a highly-active area of research since the conception of metabolic engineering. Yet, rational strategies that enable non-native substrate utilization in this yeast without the need for combinatorial and/or evolutionary techniques are underdeveloped. Herein, this review focuses on pentose metabolism in S. cerevisiae as a case study to highlight the challenges in this field. In the last three decades, work has focused on expressing exogenous pentose metabolizing enzymes as well as endogenous enzymes for effective pentose assimilation, growth, and biofuel production. The engineering strategies that are employed for pentose assimilation in this yeast are reviewed, and compared with metabolism and regulation of native sugar, galactose. In the case of galactose metabolism, multiple signals regulate and aid growth in the presence of the sugar. However, for pentoses that are non-native, it is unclear if similar growth and regulatory signals are activated. Such a comparative analysis aids in identifying missing links in xylose and arabinose utilization. While research on pentose metabolism have mostly concentrated on pathway level optimization, recent transcriptomics analyses highlight the need to consider more global regulatory, structural, and signaling components.

Prospecting for l-arabinose/d-xylose symporters from Pichia guilliermondii and Aureobasidium leucospermi

“…Conventional D-glucose transporters (Gal2p, Hxt5p, and Hxt7p) from S. cerevisiae can assimilate D-xylose/L-arabinose when D-glucose is not present. However, in lignocellulolytic hydrolysate, the sequential assimilation of sugar, being the pentoses assimilated after D-glucose, characterizes a diauxic growth pattern [8] that would require additional time for reorganization of the metabolic machinery of the yeast to assimilate pentose after consumption of D-glucose (additional lag phase). Furthermore, according to the total sugar concentration in the medium, pentose consumption would be intensified in a culture medium in which fermentative products, such as ethanol and acetic acid, may reduce the kinetics of yeast growth [17].…”

“…The global production of lignocellulosic biomass is estimated to be approximately 3-5 gigatons per year [6,7]. However, from the large contingent of hemicelluloses, little is used in fermentative processes, mostly due to the inability of wild-type Saccharomyces cerevisiae to efficiently co-utilize hexoses and pentoses [5,8].…”

“…Advances in engineering S. cerevisiae to metabolize D-xylose/L-arabinose have been achieved in recent years by the overexpression of xylose reductase and xylitol dehydrogenase capable of using NADH [8,9]. However, the co-utilization of hexoses and pentoses by industrial strain S. cerevisiae is still an obstacle.…”

“…In this context, prospecting transporters from organisms capable of metabolizing C 5 sugar has been the main strategy. By prospecting pentose transporters, some D-xylose-fermenting yeasts, such as Candida shehatae, Pichia stipitis, and Pachysolen tannophilus [3], have been well studied; however, the results of engineering competent co-transport in S. cerevisiae have been unsatisfactory [2,5,8,9].…”

d‐Xylose consumption by nonrecombinant Saccharomyces cerevisiae: A review

“…When recombinant yeast cells able to assimilate xylose were analysed, the results indicated that intracellular xylose triggers a signal similar to carbon starvation in cells that are actively metabolizing xylose, probably causing the low assimilation rates observed with this carbon source (Osiro et al, ). Thus, the mechanisms by which xylose triggers a signalling response in S. cerevisiae are still not clear, and further analyses are required with evidence pointing to the need for a more system‐level rewiring of transcription factors, signalling pathways, and regulatory and structural proteins to achieve efficient pentose assimilation by S. cerevisiae (Endalur‐Gopinarayanan & Nair, ).…”